OOP Java
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The document outlines a course on Object-Oriented Programming (OOP) in Java, covering fundamental concepts such as classes, objects, inheritance, encapsulation, polymorphism, and collaboration. It discusses the history and benefits of OOP compared to procedural programming, emphasizing the advantages for software maintenance and teamwork. Additionally, it presents the course structure, including topics and practical tests designed to enhance understanding of both OOP principles and Java programming.
![Object Oriented Programming OOP
34
Lecture 4: Java Language Structure
This section provides a brief overview of the Java language structure.
Let’s consider the following running example of a Java program
calculating BMI to illustrate the discussion.
public class BMICalculator {
// declare variables
double weight;
double height;
double BMI;
public BMICalculator(double w, double h) {
weight = w;
height = h;
}
public double calculateBMI() {
return weight / (height * height);
}
// This is our main method.
public static void main(String[] args) {
BMICalculator calculator = new BMICalculator(60, 1.70);
double bmi = calculator.calculateBMI();
// print BMI to screen
System.out.println("Your BMI is " + bmi + ".");
}
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-34-320.jpg)
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Methods
As discussed earlier, a method is a piece of code within a class definition,
and it performs a specific kind of functionality. Just as with a class, every
method definition is enclosed within brackets {...}. In the BMICalculator
example, three methods have been defined—BMICalculator,
CalculateBMI, and main. The main method is the main entry point of
program execution. So, it is the first method that runs when executing the
class BMICalculator. Consider the main method:
The first line, called the method declaration, contains several keywords ,
we will briefly introduce those you see here.
public: This method can be accessed by other classes and/or
methods without restriction.
static: This method does not need an object.
void: This method does not return any value.
(String[] args): This is a conventional way to refer to the arguments
of the method. In this case, the method takes an array of strings as
its input parameter.
The method starts by assigning the values of 60 and 1.70 to the variables
of weight and height, It then continues by calculating the BMI by calling
another method
called calculateBMI. The calculateBMI method then looks as follows:
public static void main(String[] args) {
BMICalculator calculator = new BMICalculator(60, 1.70);
double bmi = calculator.calculateBMI();
// print BMI to screen
System.out.println("Your BMI is " + bmi + ".");
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-36-320.jpg)
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38
Another Application: Adding Integers
Our next application reads (or inputs) two integers (whole numbers, such
as –22, 7, 0 and 1024) typed by a user at the keyboard, compute their sum
and display it. Programs remember numbers and other data in the
computer’s memory and access that data through program elements
called variables. The program demonstrates these concepts.
// Addition program that inputs two numbers then displays their sum.
import java.util.Scanner;
public class Addition{
public static void main(String[] args) {
// create a Scanner to obtain input from the command window
Scanner input = new Scanner(System.in);
int number1; // first number to add
int number2; // second number to add
int sum; // sum of number1 and num
System.out.print("Enter first integer: ");
number1 = input.nextInt(); // read first number from user
System.out.print("Enter second integer: "); // prompt
number2 = input.nextInt(); // read second number from user
sum = number1 + number2; // add numbers, then store total in
sum
System.out.printf("Sum is %d%n", sum); // display sum
} // end method main
} // end class Addition](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-38-320.jpg)
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42
Lecture 5: Decision Making and Repetition Statement:
If condition
A condition is an expression that can be true or false. This section
introduces Java’s if selection statement, which allows a program to
make a decision based on a condition’s value. For example, the condition
“grade is greater than or equal to 60” determines whether a student passed
a test. If the condition in an if statement is true, the body of the if
statement executes. If the condition is false, the body does not execute.
import java.util.Scanner; // program uses class Scanner
public class Comparison
{
// main method begins execution of Java application
public static void main(String[] args)
{
// create Scanner to obtain input from command line
Scanner input = new Scanner(System.in);
int number1; // first number to compare
int number2; // second number to compare
System.out.print("Enter first integer: "); // prompt
number1 = input.nextInt(); // read first number from user
System.out.print("Enter second integer: "); // prompt
number2 = input.nextInt(); // read second number from user
if (number1 == number2)
System.out.printf("%d == %d%n", number1, number2);
if (number1 != number2)
System.out.printf("%d != %d%n", number1, number2);
if (number1 < number2)
System.out.printf("%d < %d%n", number1, number2);
} // end method main
}// end class Comparison](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-42-320.jpg)
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44
<= 100—becomes false. This terminates the repetition, so the final value
of product is 243. At this point, program execution continues with the
next statement after the while statement.
for Repetition Statement
presented the essentials of counter-controlled repetition. The while
statement can be used to implement any counter-controlled loop. Java
also provides the for repetition statement, which specifies the counter-
controlled repetition details in a single line of code.
public class WhileCounter
{
public static void main( String args[] )
{
int counter = 1; // declare and initialize control variable
while ( counter <= 10 ) // loop-continuation condition
{
System.out.printf( "%d ", counter );
++counter; // increment control variable by 1
} // end while
System.out.println(); // output a newline
} // end main
} // end class WhileCounter
public class ForCounter
{
public static void main( String args[] )
{
for ( int counter = 1; counter <= 10; counter++ )
System.out.printf( "%d ", counter );
System.out.println(); // output a newline
} // end main](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-44-320.jpg)
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45
do...while Repetition Statement
The do...while repetition statement is similar to the while statement. In
the while, the program tests the loop continuation condition at the
beginning of the loop, before executing the loop's body; if the condition is
false, the body never executes. The do...while statement tests the loop-
continuation condition after executing the loop's body; therefore, the
body always executes at least once. When a do...while statement
terminates, execution continues with the next statement in sequence.
public class Interest
{
public static void main( String args[] )
{
double amount; // amount on deposit at end of each year
double principal = 1000.0; // initial amount before interest
double rate = 0.05; // interest rate
// display headers
System.out.printf( "%s%20sn", "Year", "Amount on deposit" );
// calculate amount on deposit for each of ten years
for ( int year = 1; year <= 10; year++ )
{
// calculate new amount for specified year
amount = principal * Math.pow( 1.0 + rate, year );
// display the year and the amount
System.out.printf( "%4d%,20.2fn", year, amount );
} // end for](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-45-320.jpg)
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46
Declaring Methods with Multiple Parameters
We now show how to write methods with multiple parameters. The
application uses a programmer-declared method called maximum to
determine and return the largest of three double values that are input by
the user. When the application begins execution, class
MaximumFinderTest's main method creates one object of class
MaximumFinder and calls the object's determineMaximum method to
produce the program's output. In class MaximumFinder of method
determineMaximum prompt the user to enter three double values, then
read them from the user. calls method maximum to determine the largest
of the three double values passed as arguments to the method. When
method maximum returns the result to, the program assigns maximum's
return value to local variable result. Then line 24 outputs the maximum
value. At the end of this section, we'll discuss the use of operator +.
public class DoWhileTest
{
public static void main( String args[] )
{
int counter = 1; // initialize counter
do
{
System.out.printf( "%d ", counter );
++counter;
} while ( counter <= 10 ); // end do...while
System.out.println(); // outputs a newline
} // end main
} // end class DoWhileTest](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-46-320.jpg)
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Method Overloading
Methods of the same name can be declared in the same class, as long as
they have different sets of parameters (determined by the number, types
and order of the parameters)—this is called method overloading. When
an overloaded method is called, the Java compiler selects the appropriate
method by examining the number, types and order of the arguments in the
call. Method overloading is commonly used to create several methods
with the same name that perform the same or similar tasks, but on
different types or different numbers of arguments.
public class MaximumFinderTest
{
// application starting point
public static void main( String args[] )
{
MaximumFinder maximumFinder = new MaximumFinder();
maximumFinder.determineMaximum();
} // end main
} // end class MaximumFinderTest
public class MethodOverloadTest
{
public static void main( String args[] )
{
MethodOverload methodOverload = new MethodOverload();
methodOverload.testOverloadedMethods();
} // end main
} // end class MethodOverloadTest](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-48-320.jpg)
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Add the following main method to the Student class definition as follows.
Note the use of the new keyword to create objects:
take a look at how it works.Now
1. The new keyword is used to create objects, which you will use to
create a bunch of students. Note the use of the dot (.) operator in
this code to access (read and write) the Student objects’ variables.
2. To actually execute, that is, run the program, you add a so ‐called
main method to the Student class. As Java has no way of knowing
which particular method you want to use as the entry point of the
program, a special method exists—the so called main method—
that serves exactly this purpose.
3. When running this program from Eclipse, Java will call the class’
main method. The main method will create some students, set their
variables, and print some information to Eclipse’s console.
public static void main(String[] args) {
Student firstStudent = new Student();
Student secondStudent = new Student();
firstStudent.id = 1;
firstStudent.firstName = "Marc";
secondStudent.id = 2;
secondStudent.firstName = "Sophie";
System.out.println("The student object referred to "+
"by the variable secondStudent has the first "+
"name: "+secondStudent);
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-52-320.jpg)
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54
Class variables: Variables that are not bound to an object but
instead belong to the class as such, that is, to the blueprint of the
object.
Final variables: Variables that—after their initial assignment—
cannot be modified.
Finally, I will also devote some words to the topic of variable scope. A
scope of a variable is the context in which it’s defined.
Instance Variables
In object‐oriented programming, an instance variable is a variable that’s
defined in a class. Instance variables are also commonly referred to as
member variables or fields of a class.
In object‐oriented programming, a class variable, also denoted as a static
variable, is a variable that’s been allocated “statically”—meaning that
this variable is shared between all objects (instances) belonging to this
class.
class Book {
String title;
String[] authors;
}
class Book {
static int maxAmountOfPages = 500;
}
//MAIN FUNCTION
Book.maxAmountOfPages = 2000; // Let's increase the max amount
of pages
System.out.println("We now support books with max. pages: "
+Book.maxAmountOfPages);](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-54-320.jpg)
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returned as part of the String. Recall all objects in Java have a toString
method that returns a String representation of the object. We chose to
return a String containing the time in standard-time format. Method
toString is called implicitly whenever a Time1 object appears in the code
where a String is needed, such as the value to output with a %s format
specifier in a call to System.out.printf. You may also call toString
explicitly to obtain a String representation of a Time object.
public class Time1Test
{
public static void main(String[] args)
{
Time1 time = new Time1(); // invokes Time1 constructor
displayTime("After time object is created", time);
System.out.println();
time.SetHour = 13;
time.SetMunite= 27;
time.SetSecond =6;
displayTime("After calling set for all attributes ", time);
System.out.println();
}
private static void displayTime(String header, Time1 t)
{
System.out.printf("%s%nUniversal time: %s%nStandard time: %s%n",
header, t.toUniversalString() , t.toString());
}
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-65-320.jpg)
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66
Lecture 8: Array and Collocations
An array is a list of values. Each value is stored at a specific, numbered
position in the array. The number corresponding to each position is called
an index or a subscript. In Java, arrays are objects. To create an array, the
reference to the array must be declared. The array can then be instantiated
using the new operator, which allocates memory space to store values.
public class BasicArray
{
public static void main(String[] args)
{
final int LIMIT = 15, MULTIPLE = 10;
int [] list = new int [LIMIT];
// Initialize the array values
for ( int index = 0; index < LIMIT; index++)
list[index] = index * MULTIPLE;
list[5] = 999; // change one array value
for ( int value : list)
System.out.print(value + " ");
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-66-320.jpg)
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Initialize Lists
You can use an initializer list to instantiate an array and provide the
initial values for the elements of the array. the following declaration
instantiates the array scores as an array of eight integers, indexed from 0
to 7 with the specified initial values:
int [] scores = {87, 98, 69, 87, 65, 76, 99, 83};](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-67-320.jpg)
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Arrays as parameters
An entire array can be passed as a parameter to a method. Because
an array is an object, when an entire array is passed as a parameter,
a copy of the reference to the original array is passed.
Arrays of Objects
We used arrays to store primitive types such as integers and characters.
Arrays can also store references to objects as elements. Fairly complex
information management structures can be created using only arrays and
other objects. For example, an array could contain objects, and each of
those objects could consist of several variables and the methods that use
them. When we store objects in an array, each element is a separate
object.
public class Primes
{
public static void main(String[] args)
{
int[] primeNums = {2, 3, 5, 7, 11, 13, 17, 19};
System.out.println("Array length: " +
primeNums.length);
System.out.println("The first few prime numbers
are:");
for (int prime : primeNums)
System.out.print(prime + " ");
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-68-320.jpg)
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Collections and Class ArrayList
The Java API provides several predefined data structures, called
collections, used to store groups of related objects in memory. These
classes provide efficient methods that organize, store and retrieve your
data without requiring knowledge of how the data is being stored. This
reduces application-development time.
ArrayList<T> (package java.util) provides a convenient solution to this
problem—it can dynamically change its size to accommodate more
String[] words = {“friendship" , "loyalty" , "honor" , " " , " "};
ArrayList<String> list1 = new ArrayList<String>();
ArrayList<Integer> list2 = new ArrayList< Integer >() ;](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-69-320.jpg)
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import java.util.ArrayList;
public class ArrayListCollection
{
public static void main(String[] args)
{
// create a new ArrayList of Strings with an initial capacity of 10
ArrayList<String> items = new ArrayList<String>();
items.add("red"); // append an item to the list
items.add(0, "yellow"); // insert "yellow" at index 0
// header
System.out.print( "Display list contents with counter-controlled loop:");
// display the colors in the list
for (int i = 0; i < items.size(); i++)
System.out.printf(" %s", items.get(i));
// display colors using enhanced for in the display method
display(items, "%nDisplay list contents with enhanced for statement:");
items.add("green"); // add "green" to the end of the list
items.add("yellow"); // add "yellow" to the end of the list
display(items, "List with two new elements:");
items.remove("yellow"); // remove the first "yellow"
display(items, "Remove first instance of yellow:");
items.remove(1); // remove item at index 1
display(items, "Remove second list element (green):");
// check if a value is in the List
System.out.printf(""red" is %sin the list%n",
items.contains("red") ? "": "not ");
// display number of elements in the List
System.out.printf("Size: %s%n", items.size());
}
// display the ArrayList's elements on the console
public static void display(ArrayList<String> items, String header)
{
System.out.printf(header); // display header
// display each element in items
for (String item : items)
System.out.printf(" %s", item);
} }](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-71-320.jpg)
![Object Oriented Programming OOP
73
Listing 7.1 Each DVD added to the collection is specified by its title,
director, year of release, purchase price, and whether or not it is in Blu-
ray format.
public class Movies
{
public static void main(String[] args)
{
DVDCollection movies = new DVDCollection();
movies.addDVD("The Godfather", "Francis Ford Coppola", 1972, 24.95, true);
movies.addDVD("District 9", "Neill Blomkamp", 2009, 19.95, false);
movies.addDVD("Iron Man", "Jon Favreau", 2008, 15.95, false);
movies.addDVD("All About Eve", "Joseph Mankiewicz", 1950, 17.50, false);
movies.addDVD("The Matrix", "Andy & Lana Wachowski", 1999, 19.95, true);
System.out.println(movies);
movies.addDVD("Iron Man 2", "Jon Favreau", 2010, 22.99, false);
movies.addDVD("Casablanca", "Michael Curtiz", 1942, 19.95, false);
System.out.println(movies);
}
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-73-320.jpg)
![Object Oriented Programming OOP
75
public class DVDCollection
{
private DVD[] collection;
private int count;
private double totalCost;
public DVDCollection()
{
collection = new DVD[100];
count = 0;
totalCost = 0.0;
}
public void addDVD(String title, String director, int year, double cost, boolean bluray){
if (count == collection.length)
increaseSize();
collection[count] = new DVD(title, director, year, cost, bluray);
totalCost += cost;
count++;
}
public String toString(){
NumberFormat fmt = NumberFormat.getCurrencyInstance();
String report = "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~n";
report += "My DVD Collectionnn";
report += "Number of DVDs: " + count + "n";
report += "Total cost: " + fmt.format(totalCost) + "n";
report += "Average cost: " + fmt.format(totalCost/count);
report += "nnDVD List:nn";
for (int dvd = 0; dvd < count; dvd++)
report += collection[dvd].toString() + "n";
return report;
}
private void increaseSize(){
DVD[] temp = new DVD[collection.length * 2];
for (int dvd = 0; dvd < collection.length; dvd++)
temp[dvd] = collection[dvd];
collection = temp;
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-75-320.jpg)
![Object Oriented Programming OOP
79
Inheritance is a powerful software development technique and a defining
characteristic of object-oriented programming.
In general, new classes can be created via inheritance faster, easier,
and cheaper than by writing them from scratch. Inheritance is one way
to support the idea of software reuse.
The original class that is used to derive a new one is called the parent
class, superclass, or base class. The derived class is called a child class,
or subclass. Java uses the reserved word extends to indicate that a new
class is being derived from an existing class.
Let’s look at an example. The program shown in Listing 8.1 instantiates
an object of class Dictionary , which is derived from a class called Book
. In the main method, three methods are invoked through the Dictionary
object: two that were declared locally in the Dictionary class and one that
was inherited from the Book class.
The Book class (see Listing 8.2 ) is used to derive the Dictionary class
(see Listing 8.3 ) using the reserved word extends in the header of
Dictionary .
public class Words{
public static void main(String[] args){
Dictionary webster = new Dictionary();
System.out.println("Number of pages: " + webster.getPages());
System.out.println("Number of definitions: " +
webster.getDefinitions());
System.out.println("Definitions per page: " +
webster.computeRatio());
}}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-79-320.jpg)
![Object Oriented Programming OOP
82
Constructors, however, are not inherited. Constructors are special
methods that are used to set up a particular type of object, so it doesn’t
make sense for a class called Dictionary to have a constructor called
Book . But you can imagine that a child class may want to refer to the
constructor of the parent class, which is one of the reasons for the super
reference.
The reserved word super can be used in a class to refer to its parent class.
Using the super reference, we can access a parents members. Like the
this reference, what the word super refers to depends on the class in
which it is used. One use of the super reference is to invoke a parent's
constructor. Let's look at an example. Listing 8.4 shows a modification of
the original Words program from Listing 8.1 .
public class Words2
{
public static void main(String[] args)
{
Dictionary2 webster = new Dictionary2(1500, 52500);
System.out.println("Number of pages: " + webster.getPages());
System.out.println("Number of definitions: " +
webster.getDefinitions());
System.out.println("Definitions per page: " +
webster.computeRatio());
}}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-82-320.jpg)
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88
Listing 9.3 shows a program called MiniQuiz, which uses some Question
objects.
public String getQuestion()
{
return question;
}
public String getAnswer()
{
return answer;
}
public boolean answerCorrect(String candidateAnswer)
{
return answer.equals(candidateAnswer);
}
public String toString()
{
return question + "n" + answer;
}
import java.util.Scanner;
public class MiniQuiz
{
public static void main(String[] args)
{
Question q1, q2;
String possible;
Scanner scan = new Scanner(System.in);
q1 = new Question("What is the capital of Jamaica?",
"Kingston");
q1.setComplexity(4);
q2 = new Question("Which is worse, ignorance or apathy?",
"I don't know and I don't care");
q2.setComplexity(10);](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-88-320.jpg)
![Object Oriented Programming OOP
92
Understanding the real scenario of abstract class
In this example, Bank is the abstract class, its implementation is provided
by the SBI and PNB classes.
Output
Rate of Interest is: 7 %
Rate of Interest is: 8 %
abstract class Bank{
abstract int getRateOfInterest();
}
class SBI extends Bank{
int getRateOfInterest(){return 7;}
}
class PNB extends Bank{
int getRateOfInterest(){return 8;}
}
class TestBank{
public static void main(String args[]){
Bank b;
b=new SBI();
System.out.println("Rate of Interest is: "+b.getRateOfInterest()+" %");
b=new PNB();
System.out.println("Rate of Interest is: "+b.getRateOfInterest()+" %");
}
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-92-320.jpg)
![Object Oriented Programming OOP
93
Abstract class having constructor, data member, abstract
method, method body.
Abstract classes are useful when you want to create a generic type
that is used as the superclass for two or more subclasses, but the
superclass itself doesn’t represent an actual object. If all
employees are either salaried or hourly, for example, it makes
sense to create an abstract Employee class and then use it as the
base class for
abstract class Bike{
Bike(){
System.out.println("bike is created");}
abstract void run();
void changeGear(){System.out.println("gear changed");}
}
class Honda extends Bike{
void run(){System.out.println("running safely..");}
}
class TestAbstraction2{
public static void main(String args[]){
Bike obj = new Honda();
obj.run();
obj.changeGear();
}
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-93-320.jpg)
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97
Example of polymorphism.
In below example we create two class Person an Employee, Employee
class extends Person class feature and override walk() method. We are
calling the walk() method by the reference variable of Parent class. Since
it refers to the subclass object and subclass method overrides the Parent
class method, subclass method is invoked at runtime.
Output
Running fast..
class Person
{
void walk()
{
System.out.println("Can Run....");
}
}
class Employee extends Person
{
void walk()
{
System.out.println("Running Fast...");
}
public static void main(String arg[])
{
Person p=new Employee(); //upcasting
p.walk();
}
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-97-320.jpg)
![Object Oriented Programming OOP
99
Output:
SBI Rate of Interest: 8
ICICI Rate of Interest: 7
AXIS Rate of Interest: 9
Garbage Collection
Garbage collection is a way to reclaim memory from objects once they
are no longer in use. By removing unused objects from memory, more is
available for new and existing objects. One difference between C++ and
Java is that Java incorporates automatic garbage collection, so the
programmer does not need to manage memory as much themselves. An
object is eligible for garbage collection when it is no longer accessible
through any variable. However, if one object refers to another and that
object in turn refers to the first in a cyclical dependency, they may both
be eligible for garbage collection. Once an object is eligible for garbage
collection, the garbage collector will eventually remove it from memory.
Recall how when a new object is created, the constructor method is called
to initialize it. There is also a finalize method that’s invoked just before
the object is destroyed. This method is built in to the Object superclass,
class Test3{
public static void main(String args[]){
Bank b1=new SBI();
Bank b2=new ICICI();
Bank b3=new AXIS();
System.out.println("SBI Rate of Interest: "+b1.getRateOfInterest());
System.out.println("ICICI Rate of Interest: "+b2.getRateOfInterest())
System.out.println("AXIS Rate of Interest: "+b3.getRateOfInterest())
}
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-99-320.jpg)
![Object Oriented Programming OOP
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but you can override it in your classes to perform special actions before
an object is destroyed.
A Person variable p1 is declared and initialized to a newly created Person
object with the name Adam. A second Person variable p2 is declared and
initialized to another newly created Person object with the name Robert.
See Figure below.
In the next statement, the variable p1 is reassigned to the reference object
of p2. See Figure below.
class PersonManager {
public static void main(String args[]) {
Person p1 = new Person("Adam");
Person p2 = new Person("Robert");
p1 = p2;
}
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-100-320.jpg)
![Object Oriented Programming OOP
103
when errors arise in an application that does not use exception handling.
Figure 12.1 prompts the user for two integers and passes them to method
quotient, which calculates the integer quotient and returns an int result. In
this example, you’ll see that exceptions are thrown (i.e., the exception
occurs) by a method when it detects a problem and is unable to handle it
import java.util.Scanner;
public class DivideByZeroNoExceptionHandling
{
// demonstrates throwing an exception when a divide-by-zero occurs
public static int quotient(int numerator, int denominator)
{
return numerator / denominator; // possible division by zero
}
public static void main(String[] args)
{
Scanner scanner = new Scanner(System.in);
System.out.print("Please enter an integer numerator: ");
int numerator = scanner.nextInt();
System.out.print("Please enter an integer denominator: ");
int denominator = scanner.nextInt();
int result = quotient(numerator, denominator);
System.out.printf(
"%nResult: %d / %d = %d%n", numerator, denominator,
result);
}}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-103-320.jpg)
![Object Oriented Programming OOP
105
Example: Handling ArithmeticExceptions and
InputMismatchExceptions
The application in Fig. 12.3, which is based on Fig. 12.2, uses exception
handling to process any ArithmeticExceptions and
InputMistmatchExceptions that arise. The application still prompts the
user for two integers and passes them to method quotient, which
calculates the quotient and returns an int result. This version of the
application uses exception handling so that if the user makes a mistake,
the program catches and handles (i.e., deals with) the exception—in this
case.
import java.util.Scanner;
import java.util.InputMismatchException;
public class DivideByZeroWithExceptionHandling
{
public static int quotient(int numerator, int denominator)
{
return numerator / denominator; // possible division by zero }
public static void main(String[] args)
{
Scanner scanner = new Scanner(System.in);
boolean continueLoop = true; // determines if more input is needed
do
{
try
{
System.out.print("Please enter an integer numerator: ");
int numerator = scanner.nextInt();
System.out.print("Please enter an integer denominator: ");
int denominator = scanner.nextInt();
int result = quotient(numerator, denominator);
System.out.printf("%nResult: %d / %d = %d%n", numerator,
denominator, result);
continueLoop = false; // input successful; end looping
}](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-105-320.jpg)
![Object Oriented Programming OOP
114
When a class implements an interface, the same is-a relationship
provided by inheritance applies. For example, class Employee
implements Payable, so we can say that an Employee is a Payable. In
fact, objects of any classes that extend Employee are also Payable
objects. SalariedEmployee objects, for instance, are Payable objects. As
with inheritance relationships, an object of a class that implements an
interface may be thought of as an object of the interface type. Objects of
any subclasses of the class that implements the interface can also be
thought of as objects of the interface type. Thus, just as we can assign the
reference of a SalariedEmployee object to a superclass Employee
variable, we can assign the reference of a SalariedEmployee object to an
interface Payable variable. Invoice implements Payable, so an Invoice
object also is a Payable object, and we can assign the reference of an
Invoice object to a Payable variable.
5. All methods of class Object can be called by using a
reference of an interface type. A reference refers to an
object, and all objects inherit the methods of class
Object.
public class PayableInterfaceTest
{
public static void main( String args[] )
{
// create four-element Payable array
Payable payableObjects[] = new Payable[ 4 ];
// populate array with objects that implement Payable
payableObjects[ 0 ] = new Invoice( "01234", "seat", 2,
375.00 );
payableObjects[ 1 ] = new Invoice( "56789", "tire", 4,
79.95 );
payableObjects[ 2 ] =
new SalariedEmployee( "John", "Smith", "111-11-
1111", 800.00 );](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-114-320.jpg)
![Object Oriented Programming OOP
115
payableObjects[ 3 ] =
new SalariedEmployee( "Lisa", "Barnes", "888-88-
8888",1200.00 );
System.out.println(
"Invoices and Employees processed OOP:n" );
for ( Payable currentPayable : payableObjects )
{
// output currentPayable and its appropriate
payment amount
System.out.printf( "%s n%s: $%,.2fnn",
currentPayable.toString(),
"payment due", currentPayable.getPaymentAmount()
);
} // end for
} // end main
} // end class PayableInterfaceTest
OUTPUT
Invoices and Employees processed OOP:
invoice:
part number: 01234 (seat)
quantity: 2
price per item: $375.00
payment due: $750.00
invoice:
part number: 56789 (tire)
quantity: 4
price per item: $79.95
payment due: $319.80
salaried employee: John Smith
social security number: 111-11-1111
weekly salary: $800.00
payment due: $800.00
salaried employee: Lisa Barnes
social security number: 888-88-8888
weekly salary: $1,200.00
payment due: $1,200.00](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-115-320.jpg)
![Object Oriented Programming OOP
117
6) Write a class called Course that represents a course offered to
students. It should contain instance data that represents the
course title, course code, credits and course Instructor’s name.
Define the Course constructor to accept and initialize all instance
data. Include getter and setter methods for all instance data.
Include a toString method that returns a one-line description
of the course. Create a driver class called CourseDetails whose
main method instantiates and updates several Course objects.
7) Drawing Class diagram and Create classes with all code necessary
in dependency to package below
8) Draw Class diagram for system Company. System consists of two
classes Person and Main. Person has two types Employer and
Lecture. For print information you can using Interface PrintInfo.
Please write all classes, attributes and method.
9) Write Class and Attributes depending on the Class diagram
below?Knowing that attribute numberofStudentPhone is fixed.
classTestCode{
public static void main(String args[]){
Person b1=new Employer();
System.out.println("Info Emp: "+b1.getInfo());
Personb2=new Servicer();
System.out.println(" Info Servicer: "+b2.getInfo());
}
1Student
StudentPhone
n](https://image.slidesharecdn.com/oopjava-170114181111/85/OOP-Java-117-320.jpg)
OOP Java
- 1. Object Oriented Programming OOP 1 Object oriented Programming in Java (I) Lecture: Dr. Saif Qassim Mohammed University Information Technologies and Communication The course aims to provide you with an understanding of the fundamental concepts involved in object-oriented programming (object, class, protocol, hierarchy, inheritance, encapsulation, polymorphism and collaboration). The programming language you will use is Java. However, the purpose of the course is not to teach you the minutiae of the Java language, but rather to teach you fundamental object-oriented programming concepts and skills that will be transferable to any object oriented language 2017
- 2. Object Oriented Programming OOP 2 Course outline: Week No. Details 1 Overview of Object-Oriented Programming: The History of OOP , The Characteristics of OOP, Classes, objects, Properties OOP Abstraction Encapsulation Polymorphism Inheritance , Benefits of OOP Approach. 2 Designing OOP Solutions: Identifying the Class Structure Unified Modeling Language UML(SRS, Use Cases, Class Diagrams, Relationships), Developing the Class Model 3 Introduction Java : Short Java History, Features of Java , Java Runtime Environment (JRE ), Java Platforms, Integrated Development Environments, Java Application , Java Applications Interface (API) 4 Java Language Structure: Classes, Variables , Methods , import Declarations, Creating a Scanner, Declaring Variables, Prompting the User for Input, Displaying the Result 5 Decision Making and Repetition Statement: If condition , while Repetition, for Repetition, do...while, Declaring Methods with Multiple Parameters, Overloading Methods 6 Creating Classes : Constructors by default and Constructors with parameters , Properties, Methods, , Subclasses , Collaborating objects, Workspace variables , Instance variables , Class variables 7 Class members and Encapsulation:, Access Modifiers (Getters, Setters) Overloading Methods, The this Keyword, Time Class Case Study 8 Working with Arrays and Collections: Array, Initialize Lists , Arrays as parameters , Arrays of Objects , Collections and Class ArrayList , Case Study: Creates a DVDCollection
- 3. Object Oriented Programming OOP 3 9 First test (Theoretical and Practical) 10 Creating Class Inheritance: Understanding Inheritance , Creating Base and Derived Classes, Superclass subclassv , Overriding, Keyword super. 11 Interface and abstract class : understand Interface , why use interface , use case Complexity Question, understand abstract class, Understanding the real scenario, different between interface and abstract class. 12 Polymorphism : understand polymorphism , Real life example of polymorphism, Implementing for Extensibility, Example of polymorphism, Garbage Collection 13 Exception : Uncaught exceptions, understand exception and throw ,Stack Trace, Example: Handling ArithmeticExceptions and InputMismatchExceptions 14 Programming Practice OOP : Abstraction, Encapsulation, Inheritance and Polymorphism 15 Second test (Theoretical and Practical) References • Programming with Java and UMLThe Essence of Object Oriented .Bruce E. Wampler, Ph.D. Copyright © 2010 • Beginning Java® Programming: The Object-Oriented ApproachJohn Wiley & Sons, Inc.Copyright © 2015 by John Wiley • Java Software Solutions John Lewis • William Loftus Copyright © 2015 by John Wiley
- 4. Object Oriented Programming OOP 4 Lecture 1: Overview of Object-Oriented Programming What is OOP? Object-oriented programming is an approach to software development in which the structure of the software is based on objects interacting with each other to accomplish a task. This interaction takes the form of messages passing back and forth between the objects. If you want to go to the store, for example, you interact with a car object. A car object consists of other objects that interact with each other to accomplish the task of getting you to the store. You put the key object in the ignition object and turn it. This in turn sends a message (through an electrical signal) to the starter object, which interacts with the engine object to start the car. As a driver, you are isolated from the logic of how the objects of the system work together to start the car. You just initiate the sequence of events by executing the start method of the ignition object with the key. You then wait for a response (message) of success or failure. Similarly, users of software programs are isolated from the logic needed to accomplish a task. For example, when you print a page in your word processor, you initiate the action by clicking a print button. You are isolated from the internal processing that needs to occur; you just wait for a response telling you if it printed. In the software program, the button object interacts with a printer object, which interacts with the actual printer to accomplish the task of printing the page. The History of OOP OOP concepts started surfacing in the mid-1960s with a programming language called Simula and further evolved in the 1970s with advent of
- 5. Object Oriented Programming OOP 5 Smalltalk. In the mid-1980s there was a resurgence of interest in object- oriented methodologies. Specifically, OOP languages such as C++ and Eiffel became popular with mainstream computer programmers. OOP continued to grow in popularity in the 1990s, most notably with the advent of Java and the huge following it attracted. And in 2002, in conjunction with the release of the .NET Framework, Microsoft introduced a new OOP language, C# (pronounced C-sharp) and revamped their widely popular existing language, Visual Basic, so that it is now truly object oriented. Why Use OOP? Why has OOP developed into such a widely used paradigm for solving business problems today? During the 1970s and 1980s, procedure- oriented programming languages such as C, Pascal, and Fortran were widely used to develop business-oriented software systems. Procedural languages organize the program in a linear fashion—they run from top to bottom. In other words, the program is a series of steps that run one after another. This type of programming worked fine for small programs that consisted of a few hundred code lines, but as programs became larger they became hard to manage and debug. In an attempt to manage the ever-increasing size of the programs, structured programming was introduced to break down the code into manageable segments called functions or procedures. This was an improvement, but as programs performed more complex business functionality and interacted with other systems, the following shortcomings of structural programming began to surface: • Programs became harder to maintain. • Existing functionality was hard to alter without adversely affecting all of the system’s functionality.
- 6. Object Oriented Programming OOP 6 • New programs were essentially built from scratch. Consequently, there was little return on the investment of previous efforts. • Programming was not conducive to team development. Programmers had to know every aspect of how a program worked and could not isolate their efforts on one aspect of a system. • It was hard to translate business models into programming models. As a result, many business software developers turned to object-oriented methods and programming languages. The benefits included the following: • a more intuitive transition from business-analysis models to software-implementation models • the ability to maintain and implement changes in the programs more efficiently and rapidly • the ability to create software systems more effectively using a team process, allowing specialists to work on parts of the system • the ability to reuse code components in other programs and purchase components written by third-party developers to increase the functionality of existing programs with little effort • better integration with loosely coupled distributed-computing systems • improved integration with modern operating systems • the ability to create a more intuitive graphical-user interface for the users Object-Oriented Languages There are several object-oriented programming languages available to choose from, including Smalltalk, Eiffel, C++, Objective C, Objective Pascal, Java, C#, and even a version of Lisp. There are two clear
- 7. Object Oriented Programming OOP 7 marketplace winners, C++ and Java. Today, Java is the emerging object- oriented language of choice for many programmers and software projects. One of the main reasons for Java's emergence is the World Wide Web, and Java's ability to run web applets directly on any computer or operating system with a web browser. Another reason is that Java is an excellent programming language. It is a small, well-designed language that can be used for not just web applets, but full-blown programs on almost any computer available today. Java was somewhat hampered in its early days because of its speed, but this is really no longer an issue. Because it is such a good language, Java has been widely adopted as the main language used to teach computer science at colleges and universities all over the world. In the whole history of computer The Characteristics of OOP In this section you are going to examine some fundamental concepts and terms common to all OOP languages. My goal is to familiarize you with the concepts and relate them to your everyday experiences so that they make more sense later when you look at OOP design and implementation. For example, you could model a temperature sensor as an object. Sensor attributes might include a state such as active or inactive, an attribute indicating its current value, and information about its physical location. Objects usually provide the ability to modify their state as well. In addition to keeping track of the current temperature, for example, a temperature sensor object might provide a way to turn the sensor on or off. An object also has a set of responsibilities that it carries out by providing services to other objects. For example, it is the responsibility of a sensor
- 8. Object Oriented Programming OOP 8 object to keep track of the state of the sensor. A sensor object might respond to requests from other objects that use sensors to check the status of a sensor, to turn a sensor on or off, or to report on the sensor's values. A sensor object could also maintain a history of its values as part of its responsibilities. The outside objects really don't care how a sensor object implements its attributes internally, but rather what services the sensor object can provide - its responsibilities. While a program is running, individual objects usually don't stand alone. They belong to a collection of other similar objects that all are members of the same group, or class. A program will be made up of many different classes, each class made up of similar objects. A class may have just a single object or any number of member objects existing at any given time. All members of a class have similar behavior. For example, consider a software system used to monitor various sensors in a factory. One obvious kind of object present is such a system is a sensor. A class called Sensor would be defined and used to model physical sensors. The class would define the basic characteristics of any Sensor, such as its location, value, and identification number, as well as a set of services used to carry out its responsibilities. Each individual Class is a description of a set of objects. The set of objects share common attributes and common behavior. Object The basic unit of object orientation. An object is an entity that has attributes, behavior, and identity. Objects are members of a class, and the attributes and behavior of an
- 9. Object Oriented Programming OOP 9 physical sensor in the system would be represented as an object belonging to the class Sensor, and have specific values for the attributes described by the class definition. Any object-oriented software system will have the following Properties: Abstraction When you interact with objects in the world, you are often only concerned with a subset of their properties. Without this ability to abstract or filter out the extraneous properties of objects, you would find it hard to process the plethora of information bombarding you and concentrate on the task at hand. As a result of abstraction, when two different people interact with the same object, they often deal with a different subset of attributes. When I drive my car, for example, I need to know the speed of the car and the direction it is going. Because the car is using an automatic transmission, I do not need to know the revolutions per minute (RPMs) of the engine, so I filter this information out. On the other hand, this information would be critical to a racecar driver, who would not filter it out. When constructing objects in OOP applications, it is important to incorporate this concept of abstraction. The objects include only the information that is relevant in the context of the application. If you were building a shipping application, you would construct a product object with attributes such as size and weight. The color of the item would be extraneous information and would be ignored. Encapsulation Abstraction A model of a real-world object or concept.
- 10. Object Oriented Programming OOP 10 Another important feature of OOP is encapsulation. Encapsulation is the process in which no direct access is granted to the data; instead, it is hidden. If you want to gain access to the data, you have to interact with the object responsible for the data. In the previous inventory example, if you wanted to view or update information on the products, you would have to work through the product object. To read the data, you would send the product object a message. The product object would then read the value and send back a message telling you what the value is. The product object defines which operations can be performed on the product data. If you send a message to modify the data and the product object determines it is a valid request, it will perform the operation for you and send a message back with the result. Think about a human resources department. They encapsulate (hide) the information about employees. They determine how this data can be used and manipulated. Any request for the employee data or request to update the data has to be routed through them. Another example is network security. Any request for security information or a change to a security policy must be made through a network security administrator. The security data is encapsulated from the users of the network. By encapsulating data, you make the data of your system more secure and reliable. You know how the data is being accessed and what operations are being performed on the data. Encapsulation The process of hiding all the internal details of an object from the outside world. In Java, encapsulation is enforced by having the definitions for attributes and methods inside a class definition.
- 11. Object Oriented Programming OOP 11 Polymorphism Polymorphism is the ability of two different objects to respond to the same request message in their own unique way. For example, I could train my dog to respond to the command bark and my bird to respond to the command chirp. On the other hand, I could train them to both respond to the command speak. Through polymorphism I know that the dog will respond with a bark and the bird will respond with a chirp. How does this relate to OOP? You can create objects that respond to the same message in their own unique implementations. For example, you could send a print message to a printer object that would print the text on a printer, and you could send the same message to a screen object that would print the text to a window on your computer screen. In OOP you implement this type of polymorphism through a process called overloading. You can implement different methods of an object that have the same name. The object can then tell which method to implement depending on the context (in other words, the number and type of arguments passed) of the message. For example, you could create two methods of an inventory object to look up the price of a product. Both these methods would be named getPrice. Another object could call this method and pass either the name of the product or the product ID. The inventory object could tell which getPrice method to run by whether a string value or an integer value was passed with the request.
- 12. Object Oriented Programming OOP 12 Inheritance Most real-life objects can be classified into hierarchies. For example, you can classify all dogs together as having certain common characteristics such as having four legs and fur. Their breeds further classify them into subgroups with common attributes such as size and demeanor. You also classify objects according to their function. For example, there are commercial vehicles and recreational vehicles. There are trucks and passenger cars. You classify cars according to their make and model. To make sense of the world, you need to use object hierarchies and classifications. You use inheritance in OOP to classify the objects in your programs according to common characteristics and function. This makes working with the objects easier and more intuitive. It also makes programming easier because it enables you to combine general characteristics into a parent object and inherit these characteristics in the child objects. For example, you can define an employee object that defines all the general characteristics of employees in your company. You can then define a manager object that inherits the characteristics of the employee object but also adds characteristics unique to managers in your company. Because of inheritance the manager object will automatically reflect any changes to the characteristics of the employee object. To produce successful OO programs, it is important to always "think objects."
- 13. Object Oriented Programming OOP 13 Lecture 2: Object-Oriented Design and the UML There are several different object-oriented development methodologies in use today. Each has its strengths and weaknesses. The older, more traditional methodologies are often called "heavyweight" methodologies, and are most useful for large software projects involving tens or even hundreds of programmers over years of development effort. The newer methodologies are called "lightweight" or "agile" methodologies, and are more appropriate for smaller projects. Design and development methodologies have always needed a graphical notation to express the designs. In the past, one of the major problems has been that each major methodology has had its own graphical notation. UML (Unified Modeling Language) as the standard notation. Any of the current design methodologies, heavyweight or agile, use or can benefit from the UML. The UML originated in the mid-1990'. There is a standard specification of the UML coordinated by the Object Management Group (www.omg.org). Its ability to show objects and object relationships is especially useful. UML consists of a series of textual and graphical models of the proposed solution. These models define the system scope, components of the system, user interaction with the system, and how the system components interact with each other to implement the system functionality. Some common models used in UML are the following: • Software Requirement Specification (SRS): a textual description of the overall responsibilities and scope of the system. • Use Case: a textual/graphical description of how the system will behave from the user’s perspective. Users can be human or other systems.
- 14. Object Oriented Programming OOP 14 • Class Diagram: a visual blueprint of the objects that will be used to construct the system. • Sequence Diagram: a model of the sequence of object interaction as the program executes. Emphasis is placed on the order of the interactions and how they proceed over time. • Collaboration Diagram: a view of how objects are organized to work together as the program executes. Emphasis is placed on the communications that occur between the objects. • Activity Diagram: a visual representation of the flow of execution of a process or operation. Introducing Use Cases Use cases describe how external entities will use a system. These external entities can be human or other systems (called actors in UML terminology). The description emphasizes the users’ view of the system and the interaction between the users and the system. Use cases help to further define system scope and boundaries. Figure 2-1 shows a generic diagram that consists of two actors represented by stick figures, the system represented by a rectangle, and use cases depicted by ovals inside the system boundaries.
- 15. Object Oriented Programming OOP 15 Use cases are developed from the SRS (Software Requirement Specification) textual description of the overall responsibilities and scope of the system. The actor is any outside entity that interacts with the system. An actor could be a human user (for instance, a rental agent), another software system (for instance, a software billing system), or an interface device (for instance, a temperature probe). Each interaction that occurs between an actor and the system is modeled as a use case. The sample use case shown in Figure 2-2 was developed for the flight- booking application. It shows the use case diagram for the requirement “Customers can search for flights based on destination and departure times.” Figure 2-1. Generic use case diagram with two actors and three use cases
- 16. Object Oriented Programming OOP 16 Along with the graphical depiction of the use case, many designers and software developers find it helpful to provide a textual description of the use case. Sometimes any preconditions or postconditions associated with the use case are also identified. The following text further describes the use case diagram shown in Figure 2-2. • Description: A customer views the flight information page. The customer enters flight search information. After submitting the search request, the customer views a list of flights matching the search criteria. • Preconditions: None. • Postconditions: The customer has the opportunity to log in and proceed to the flight-booking page. Figure 2-2. View Flight Info use case
- 17. Object Oriented Programming OOP 17 As another example, take a look at the Reserve Seat use case shown in Figure 2-3. The following text further describes the use case diagram shown in Figure 2-3. • Description: The customer enters the flight number and indicates the seats being requested. After the customer submits the request, some confirmation information is displayed. • Preconditions: The customer has looked up the flight information. The customer has logged in and is viewing the flight-booking screen. • Postconditions: The customer is sent a confirmation e-mail outlining the flight details and the cancellation policy. As you can see from Figure 2-3, certain relationships can exist between use cases. The Reserve Seat use case includes the View Flight Info use case. This relationship is useful because you can use the View Flight Info Figure 2-3. Reserve Seat use case diagram
- 18. Object Oriented Programming OOP 18 use case independently of the Reserve Flight use case. This is called inclusion. You cannot use the Reserve Seat use case independently of the View Flight Info use case, however. Another way that use cases relate to each other is through extension. You might have a general use case that is the base for other use cases. The base use case is extended by other use cases. For example, you might have a Register Customer use case that describes the core process of registering customers. You could then develop Register Corporate Customer and Register Retail Customer use cases that extend the base use case. The difference between extension and inclusion is that, in extension, the base use case being extended is not used on its own. Figure 2-4 demonstrates how you model this in a use case diagram. After you have developed the use cases of the system, you can begin to identify the internal system objects that will carry out the system’s functional requirements. You do this using a class diagram. Figure 2-4. Extending use cases
- 19. Object Oriented Programming OOP 19 Understanding Class Diagrams Designers identify a potential list of classes that they will need to develop from the SRS and the use case diagrams. If you look at the documentation developed thus far for the airline booking application, you can begin to identify the classes that will make up the system. For example, you can develop a Customer class to work with the customer data and a Flight class to work with the flight data. The class attributes define this information. For example, the Flight class will have attributes for identifying the flight number, departure time and date, flight duration, destination, capacity, and seats available. The class structure must also define any operations that will be performed on the data. An example of an operation the Flight class is responsible for is updating the seats available when a seat is reserved. Figure 2-5 is an example of the class diagram for the Flight class used in the flight booking system example. A rectangle divided into three sections represents the class. The top section of the rectangle shows the name of the class, the middle section lists the attributes of the class, and the bottom section lists the operations performed by the class. Figure 2-5. Flight class diagram
- 20. Object Oriented Programming OOP 20 Modeling Object Relationships In OOP, when the program executes, the various objects work together to accomplish the programming tasks. For example, in the flight booking application, in order to reserve a seat on the flight, a Reservation object must interact with the Flight object. A relationship exists between the two objects. The relationships among the classes that make up the program are modeled in the class diagram. The common relationships that can occur between classes and how the class diagram represents them. Association When one class refers to or uses another class, the classes form an association. You draw a line between the two classes to represent the association and add a label to indicate the name of the association. For example, a seat is associated with a flight in the flight-booking- application, as shown in Figure 2-6. Sometimes a single instance of one class associates with multiple instances of another class. This is indicated on the line connecting the two classes. For example, when a customer makes a reservation, there is an association between the Customer class and the Reservation class. A single instance of the Customer class may be associated with multiple instances of the Reservation class. The n placed near the Reservation class indicates this multiplicity, as shown in Figure 2-7. Figure 2-6. Class associations Figure 2-7. Indicating multiplicity in a class diagram
- 21. Object Oriented Programming OOP 21 A situation may also exist where an instance of a class may be associated with multiple instances of the same class. For example, an instance of the Pilot class represents the captain while another instance of the Pilot class represents the co-pilot. The pilot manages the co-pilot. This scenario is referred to as a self-association and is modeled by drawing the association line from the class back to itself, as shown in Figure 2-8. Inheritance When multiple classes share some of the same operations and attributes, a base class can encapsulate the commonality. The child class then inherits from the base class. This is represented in the class diagram by a solid line with an open arrowhead pointing to the base class. For example, a CorporateCustomer class and a RetailCustomer class could inherit common attributes and operations from a base Customer class, as shown in Figure 2-9. Figure 2-8. A self-associating class Figure 2-9. Documenting inheritance
- 22. Object Oriented Programming OOP 22 Association Classes As the classes and the associations for a program are developed, there may be a situation where an attribute can’t be assigned to any one class, but is a result of an association between classes. For example, the parts inventory application mentioned previously may have a Part class and a Supplier class. Because a part can have more than one supplier and the supplier supplies more than one part, where should the price attribute be located? It does not fit nicely as an attribute for either class, and it should not be duplicated in both classes. The solution is to develop an association class that manages the data that is a product of the association. In this case, you would develop a PartPrice class. The relationship is modeled with a dashed line drawn between the association and the association class, as shown in Figure 2-10. Figure 2-11 shows the evolving class diagram for the flight booking application. It includes the classes, attributes, and relationships that have been identified for the system. Figure 2-10. An association class
- 23. Object Oriented Programming OOP 23 Figure 2-11. Flight booking class diagram
- 24. Object Oriented Programming OOP 24 Lecture 3: Introduction Java The microprocessor revolution’s most important contribution to date is that it enabled the development of personal computers. Microprocessors have had a profound impact in intelligent consumer-electronic devices. Recognizing this, Sun Microsystems in 1991 funded an internal corporate research project led by James Gosling, which resulted in a C++- based object-oriented programming language that Sun called Java. This is sometimes called “write once, run anywhere.” Java is a relatively new programming language as compared with many others. It was initiated in 1991 by James Gosling at Sun Microsystems as one of his many set-top box projects. The language initially was called Oak, then Green, and ultimately Java. Java was introduced to the public in 1995 and has gained tremendous popularity since. In 2010, Sun Microsystems was purchased by Oracle. Features of Java The key characteristics of the Java programming language that have made it so popular include the following: Simple: Java omits some of the vaguely defined features of C++. It has facilities for automatic garbage collection to automatically release unused memory while a program is running. It also includes a rich predefined set of packages (such as for mathematics, statistics, database access, GUI design, and so on) that can be easily reused by application developers. Its syntax looks very similar to C/C. Platform independent and portable: By using a hybrid compilation/interpretation approach, Java programs can be
- 25. Object Oriented Programming OOP 25 executed in a networked environment with different hardware platforms and architectures. Object-oriented: Java implements the object-oriented programming paradigm by grouping data and operations into classes and/or objects. Secure: Java has many facilities to guarantee security in a networked environment. It imposes various types of access restrictions to (networked) resources and carefully supervises memory allocation. It allows code to be downloaded over a network Multi-threaded: Java delivers the power of advanced multi- threaded capabilities to the developer in an environment without complexity. Dynamic: Java allows code to be added to libraries dynamically and then can determine which code should run at execution time. Java Runtime Environment (JRE ) The Java Runtime Environment (JRE) is the software environment in which Java programs run. It consists of various components. Java API (Application Programming Interface) Class loader Bytecode verifier Java Virtual Machine (JVM) A Typical Java Development Environment We now explain the steps to create and execute a Java application. Normally there are five phases—edit, compile, load, verify and execute. We discuss them in the context of the Java SE 8 Development Kit (JDK).
- 26. Object Oriented Programming OOP 26 Phase 1: Creating a Program Phase 1 consists of editing a file with an editor program, normally known simply as an editor (Fig. 5.1). Using the editor, you type a Java program (typically referred to as source code), make any necessary corrections and save it on a secondary storage device, such as your hard drive. Java source code files are given a name ending with the .java extension Phase 2: Compiling a Java Program into Bytecodes In Phase 2, you use the command javac (the Java compiler) to compile a program. IDEs typically provide a menu item, such as Build or Make, that invokes the javac command for you. If the compiler detects errors, you’ll need to go back to Phase 1 and correct them. Figure (5-2) Integrated development environments (IDEs) provide tools that support the software development process, such as editors, debuggers for locating logic errors (errors that cause programs to execute incorrectly) and more. There are many popular Java IDEs, Including: • Eclipse (www.eclipse.org) • NetBeans (www.netbeans.org) • IntelliJ IDEA (www.jetbrains.com)
- 27. Object Oriented Programming OOP 27 The Java compiler translates Java source code into bytecodes that represent the tasks to execute in the execution phase (Phase 5). The Java Virtual Machine (JVM)—a part of the JDK and the foundation of the Java platform—executes bytecodes. A virtual machine (VM) is a software application that simulates a computer but hides the underlying operating system and hardware from the programs that interact with it. This begins Phase 3. IDEs typically provide a menu item, such as Run, that invokes the java command for you. Phase 3: Loading a Program into Memory In Phase 3, the JVM places the program in memory to execute it—this is known as loading (Fig. 5.3).The JVM’s class loader takes the .class files containing the program’s bytecodes and transfers them to primary memory. It also loads any of the .class files provided by Java that your program uses. The .class files can be loaded from a disk on your system or over a network
- 28. Object Oriented Programming OOP 28 Phase 4: Bytecode Verification In Phase 4, as the classes are loaded, the bytecode verifier examines their bytecodes to ensure that they’re valid and do not violate Java’s security restrictions (Fig. 5.4). Java enforces strong security to make sure that Java programs arriving over the network do not damage your files or your system. Phase 5: Execution In Phase 5, the JVM executes the program’s bytecodes, thus performing the actions specified by the program (Fig. 5.5). In early Java versions, the JVM was simply an interpreter for Java bytecodes. during execution, the bytecodes are translated into machine language for the actual computer on which the program executes.
- 29. Object Oriented Programming OOP 29 Java Application Programming Interface (API) The Java Application Programming Interface (API) is the set of prepackaged, ready-made Java components grouped into libraries. It provides programmers with many useful capabilities. Some popular API libraries are listed in Table 5-1.
- 30. Object Oriented Programming OOP 30 Java Platforms A Java platform or edition consists of a JRE with a specific set of libraries for a specific application environment. Table 5-2 gives an overview of the most important Java platforms. Java Applications In this section, we will discuss various types of Java applications such as standalone applications, Java applets, Java servlets, and Java beans. Standalone Applications A standalone application is one that can run on its own without needing to be embedded in a particular host environment (such as a web browser). Standalone applications only need a JVM to execute. They can be command-line applications or graphical user interface (GUI) applications. A command line application uses the command prompt for the input and output. This is illustrated in Figure 5-1.
- 31. Object Oriented Programming OOP 31 A GUI application uses graphical components to facilitate the input and output of the program. An example GUI for calculating BMI is shown in Figure 5-2. Java Applets An applet is a Java application that’s typically embedded in an HTML page and run by the client web browser. Applets run in a sandbox, which
- 32. Object Oriented Programming OOP 32 is a confined space in memory that guarantees their execution is secure. Applets can make use of the full expressive power of the Java language. They are defined in HTML by means of the <applet> tag, as follows: <Applet> </Applet> Java Servlets Servlets are part of the J2EE platform and are small Java applications that run on a Java-enabled Application server. Servlets can read and process data (originated from, for example, an HTML form) sent by the client browser through HTTP. They can interact with a server database, invoke web services, or call other servlets or server-side functionality. The results can then be communicated back to the client browser in a variety of formats (such as in HTML, XML, Word, and so on). Java Beans A Java bean is a reusable software component that can be visually manipulated in a builder tool. These tools allow beans to be customized by setting their properties and specifying how they react to events.
- 33. Object Oriented Programming OOP 33 Java™ 2 Platform Lecture 4: Java Language Structure Differences between Java EE and Java SE is available on Oracle website. Java SE When most people think of the Java programming language, they think of the Java SE API. Java SE's API provides the core functionality of the Java programming language. It defines everything from the basic types and objects of the Java programming language to high-level classes that are used for networking, security, database access, graphical user interface (GUI) development, and XML parsing. In addition to the core API, the Java SE platform consists of a virtual machine, development tools, deployment technologies, and other class libraries and toolkits commonly used in Java technology applications. Java EE The Java EE platform is built on top of the Java SE platform. The Java EE platform provides an API and runtime environment for developing and running large-scale, multi-tiered, scalable, reliable, and secure network applications.
- 34. Object Oriented Programming OOP 34 Lecture 4: Java Language Structure This section provides a brief overview of the Java language structure. Let’s consider the following running example of a Java program calculating BMI to illustrate the discussion. public class BMICalculator { // declare variables double weight; double height; double BMI; public BMICalculator(double w, double h) { weight = w; height = h; } public double calculateBMI() { return weight / (height * height); } // This is our main method. public static void main(String[] args) { BMICalculator calculator = new BMICalculator(60, 1.70); double bmi = calculator.calculateBMI(); // print BMI to screen System.out.println("Your BMI is " + bmi + "."); } }
- 35. Object Oriented Programming OOP 35 Classes In Java, all code is grouped into classes. A class is thus a code container. The definition of the class starts with an access modifier (public in this case), which specifies which classes have access to it. This is followed by the keyword class and the name of the class (BMICalculator). Every class definition is enclosed within brackets {}. It has both variables (weight, height, and BMI) and methods (BMICalculator, calculateBMI, and main). The main method is a special method since it is the entry point of program execution. In other words, when the class BMICalculator is run by the Java Runtime Environment, it will start by executing the main method. Variables As stated earlier, every class definition consists of both variables and methods. A variable is a name for a memory location that stores a specific value. This value may change during program execution, which is why it’s called a “variable.” The BMICalculator class starts by defining the following variables: // declare variables double weight; double height; double BMI; The weight, height, and BMI variables are defined using the data type double, which represents a floating point number. Other data types exist in Java and will be covered in a subsequent section. In Java, variables must always be defined in a class.
- 36. Object Oriented Programming OOP 36 Methods As discussed earlier, a method is a piece of code within a class definition, and it performs a specific kind of functionality. Just as with a class, every method definition is enclosed within brackets {...}. In the BMICalculator example, three methods have been defined—BMICalculator, CalculateBMI, and main. The main method is the main entry point of program execution. So, it is the first method that runs when executing the class BMICalculator. Consider the main method: The first line, called the method declaration, contains several keywords , we will briefly introduce those you see here. public: This method can be accessed by other classes and/or methods without restriction. static: This method does not need an object. void: This method does not return any value. (String[] args): This is a conventional way to refer to the arguments of the method. In this case, the method takes an array of strings as its input parameter. The method starts by assigning the values of 60 and 1.70 to the variables of weight and height, It then continues by calculating the BMI by calling another method called calculateBMI. The calculateBMI method then looks as follows: public static void main(String[] args) { BMICalculator calculator = new BMICalculator(60, 1.70); double bmi = calculator.calculateBMI(); // print BMI to screen System.out.println("Your BMI is " + bmi + "."); }
- 37. Object Oriented Programming OOP 37 It calculates the BMI as BMI = weight/(height*height). The main method then prints the BMI to the screen using the following statement: // method calculating BMI public static void calculateBMI(){ BMI = weight/(height*height); } System.out.println("Your BMI is " + BMI +".");
- 38. Object Oriented Programming OOP 38 Another Application: Adding Integers Our next application reads (or inputs) two integers (whole numbers, such as –22, 7, 0 and 1024) typed by a user at the keyboard, compute their sum and display it. Programs remember numbers and other data in the computer’s memory and access that data through program elements called variables. The program demonstrates these concepts. // Addition program that inputs two numbers then displays their sum. import java.util.Scanner; public class Addition{ public static void main(String[] args) { // create a Scanner to obtain input from the command window Scanner input = new Scanner(System.in); int number1; // first number to add int number2; // second number to add int sum; // sum of number1 and num System.out.print("Enter first integer: "); number1 = input.nextInt(); // read first number from user System.out.print("Enter second integer: "); // prompt number2 = input.nextInt(); // read second number from user sum = number1 + number2; // add numbers, then store total in sum System.out.printf("Sum is %d%n", sum); // display sum } // end method main } // end class Addition
- 39. Object Oriented Programming OOP 39 Output Enter first integer: 45 Enter second integer: 72 Sum is 117 import Declarations import java.util.Scanner; Is an import declaration that helps the compiler locate a class that’s used in this program. It indicates that the program uses the predefined Scanner class (discussed shortly) from the package named java.util. The compiler then ensures that you use the class correctly. Declaring Class Addition public class Addition Begins the declaration of class Addition. The filename for this public class must be Addition.java.Remember that the body of each class declaration starts with an opening left brace and ends with a closing right brace Declaring and Creating a Scanner to Obtain User Input from the Keyboard All Java variables must be declared with a name and a type before they can be used. A variable’s name enables the program to access the value of the variable in memory. Scanner input = new Scanner(System.in); Is a variable declaration statement that specifies the name (input) and type (Scanner) of a variable that’s used in this program. A Scanner enables a program to read data (e.g., numbers and strings) for use in a program. The data can come from many sources, such
- 40. Object Oriented Programming OOP 40 as the user at the keyboard or a file on disk. This expression uses the new keyword to create a Scanner object that reads characters typed by the user at the keyboard. The standard input object, System.in, enables applications to read bytes of data typed by the user. The Scanner translates these bytes into types (like ints) that can be used in a program. Declaring Variables to Store Integers The variable declaration statements in lines int number1; int number2; int sum; declare that variables number1, number2 and sum hold data of type int—they can hold integer values (whole numbers such as 72, – 1127 and 0). These variables are not yet initialized. The range of values for an int is –2,147,483,648 to +2,147,483,647. The types int, float, double and char are called primitive types. Prompting the User for Input System.out.print("Enter first integer: "); Uses System.out.print to display the message "Enter first integer: ". This message is called a prompt because it directs the user to take a specific action. We use method print here rather than println so that the user’s input appears on the same line as the prompt. Class System is part of package java.lang. Notice that class System is not imported with an import declaration at the beginning of the program.
- 41. Object Oriented Programming OOP 41 Obtaining an int as Input from the User number1 = input.nextInt(); uses Scanner object input’s nextInt method to obtain an integer from the user at the keyboard.At this point the program waits for the user to type the number and press the Enter key to submit the number to the program. Sing Variables in a Calculation sum = number1 + number2; is an assignment statement that calculates the sum of the variables number1 and number2 then assigns the result to variable sum by using the assignment operator, =. Displaying the Result of the Calculation After the calculation has been performed System.out.printf("Sum is %d%n", sum); uses method System.out.printf to display the sum. The format specifier %d is a placeholder for an int value (in this case the value of sum)—the letter d stands for “decimal integer.” The remaining characters in the format string are all fixed text. So, method printf displays "Sum is ", followed by the value of sum (in the position of the %d format specifier) and a newline.
- 42. Object Oriented Programming OOP 42 Lecture 5: Decision Making and Repetition Statement: If condition A condition is an expression that can be true or false. This section introduces Java’s if selection statement, which allows a program to make a decision based on a condition’s value. For example, the condition “grade is greater than or equal to 60” determines whether a student passed a test. If the condition in an if statement is true, the body of the if statement executes. If the condition is false, the body does not execute. import java.util.Scanner; // program uses class Scanner public class Comparison { // main method begins execution of Java application public static void main(String[] args) { // create Scanner to obtain input from command line Scanner input = new Scanner(System.in); int number1; // first number to compare int number2; // second number to compare System.out.print("Enter first integer: "); // prompt number1 = input.nextInt(); // read first number from user System.out.print("Enter second integer: "); // prompt number2 = input.nextInt(); // read second number from user if (number1 == number2) System.out.printf("%d == %d%n", number1, number2); if (number1 != number2) System.out.printf("%d != %d%n", number1, number2); if (number1 < number2) System.out.printf("%d < %d%n", number1, number2); } // end method main }// end class Comparison
- 43. Object Oriented Programming OOP 43 while Repetition Statement A repetition statement allows the programmer to specify that a program should repeat an action while some condition remains true. As an example of Java's while repetition statement, consider a program segment designed to find the first power of 3 larger than 100. Suppose that the int variable product is initialized to 3. When the following while statement finishes executing, product contains the result: When this while statement begins execution, the value of variable product is 3. Each iteration of the while statement multiplies product by 3, so product takes on the values 9, 27, 81 and 243 successively. When variable product becomes 243, the while statement condition—product int product = 3; while ( product <= 100 ) product = 3 * product;
- 44. Object Oriented Programming OOP 44 <= 100—becomes false. This terminates the repetition, so the final value of product is 243. At this point, program execution continues with the next statement after the while statement. for Repetition Statement presented the essentials of counter-controlled repetition. The while statement can be used to implement any counter-controlled loop. Java also provides the for repetition statement, which specifies the counter- controlled repetition details in a single line of code. public class WhileCounter { public static void main( String args[] ) { int counter = 1; // declare and initialize control variable while ( counter <= 10 ) // loop-continuation condition { System.out.printf( "%d ", counter ); ++counter; // increment control variable by 1 } // end while System.out.println(); // output a newline } // end main } // end class WhileCounter public class ForCounter { public static void main( String args[] ) { for ( int counter = 1; counter <= 10; counter++ ) System.out.printf( "%d ", counter ); System.out.println(); // output a newline } // end main
- 45. Object Oriented Programming OOP 45 do...while Repetition Statement The do...while repetition statement is similar to the while statement. In the while, the program tests the loop continuation condition at the beginning of the loop, before executing the loop's body; if the condition is false, the body never executes. The do...while statement tests the loop- continuation condition after executing the loop's body; therefore, the body always executes at least once. When a do...while statement terminates, execution continues with the next statement in sequence. public class Interest { public static void main( String args[] ) { double amount; // amount on deposit at end of each year double principal = 1000.0; // initial amount before interest double rate = 0.05; // interest rate // display headers System.out.printf( "%s%20sn", "Year", "Amount on deposit" ); // calculate amount on deposit for each of ten years for ( int year = 1; year <= 10; year++ ) { // calculate new amount for specified year amount = principal * Math.pow( 1.0 + rate, year ); // display the year and the amount System.out.printf( "%4d%,20.2fn", year, amount ); } // end for
- 46. Object Oriented Programming OOP 46 Declaring Methods with Multiple Parameters We now show how to write methods with multiple parameters. The application uses a programmer-declared method called maximum to determine and return the largest of three double values that are input by the user. When the application begins execution, class MaximumFinderTest's main method creates one object of class MaximumFinder and calls the object's determineMaximum method to produce the program's output. In class MaximumFinder of method determineMaximum prompt the user to enter three double values, then read them from the user. calls method maximum to determine the largest of the three double values passed as arguments to the method. When method maximum returns the result to, the program assigns maximum's return value to local variable result. Then line 24 outputs the maximum value. At the end of this section, we'll discuss the use of operator +. public class DoWhileTest { public static void main( String args[] ) { int counter = 1; // initialize counter do { System.out.printf( "%d ", counter ); ++counter; } while ( counter <= 10 ); // end do...while System.out.println(); // outputs a newline } // end main } // end class DoWhileTest
- 47. Object Oriented Programming OOP 47 import java.util.Scanner; public class MaximumFinder { // obtain three floating-point values and locate the maximum value public void determineMaximum() { // create Scanner for input from command window Scanner input = new Scanner( System.in ); // obtain user input System.out.print( "Enter three floating-point values separated by spaces: " ); double number1 = input.nextDouble(); // read first double double number2 = input.nextDouble(); // read second double double number3 = input.nextDouble(); // read third double // determine the maximum value double result = maximum( number1, number2, number3 ); // display maximum value System.out.println( "Maximum is: " + result ); } // end method determineMaximum // returns the maximum of its three double parameters public double maximum( double x, double y, double z ) { double maximumValue = x; // assume x is the largest to start // determine whether y is greater than maximumValue if ( y > maximumValue ) maximumValue = y; // determine whether z is greater than maximumValue if ( z > maximumValue ) maximumValue = z; return maximumValue; } // end method maximum } // end class MaximumFinder
- 48. Object Oriented Programming OOP 48 Method Overloading Methods of the same name can be declared in the same class, as long as they have different sets of parameters (determined by the number, types and order of the parameters)—this is called method overloading. When an overloaded method is called, the Java compiler selects the appropriate method by examining the number, types and order of the arguments in the call. Method overloading is commonly used to create several methods with the same name that perform the same or similar tasks, but on different types or different numbers of arguments. public class MaximumFinderTest { // application starting point public static void main( String args[] ) { MaximumFinder maximumFinder = new MaximumFinder(); maximumFinder.determineMaximum(); } // end main } // end class MaximumFinderTest public class MethodOverloadTest { public static void main( String args[] ) { MethodOverload methodOverload = new MethodOverload(); methodOverload.testOverloadedMethods(); } // end main } // end class MethodOverloadTest
- 49. Object Oriented Programming OOP 49 public class MethodOverload { // test overloaded square methods public void testOverloadedMethods() { System.out.printf( "Square of integer 7 is %dn", square( 7 ) ); System.out.printf( "Square of double 7.5 is %fn", square( 7.5 ) ); } // end method testOverloadedMethods // square method with int argument public int square( int intValue ) { System.out.printf( "nCalled square with int argument: %dn", intValue ); return intValue * intValue; } // end method square with int argument // square method with double argument public double square( double doubleValue ) { System.out.printf( "nCalled square doubleValue ); return doubleValue * doubleValue; } // end method square with double argument } // end class MethodOverload
- 50. Object Oriented Programming OOP 50 Lecture 6: Basic Concepts of Object-Oriented Programming in JAVA Java is a “pure” object‐oriented programming language, meaning that there are no standalone constants, variables, or functions. It is not possible to define such standalone elements, and everything is thus accessed through classes and objects. Class definitions start with the keyword class, followed by the class name. Java imposes almost no restrictions on which characters or names you can use for class names, except for the fact that class names should start with a letter The class’ body is surrounded by curly brackets, { and }. Within this body, you define the variables (the data) and the methods (the behaviors) of the class. Variable definitions start with a variable type, followed by one or more variable names Objects are created using the new keyword. For example, to create a new student, you would simply write the following statement: The reason is that when you instantiate a class, you are in fact actually calling a special method of the class called a constructor. The compiler create constructor by default directly when you create instance. Class Student { String firstName; String lastName; } Student myFirstStudent = new Student(); Class Student() {}
- 51. Object Oriented Programming OOP 51 Since it is a method, it can take a number of arguments. For example, you might have defined the class so that you immediately need to pass a name for the new student: You will use the Student class definition as a blueprint to create a number of Student objects. Student myFirstStudent = new Student ("Ahmed", "Last Name"); Class Student (String firstName, String lastName) { firstName = firstName; lastName = lastName; } class Student { int id; String firstName; String lastName; int birthYear, birthMonth, birthDay; boolean isBirthday() { // Return true if it's the student's birthday today. return false; } void giveWarning(boolean isFinalWarning) { // You should study harder! } int numberOfFriends() { // Return the number of friends the student has. return 0; }
- 52. Object Oriented Programming OOP 52 Add the following main method to the Student class definition as follows. Note the use of the new keyword to create objects: take a look at how it works.Now 1. The new keyword is used to create objects, which you will use to create a bunch of students. Note the use of the dot (.) operator in this code to access (read and write) the Student objects’ variables. 2. To actually execute, that is, run the program, you add a so ‐called main method to the Student class. As Java has no way of knowing which particular method you want to use as the entry point of the program, a special method exists—the so called main method— that serves exactly this purpose. 3. When running this program from Eclipse, Java will call the class’ main method. The main method will create some students, set their variables, and print some information to Eclipse’s console. public static void main(String[] args) { Student firstStudent = new Student(); Student secondStudent = new Student(); firstStudent.id = 1; firstStudent.firstName = "Marc"; secondStudent.id = 2; secondStudent.firstName = "Sophie"; System.out.println("The student object referred to "+ "by the variable secondStudent has the first "+ "name: "+secondStudent); }
- 53. Object Oriented Programming OOP 53 A class contains a block of variable definitions (data) and methods (behaviors), like so: class CLASSNAME { // VARIABLE DEFINITIONS // METHOD DEFINITIONS } You will now zoom in further on the aspect of defining variables within a class. Specifically, this section discusses: Instance variables: Variables that will be used to hold data of objects.
- 54. Object Oriented Programming OOP 54 Class variables: Variables that are not bound to an object but instead belong to the class as such, that is, to the blueprint of the object. Final variables: Variables that—after their initial assignment— cannot be modified. Finally, I will also devote some words to the topic of variable scope. A scope of a variable is the context in which it’s defined. Instance Variables In object‐oriented programming, an instance variable is a variable that’s defined in a class. Instance variables are also commonly referred to as member variables or fields of a class. In object‐oriented programming, a class variable, also denoted as a static variable, is a variable that’s been allocated “statically”—meaning that this variable is shared between all objects (instances) belonging to this class. class Book { String title; String[] authors; } class Book { static int maxAmountOfPages = 500; } //MAIN FUNCTION Book.maxAmountOfPages = 2000; // Let's increase the max amount of pages System.out.println("We now support books with max. pages: " +Book.maxAmountOfPages);
- 55. Object Oriented Programming OOP 55 However, for the last line, when accessing the static variable, Eclipse complains about the fact that "The static field superLargeBook.maxAmountOfPages should be accessed in a static way". What is meant by this? Well, it means that it’s generally preferred to access and modify static variables not by accessing them through an object variable, but by using the class name directly. Final Variables In Java, final variables are variables that can be initialized only once, either directly when defining the variable, or later in one of the class methods. Once a value has been given to the variable, however, it cannot be modified any longer. Note the two final static variables at the top of the class body. This pattern is very heavily used by Java programmers to define constants, meaning that you desire to set the maximum and minimum amount of pages only once (they are final), and also to keep them shared by all objects (they are part of the class blueprint). Note by the way that Java has a const keyword, but it currently remains unused. class Book { final int max_amount_of_pages= 500; } class Book { final static int MAX_AMOUNT_OF_PAGES = 500; final static int MIN_AMOUNT_OF_PAGES = 50; }
- 56. Object Oriented Programming OOP 56 Class Methods Just as you’ve seen for class variables, a class method, also denoted as a static method, is a method that has been defined statically, meaning that this method is shared between all objects (instances) belonging to the class. Note the static modifier in front of the method declaration is similar to what you’ve seen with static variables. preferredFood class Cat { static String preferredFood() { return "Fish"; } } //MAIN FUNCTION System.out.println("A cat's preferred food is: "+Cat.getPreferredFood());
- 57. Object Oriented Programming OOP 57 Lecture 7: Encapsulation Information hiding, also referred to as encapsulation, is an object-oriented practice that hides the internal representation of objects. The main idea is to make member variables private, so they are not directly accessible from outside of the class. Accessory methods are created to grant access to view or modify the values of the variables Encapsulation is depicted graphically in Figure 6.1 . The code that uses an object, sometimes called the client of an object, should not be allowed to access variables directly. The client should call an object’s methods, and those methods then interact with the data encapsulated within the object.
- 58. Object Oriented Programming OOP 58 Access Modifiers The first step to restricting access to variables or methods is to adjust the access modifiers. There are four access modifiers that can be applied to methods and variables in Java: • public: Can be accessed by any class • protected: Can be accessed by subclasses or classes in the same package • no modifier: Can be accessed by classes in the same package • private: Can be accessed only from within the same class You can see the access levels in Figure 6-2. Getters Getters are used to access variables’ values for viewing only. You should create a getter method for every private variable. Depending on the access level you want to give to the variable, you can set the access modifier of its getter method. This is demonstrated with the Product class.
- 59. Object Oriented Programming OOP 59 Setters Setters are the methods you use to modify the values of variables. Just like with getters, you should create setters for every variable. public String getName(){ return this.productName; } public void setName(String name){ this.productName = name; } public String getID(){ return this.productID; } public void setID(String id){ this.productID = id; Encapsulation is a term often heard when describing modern programming techniques. Encapsulation is the process of hiding (encapsulating) all the details of how a piece of software works and describing only enough about the software to enable a programmer to use it. Encapsulation means that data and actions are combined into a single item—in our case, a class object—that hides the details of the implementation.
- 60. Object Oriented Programming OOP 60 Overloading Methods Overloading simply refers to using the same name for more than one method in the same class. Java, and many other languages, can determine which method you’re calling as long as the number or type of parameters is different in each method. This is illustrated with a few examples. If you try this first example in Eclipse, you’ll see there are errors indicating the read() method is duplicated. The problem is you have two methods with the same name and neither has any parameters. Try the following instead: public class Book { String title; String author; boolean isRead; int numberOfReadings; public void read(){ isRead = true; } public void read(){ numberOfReadings++; } }
- 61. Object Oriented Programming OOP 61 To Java, these are two completely different methods, so you won’t have any duplication errors. If you want to create more than one constructor, you need to use the overloading principle. In the Book class, you would naturally want a constructor with two String parameters to set the title and author of the book. public class Book { String title; String author; boolean isRead; int numberOfReadings; public void read(){ isRead = true; numberOfReadings++; } public void read(int i){ isRead = true; numberOfReadings += i; } public Book(String bookTitle){ title = bookTitle; author = "Unknown"; }
- 62. Object Oriented Programming OOP 62 You might also need to create a book with an unknown author, perhaps for a publisher who has a particular book in mind but hasn’t hired an author to write it. The this Keyword This section covers how and why to use the Java this keyword. The this keyword is placed inside an instance method or constructor in order to refer to the object whose method is being called. This is perhaps best explained with an example: there are cases where you must use the this keyword. This is usually when a local variable and an instance variable have the same name. If you try this in Eclipse, you’ll see a warning: The assignment to variable name has no effect. public Book(String bookTitle, String authorName){ title = bookTitle; author = authorName; } public class Person { String name; public Person(String name){ name = name; } public void printName(){ System.out.println(name); }
- 63. Object Oriented Programming OOP 63 Time Class Case Study Our first example consists of two classes—Time1 and Time1Test .Class Time1 represents the time of day. Class Time1Test’s main method creates one object of class Time1 and invokes its methods. The output of this program appears in figure public class Person { String name; public Person(String name){ this.name = name; } public void printName(){ System.out.println(name); public class Time1 { private int hour; // 0 - 23 private int minute; // 0 - 59 private int second; // 0 – 59 public String toUniversalString() { return String.format("%02d:%02d:%02d", hour, minute, second);} public String toString() { return String.format("%d:%02d:%02d %s", ((hour == 0 || hour == 12) ? 12 : hour % 12), minute, second, (hour < 12 ? "AM" : "PM")); }
- 64. Object Oriented Programming OOP 64 Method toString Method toString takes no arguments and returns a String in standard- time format, consisting of the hour, minute and second values separated by colons and followed by AM or PM (e.g., 11:30:17 AM or 1:27:06 PM). Like method toUniversalString, method toString uses static String method format to format the minute and second as two digit values, with leading zeros if necessary. Uses a conditional operator (?:) to determine the value for hour in the String—if the hour is 0 or 12 (AM or PM), it appears as 12; otherwise, it appears as a value from 1 to 11. The conditional operator in line 30 determines whether AM or PM will be Public void SetHour(int hour) { this.hour = hour; } Public int GetHour() { Return this.hour } Public void SetMunite(int munite) { this.minute = minute; } Public int GetMunite() { Return this.minute ; } Public void SetSecond(int second) { this.second = second; } Public int GetSecond() { Return this.second; }
- 65. Object Oriented Programming OOP 65 returned as part of the String. Recall all objects in Java have a toString method that returns a String representation of the object. We chose to return a String containing the time in standard-time format. Method toString is called implicitly whenever a Time1 object appears in the code where a String is needed, such as the value to output with a %s format specifier in a call to System.out.printf. You may also call toString explicitly to obtain a String representation of a Time object. public class Time1Test { public static void main(String[] args) { Time1 time = new Time1(); // invokes Time1 constructor displayTime("After time object is created", time); System.out.println(); time.SetHour = 13; time.SetMunite= 27; time.SetSecond =6; displayTime("After calling set for all attributes ", time); System.out.println(); } private static void displayTime(String header, Time1 t) { System.out.printf("%s%nUniversal time: %s%nStandard time: %s%n", header, t.toUniversalString() , t.toString()); } }
- 66. Object Oriented Programming OOP 66 Lecture 8: Array and Collocations An array is a list of values. Each value is stored at a specific, numbered position in the array. The number corresponding to each position is called an index or a subscript. In Java, arrays are objects. To create an array, the reference to the array must be declared. The array can then be instantiated using the new operator, which allocates memory space to store values. public class BasicArray { public static void main(String[] args) { final int LIMIT = 15, MULTIPLE = 10; int [] list = new int [LIMIT]; // Initialize the array values for ( int index = 0; index < LIMIT; index++) list[index] = index * MULTIPLE; list[5] = 999; // change one array value for ( int value : list) System.out.print(value + " "); }
- 67. Object Oriented Programming OOP 67 Initialize Lists You can use an initializer list to instantiate an array and provide the initial values for the elements of the array. the following declaration instantiates the array scores as an array of eight integers, indexed from 0 to 7 with the specified initial values: int [] scores = {87, 98, 69, 87, 65, 76, 99, 83};
- 68. Object Oriented Programming OOP 68 Arrays as parameters An entire array can be passed as a parameter to a method. Because an array is an object, when an entire array is passed as a parameter, a copy of the reference to the original array is passed. Arrays of Objects We used arrays to store primitive types such as integers and characters. Arrays can also store references to objects as elements. Fairly complex information management structures can be created using only arrays and other objects. For example, an array could contain objects, and each of those objects could consist of several variables and the methods that use them. When we store objects in an array, each element is a separate object. public class Primes { public static void main(String[] args) { int[] primeNums = {2, 3, 5, 7, 11, 13, 17, 19}; System.out.println("Array length: " + primeNums.length); System.out.println("The first few prime numbers are:"); for (int prime : primeNums) System.out.print(prime + " "); }
- 69. Object Oriented Programming OOP 69 Collections and Class ArrayList The Java API provides several predefined data structures, called collections, used to store groups of related objects in memory. These classes provide efficient methods that organize, store and retrieve your data without requiring knowledge of how the data is being stored. This reduces application-development time. ArrayList<T> (package java.util) provides a convenient solution to this problem—it can dynamically change its size to accommodate more String[] words = {“friendship" , "loyalty" , "honor" , " " , " "}; ArrayList<String> list1 = new ArrayList<String>(); ArrayList<Integer> list2 = new ArrayList< Integer >() ;
- 70. Object Oriented Programming OOP 70 elements. Java SE 7 introduced the diamond (<>) notation to simplify statements like this. Using <> in a class instance creation expression for an object of a generic class tells the compiler to determine what belongs in the angle brackets. Below Figure shows some common methods of class ArrayList<T>. creates a new empty ArrayList of Strings with a default initial capacity of 10 elements. The capacity indicates how many items the ArrayList can hold without growing. ArrayList is implemented using a conventional array behind the scenes. When the ArrayList grows, it must create a larger internal array and copy each element to the new array. This is a time-consuming operation. It would be inefficient for the ArrayList to grow each time an element is added. Instead, it grows only when an element is added and the number of elements is equal to the capacity— i.e., there’s no space for the new element.
- 71. Object Oriented Programming OOP 71 import java.util.ArrayList; public class ArrayListCollection { public static void main(String[] args) { // create a new ArrayList of Strings with an initial capacity of 10 ArrayList<String> items = new ArrayList<String>(); items.add("red"); // append an item to the list items.add(0, "yellow"); // insert "yellow" at index 0 // header System.out.print( "Display list contents with counter-controlled loop:"); // display the colors in the list for (int i = 0; i < items.size(); i++) System.out.printf(" %s", items.get(i)); // display colors using enhanced for in the display method display(items, "%nDisplay list contents with enhanced for statement:"); items.add("green"); // add "green" to the end of the list items.add("yellow"); // add "yellow" to the end of the list display(items, "List with two new elements:"); items.remove("yellow"); // remove the first "yellow" display(items, "Remove first instance of yellow:"); items.remove(1); // remove item at index 1 display(items, "Remove second list element (green):"); // check if a value is in the List System.out.printf(""red" is %sin the list%n", items.contains("red") ? "": "not "); // display number of elements in the List System.out.printf("Size: %s%n", items.size()); } // display the ArrayList's elements on the console public static void display(ArrayList<String> items, String header) { System.out.printf(header); // display header // display each element in items for (String item : items) System.out.printf(" %s", item); } }
- 72. Object Oriented Programming OOP 72 Case Study: Creates a DVDCollection object and adds some DVDs to it. Prints reports on the status of the collection. Figure below shows a UML class diagram of the Movies program.The cardinality of the relationship is also noted: a DVDCollection object contains zero or more DVD objects.
- 73. Object Oriented Programming OOP 73 Listing 7.1 Each DVD added to the collection is specified by its title, director, year of release, purchase price, and whether or not it is in Blu- ray format. public class Movies { public static void main(String[] args) { DVDCollection movies = new DVDCollection(); movies.addDVD("The Godfather", "Francis Ford Coppola", 1972, 24.95, true); movies.addDVD("District 9", "Neill Blomkamp", 2009, 19.95, false); movies.addDVD("Iron Man", "Jon Favreau", 2008, 15.95, false); movies.addDVD("All About Eve", "Joseph Mankiewicz", 1950, 17.50, false); movies.addDVD("The Matrix", "Andy & Lana Wachowski", 1999, 19.95, true); System.out.println(movies); movies.addDVD("Iron Man 2", "Jon Favreau", 2010, 22.99, false); movies.addDVD("Casablanca", "Michael Curtiz", 1942, 19.95, false); System.out.println(movies); } }
- 74. Object Oriented Programming OOP 74 Output Listing 7.2 shows the DVDCollection class. It contains an array of DVD objects representing the collection. It maintains a count of the DVDs in the collection and their combined value. It also keeps track of the current size of the collection array so that a larger array can be created if too many DVDs are added to the collection.
- 75. Object Oriented Programming OOP 75 public class DVDCollection { private DVD[] collection; private int count; private double totalCost; public DVDCollection() { collection = new DVD[100]; count = 0; totalCost = 0.0; } public void addDVD(String title, String director, int year, double cost, boolean bluray){ if (count == collection.length) increaseSize(); collection[count] = new DVD(title, director, year, cost, bluray); totalCost += cost; count++; } public String toString(){ NumberFormat fmt = NumberFormat.getCurrencyInstance(); String report = "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~n"; report += "My DVD Collectionnn"; report += "Number of DVDs: " + count + "n"; report += "Total cost: " + fmt.format(totalCost) + "n"; report += "Average cost: " + fmt.format(totalCost/count); report += "nnDVD List:nn"; for (int dvd = 0; dvd < count; dvd++) report += collection[dvd].toString() + "n"; return report; } private void increaseSize(){ DVD[] temp = new DVD[collection.length * 2]; for (int dvd = 0; dvd < collection.length; dvd++) temp[dvd] = collection[dvd]; collection = temp; }
- 76. Object Oriented Programming OOP 76 The collection array is instantiated in the DVDCollection constructor. Every time a DVD is added to the collection (using the addDVD method), a new DVD object is created and a reference to it is stored in the collection array. Each time a DVD is added to the collection, we check to see whether we have reached the current capacity of the collection array. If we didn’t perform this check, an exception would eventually be thrown when we try to store a new DVD object at an invalid index. If the current capacity has been reached, the private increaseSize method is invoked, which first creates an array that is twice as big as the current collection array. Each DVD in the existing collection is then copied into the new array. Finally, the collection reference is set to the larger array. Using this technique, we theoretically never run out of room in our DVD collection. The user of the DVDCollection object (the main method in this case) never has to worry about running out of space, because it’s all handled internally. The toString method of the DVDCollection class returns an entire report summarizing the collection. The report is created, in part, using calls to the toString method of each DVD object stored in the collection. Listing 7.3 shows the DVD class.
- 77. Object Oriented Programming OOP 77 import java.text.NumberFormat; public class DVD { private String title, director; private int year; private double cost; private boolean bluray; public DVD(String title, String director, int year, double cost, boolean bluray) { this.title = title; this.director = director; this.year = year; this.cost = cost; this.bluray = bluray; } public String toString() { NumberFormat fmt = NumberFormat.getCurrencyInstance(); String description; description = fmt.format(cost) + "t" + year + "t"; description += title + "t" + director; if (bluray) description += "t" + "Blu-ray"; return description; } }
- 78. Object Oriented Programming OOP 78 Lecture 9: Class Inheritance A class is the model or blueprint from which an object is created. Consider the blueprint created by an architect when designing a house. The blueprint defines the important characteristics of the house—its walls, windows, doors, electrical outlets, and so on. Once the blueprint is created, several houses can be built using it, as depicted in Figure 8.1 . In one sense, the houses built from the blueprint are different. They are in different locations, have different addresses, contain different furniture, and are inhabited by different people. Many houses can be created from the same blueprint. They are essentially the same house in different locations with different people living in them. Now suppose you want a house that is similar to another but with some different or additional features. You want to start with the same basic blueprint but modify it to suit new, slightly different, needs. The houses in the development have the same core layout, but they have unique features. Creating a new blueprint that is based on an existing blueprint is analogous to the object-oriented concept of inheritance , which is the process in which a new class is derived from an existing one.
- 79. Object Oriented Programming OOP 79 Inheritance is a powerful software development technique and a defining characteristic of object-oriented programming. In general, new classes can be created via inheritance faster, easier, and cheaper than by writing them from scratch. Inheritance is one way to support the idea of software reuse. The original class that is used to derive a new one is called the parent class, superclass, or base class. The derived class is called a child class, or subclass. Java uses the reserved word extends to indicate that a new class is being derived from an existing class. Let’s look at an example. The program shown in Listing 8.1 instantiates an object of class Dictionary , which is derived from a class called Book . In the main method, three methods are invoked through the Dictionary object: two that were declared locally in the Dictionary class and one that was inherited from the Book class. The Book class (see Listing 8.2 ) is used to derive the Dictionary class (see Listing 8.3 ) using the reserved word extends in the header of Dictionary . public class Words{ public static void main(String[] args){ Dictionary webster = new Dictionary(); System.out.println("Number of pages: " + webster.getPages()); System.out.println("Number of definitions: " + webster.getDefinitions()); System.out.println("Definitions per page: " + webster.computeRatio()); }}
- 80. Object Oriented Programming OOP 80 The Dictionary class automatically inherits the definition of the setPages and getPages methods. public class Book { protected int pages = 1500; public void setPages(int numPages) { pages = numPages; } public int getPages() { return pages;} } public class Dictionary extends Book { private int definitions = 52500; public double computeRatio() { return (double) definitions / pages; } public void setDefinitions (int numDefinitions) { definitions = numDefinitions; } public int getDefinitions() { return definitions;} }
- 81. Object Oriented Programming OOP 81 Inheritance is a one-way street. The Book class cannot use variables or methods that are declared explicitly in the Dictionary class. Inheritance relationships are often represented in UML class diagrams. Figure 8.2 shows the inheritance relationship between the Book and Dictionary classes. An arrow with an open arrowhead is used to show inheritance in a UML diagram, with the arrow pointing from the child class to the parent class. If we declare a variable with public visibility so that a derived class can reference it, we violate the principle of encapsulation. Therefore, Java provides a third visibility modifier: protected. Note that the variable pages is declared with protected visibility in the Book class. In a UML diagram, protected visibility can be indicated by preceding the protected member with a hash mark (#).
- 82. Object Oriented Programming OOP 82 Constructors, however, are not inherited. Constructors are special methods that are used to set up a particular type of object, so it doesn’t make sense for a class called Dictionary to have a constructor called Book . But you can imagine that a child class may want to refer to the constructor of the parent class, which is one of the reasons for the super reference. The reserved word super can be used in a class to refer to its parent class. Using the super reference, we can access a parents members. Like the this reference, what the word super refers to depends on the class in which it is used. One use of the super reference is to invoke a parent's constructor. Let's look at an example. Listing 8.4 shows a modification of the original Words program from Listing 8.1 . public class Words2 { public static void main(String[] args) { Dictionary2 webster = new Dictionary2(1500, 52500); System.out.println("Number of pages: " + webster.getPages()); System.out.println("Number of definitions: " + webster.getDefinitions()); System.out.println("Definitions per page: " + webster.computeRatio()); }}
- 83. Object Oriented Programming OOP 83 public class Book2 { protected int pages; public Book2(int numPages) { pages = numPages;} public void setPages(int numPages) { pages = numPages;} public int getPages() { return pages;} } public class Dictionary2 extends Book2 { private int definitions; public Dictionary2(int numPages, int numDefinitions) { super(numPages); definitions = numDefinitions; } public double computeRatio() { return (double) definitions/pages;} public void setDefinitions(int numDefinitions) { definitions = numDefinitions;} public int getDefinitions() { return definitions; }}
- 84. Object Oriented Programming OOP 84 In this case, it would have been just as easy to set the pages variable explicitly in the Dictionary2 constructor instead of using super to call the Book2 constructor. The output of the Words2 program is the same as is for the original Words program. Java’s approach to inheritance is called single inheritance. This term means that a derived class can have only one parent. Some object- oriented languages allow a child class to have multiple parents. The designers of the Java language explicitly decided not to support multiple inheritance. Instead, we can rely on interfaces to provide the best features of multiple inheritance without the added complexity.
- 85. Object Oriented Programming OOP 85 Lecture 10: Interfaces A Java interface is a collection of constants and abstract methods. An abstract method is a method that does not have an implementation. That is, there is no body of code defined for an abstract method. The header of the method, including its parameter list, is simply followed by a semicolon. An interface cannot be instantiated. Why Use Interfaces: Interfaces provide an alternative to multiple inheritance. Java programming language does not support multiple inheritance. But interfaces provide a good solution. Any class can implement a particular interface and importantly the interfaces are not a part of class hierarchy. So, the general rule is extend one but implement many. A class can extend just one class but it can implement many interfaces. So, here we have multiple types for a class. It can be of the type of its super class and all the interfaces it implements. Interface is a collection of function which defines a contract and any 'implementing' class should override those functions Let us say you're writing a service which can be called by anyone. First thing that comes to our mind is what does that service does? What functionalities does it expose? The answer to it is a collection of functions. In this example let us say the service is calculator service and the collection of function is {add, subtract, mutiply, divide} and you've got yourself a interface, which the service should "implement" and your client should use the interface to make calls.
- 86. Object Oriented Programming OOP 86 Example: Let us say we have two interfaces A & B and two classes C & D. So, we can have 3 types for an object of class D as following: interface A{ } interface B{ } class C{ } class D extends C implements A,B { } A a=new D(); B b=new D(); C c=new D();
- 87. Object Oriented Programming OOP 87 Listing 9.1 shows an interface called Complexity . It contains two abstract methods: setComplexity and getComplexity . The Question class, shown in Listing 9.2 , implements the Complexity interface. Both the setComplexity and getComplexity methods are implemented. They must be declared with the same signatures as their abstract counterparts in the interface. In the Question class, the methods are defined simply to set or return a numeric value representing the complexity level of the question that the object represents. public interface Complexity{ public void setComplexity( int complexity); public int getComplexity();} public class Question implements Complexity { private String question, answer; private int complexityLevel; public Question(String query, String result) { question = query; answer = result; complexityLevel = 1; } public void setComplexity(int level) { complexityLevel = level; } public int getComplexity() { return complexityLevel; }
- 88. Object Oriented Programming OOP 88 Listing 9.3 shows a program called MiniQuiz, which uses some Question objects. public String getQuestion() { return question; } public String getAnswer() { return answer; } public boolean answerCorrect(String candidateAnswer) { return answer.equals(candidateAnswer); } public String toString() { return question + "n" + answer; } import java.util.Scanner; public class MiniQuiz { public static void main(String[] args) { Question q1, q2; String possible; Scanner scan = new Scanner(System.in); q1 = new Question("What is the capital of Jamaica?", "Kingston"); q1.setComplexity(4); q2 = new Question("Which is worse, ignorance or apathy?", "I don't know and I don't care"); q2.setComplexity(10);
- 89. Object Oriented Programming OOP 89 System.out.print(q1.getQuestion()); System.out.println(" (Level: " + q1.getComplexity() + ")"); possible = scan.nextLine(); if (q1.answerCorrect(possible)) System.out.println("Correct"); else System.out.println("No, the answer is " + q1.getAnswer()); System.out.println(); System.out.print(q2.getQuestion()); System.out.println(" (Level: " + q2.getComplexity() + ")"); possible = scan.nextLine(); if (q2.answerCorrect(possible)) System.out.println("Correct"); Else System.out.println("No, the answer is " + q2.getAnswer()); }
- 90. Object Oriented Programming OOP 90 An interface and its relationship to a class that implements it can be shown in a UML class diagram. An interface is represented similarly to a class node except that the designation <<interface>> is inserted above the interface name. A dotted arrow with a closed arrowhead is drawn from the class to the interface that it implements. Figure 9.3 shows a UML class diagram for the MiniQuiz program.
- 91. Object Oriented Programming OOP 91 Abstract class An abstract class in Java is a class that contains one or more abstract methods, which are simply method declarations without a body — that is, without executable code that implements the class or method. An abstract method is like a prototype for a method, declaring the method’s return type and parameter list but not providing an actual implementation of the method. You can’t instantiate an abstract class. However, you can create a subclass that extends an abstract class and provides an implementation of the abstract methods defined by the abstract class. To create an abstract class, you use the abstract on the class declaration and include at least one abstract method. For example: Remember two rules: 1) If the class is having few abstract methods and few concrete methods: declare it as abstract class. 2) If the class is having only abstract methods: declare it as interface. Public abstract class Ball { Public abstract int hit ( int speed) }
- 92. Object Oriented Programming OOP 92 Understanding the real scenario of abstract class In this example, Bank is the abstract class, its implementation is provided by the SBI and PNB classes. Output Rate of Interest is: 7 % Rate of Interest is: 8 % abstract class Bank{ abstract int getRateOfInterest(); } class SBI extends Bank{ int getRateOfInterest(){return 7;} } class PNB extends Bank{ int getRateOfInterest(){return 8;} } class TestBank{ public static void main(String args[]){ Bank b; b=new SBI(); System.out.println("Rate of Interest is: "+b.getRateOfInterest()+" %"); b=new PNB(); System.out.println("Rate of Interest is: "+b.getRateOfInterest()+" %"); } }
- 93. Object Oriented Programming OOP 93 Abstract class having constructor, data member, abstract method, method body. Abstract classes are useful when you want to create a generic type that is used as the superclass for two or more subclasses, but the superclass itself doesn’t represent an actual object. If all employees are either salaried or hourly, for example, it makes sense to create an abstract Employee class and then use it as the base class for abstract class Bike{ Bike(){ System.out.println("bike is created");} abstract void run(); void changeGear(){System.out.println("gear changed");} } class Honda extends Bike{ void run(){System.out.println("running safely..");} } class TestAbstraction2{ public static void main(String args[]){ Bike obj = new Honda(); obj.run(); obj.changeGear(); } }
- 94. Object Oriented Programming OOP 94 Output bike is created running safely.. gear changed We will discuss difference between Abstract Class and Interface in Java with examples. No abstract Classes Interfaces 1 abstract class can extend only one class or one abstract class at a time interface can extend any number of interfaces at a time 2 abstract class can have both abstract and concrete methods interface can have only abstract methods 3 In abstract class keyword ‘abstract’ is mandatory to declare a method as an abstract In an interface keyword ‘abstract’ is optional to declare a method as an abstract 4 abstract class can have protected , public and public abstract methods Interface can have only public abstract methods i.e. by default 5 abstract class can have static, final or static final variable with any access specifier interface can have only static final (constant) variable i.e. by default
- 95. Object Oriented Programming OOP 95 Lecture 11: Polymorphism The term polymorphism can be defined as “having many forms.” A polymorphic reference is a reference variable that can refer to different types of objects at different points in time. Polymorphism enables you to “program in the general” rather than “program in the specific.” Real life example of polymorphism Suppose if you are in class room that time you behave like a student, when you are in market at that time you behave like a customer, when you at your home at that time you behave like a son or daughter, Here one person present in different-different behaviors.
- 96. Object Oriented Programming OOP 96 Implementing for Extensibility With polymorphism, we can design and implement systems that are easily extensible—new classes can be added with little or no modification to the general portions of the program, as long as the new classes are part of the inheritance hierarchy that the program processes generically. The new classes simply “plug right in.” The only parts of a program that must be altered are those that require direct knowledge of the new classes that we add to the hierarchy. For example, if we extend class Animal to create class Tortoise (which might respond to a move message by crawling one inch), we need to write only the Tortoise class and the part of the simulation that instantiates a Tortoise object. The portions of the simulation that tell each Animal to move generically can remain the same. Suppose we create a program that simulates the movement of several types of animals for a biological study. Classes Fish, Frog and Bird represent the types of animals under investigation. Imagine that each class extends superclass Animal, which contains a method move and maintains an animal’s current location as x-y coordinates. Each subclass implements method move. Our program maintains an Animal array containing references to objects of the various Animal subclasses. To simulate the animals’ movements, the program sends each object the same message once per second—namely, move. Each specific type of Animal responds to a move message in its own way—a Fish might swim three feet, a Frog might jump five feet and a Bird might fly ten feet. Each object knows how to modify its x-y coordinates appropriately for its specific type of movement. Relying on each object to know how to “do the right thing” (i.e., do what’s appropriate for that type of object) in response to the same method call is the key concept of polymorphism. The same message (in this case, move) sent to a variety of objects has many forms of results— hence the term polymorphism.
- 97. Object Oriented Programming OOP 97 Example of polymorphism. In below example we create two class Person an Employee, Employee class extends Person class feature and override walk() method. We are calling the walk() method by the reference variable of Parent class. Since it refers to the subclass object and subclass method overrides the Parent class method, subclass method is invoked at runtime. Output Running fast.. class Person { void walk() { System.out.println("Can Run...."); } } class Employee extends Person { void walk() { System.out.println("Running Fast..."); } public static void main(String arg[]) { Person p=new Employee(); //upcasting p.walk(); } }
- 98. Object Oriented Programming OOP 98 Real example of Java Polymorphism Consider a scenario, Bank is a class that provides method to get the rate of interest. But, rate of interest may differ according to banks. For example, SBI, ICICI and AXIS banks could provide 8%, 7% and 9% rate of interest. class Bank{ int getRateOfInterest(){return 0;} } class SBI extends Bank{ int getRateOfInterest(){return 8;} } class ICICI extends Bank{ int getRateOfInterest(){return 7;} } class AXIS extends Bank{ int getRateOfInterest(){return 9;} }
- 99. Object Oriented Programming OOP 99 Output: SBI Rate of Interest: 8 ICICI Rate of Interest: 7 AXIS Rate of Interest: 9 Garbage Collection Garbage collection is a way to reclaim memory from objects once they are no longer in use. By removing unused objects from memory, more is available for new and existing objects. One difference between C++ and Java is that Java incorporates automatic garbage collection, so the programmer does not need to manage memory as much themselves. An object is eligible for garbage collection when it is no longer accessible through any variable. However, if one object refers to another and that object in turn refers to the first in a cyclical dependency, they may both be eligible for garbage collection. Once an object is eligible for garbage collection, the garbage collector will eventually remove it from memory. Recall how when a new object is created, the constructor method is called to initialize it. There is also a finalize method that’s invoked just before the object is destroyed. This method is built in to the Object superclass, class Test3{ public static void main(String args[]){ Bank b1=new SBI(); Bank b2=new ICICI(); Bank b3=new AXIS(); System.out.println("SBI Rate of Interest: "+b1.getRateOfInterest()); System.out.println("ICICI Rate of Interest: "+b2.getRateOfInterest()) System.out.println("AXIS Rate of Interest: "+b3.getRateOfInterest()) } }
- 100. Object Oriented Programming OOP 100 but you can override it in your classes to perform special actions before an object is destroyed. A Person variable p1 is declared and initialized to a newly created Person object with the name Adam. A second Person variable p2 is declared and initialized to another newly created Person object with the name Robert. See Figure below. In the next statement, the variable p1 is reassigned to the reference object of p2. See Figure below. class PersonManager { public static void main(String args[]) { Person p1 = new Person("Adam"); Person p2 = new Person("Robert"); p1 = p2; } }
- 101. Object Oriented Programming OOP 101 The Person object with name Adam no longer has any references pointing to it. So it becomes eligible for garbage collection. See Figure
- 102. Object Oriented Programming OOP 102 Lecture 12: Exception An exception is thrown by a program or the run-time environment and can be caught and handled appropriately if desired. An error is similar to an exception except that an error generally represents an unrecoverable situation and should not be caught. Java has a predefined set of exceptions and errors that may occur during the execution of a program. Problem situations represented by exceptions and errors can have various kinds of root causes. Here are some situations that cause exceptions to be thrown: • Attempting to divide by zero. • An array index that is out of bounds. • A specified file that could not be found. • A requested I/O operation that could not be completed normally. • An attempt was made to follow a null reference. • An attempt was made to execute an operation that violates some kind of security measure. Uncaught exceptions If a program does not handle the exception at all, it will terminate abnormally and produce a message that describes what exception occurred and where it was produced. First we demonstrate what happens Exception handling enables you to create applications that can resolve (or handle) exceptions. In many cases, handling an exception allows a program to continue executing as if no problem had been encountered.
- 103. Object Oriented Programming OOP 103 when errors arise in an application that does not use exception handling. Figure 12.1 prompts the user for two integers and passes them to method quotient, which calculates the integer quotient and returns an int result. In this example, you’ll see that exceptions are thrown (i.e., the exception occurs) by a method when it detects a problem and is unable to handle it import java.util.Scanner; public class DivideByZeroNoExceptionHandling { // demonstrates throwing an exception when a divide-by-zero occurs public static int quotient(int numerator, int denominator) { return numerator / denominator; // possible division by zero } public static void main(String[] args) { Scanner scanner = new Scanner(System.in); System.out.print("Please enter an integer numerator: "); int numerator = scanner.nextInt(); System.out.print("Please enter an integer denominator: "); int denominator = scanner.nextInt(); int result = quotient(numerator, denominator); System.out.printf( "%nResult: %d / %d = %d%n", numerator, denominator, result); }}
- 104. Object Oriented Programming OOP 104 Stack Trace The first sample execution in Fig. 12.2 shows a successful division. In the second execution, the user enters the value 0 as the denominator. Several lines of information are displayed in response to this invalid input. This information is known as a stack trace, which includes the name of the exception (java.lang.ArithmeticException) in a descriptive message that indicates the problem that occurred and the method-call stack (i.e., the call chain) at the time it occurred. The stack trace includes the path of execution that led to the exception method by method. This helps you debug the program.
- 105. Object Oriented Programming OOP 105 Example: Handling ArithmeticExceptions and InputMismatchExceptions The application in Fig. 12.3, which is based on Fig. 12.2, uses exception handling to process any ArithmeticExceptions and InputMistmatchExceptions that arise. The application still prompts the user for two integers and passes them to method quotient, which calculates the quotient and returns an int result. This version of the application uses exception handling so that if the user makes a mistake, the program catches and handles (i.e., deals with) the exception—in this case. import java.util.Scanner; import java.util.InputMismatchException; public class DivideByZeroWithExceptionHandling { public static int quotient(int numerator, int denominator) { return numerator / denominator; // possible division by zero } public static void main(String[] args) { Scanner scanner = new Scanner(System.in); boolean continueLoop = true; // determines if more input is needed do { try { System.out.print("Please enter an integer numerator: "); int numerator = scanner.nextInt(); System.out.print("Please enter an integer denominator: "); int denominator = scanner.nextInt(); int result = quotient(numerator, denominator); System.out.printf("%nResult: %d / %d = %d%n", numerator, denominator, result); continueLoop = false; // input successful; end looping }
- 106. Object Oriented Programming OOP 106 catch (InputMismatchException inputMismatchException) {System.err.printf("%nException: %s%n", inputMismatchException); scanner.nextLine(); // discard input so user can try again System.out.printf( "You must enter integers. Please try again.%n%n"); } catch (ArithmeticException arithmeticException){ System.err.printf("%nException: %s%n", arithmeticException); System.out.printf( "Zero is an invalid denominator. Please try again.%n%n"); } } while (continueLoop); }}
- 107. Object Oriented Programming OOP 107 The first sample execution in Fig. 12.3 does not encounter any problems. In the second execution the user enters a zero denominator, and an ArithmeticException exception occurs. In the third execution the user enters the string "hello" as the denominator, and an InputMismatchException occurs. For each exception, the user is informed of the mistake and asked to try again, then is prompted for two new integers. In each sample exec
- 108. Object Oriented Programming OOP 108 Lecture 13: Programming Practice OOP Abstraction, Encapsulation, Inheritance and Polymorphism Classes Invoice and Employee both represent things for which the company must be able to calculate a payment amount. Both classes implement Payable, so a program can invoke method getPaymentAmount on Invoice objects and Employee objects alike. As we'll soon see, this enables the polymorphic processing of Invoices and Employees required for our company's accounts payable application. The UML class diagram in Figure 13.1 shows the hierarchy used in our accounts payable application. The hierarchy begins with interface Payable. The diagram in Figure 13.1 indicates that classes Invoice and Employee each realize (i.e., implement) interface Payable. Note that, as in the class diagram of class Employee appears in italics, indicating that it is an abstract class. Concrete class SalariedEmployee extends Employee and inherits its superclass's realization relationship with interface Payable.
- 109. Object Oriented Programming OOP 109 1. Declaring Interface Payable Interface Payable contains public abstract method getPaymentAmount , in addition, method getPaymentAmount has no parameters, but interface methods can have parameters. public interface Payable { double getPaymentAmount(); // calculate payment; } 2. Creating Class Invoice We now create class Invoice (Figure 13.1) to represent a simple invoice that contains billing information for only one kind of part. The class declares private instance variables partNumber, partDescription, quantity and pricePerItem that indicate the part number, a description of the part, the quantity of the part ordered and the price per item. Class Invoice also contains a constructor . Note that methods setQuantity and setPricePerItem ensure that quantity and pricePerItem obtain only nonnegative values. public class Invoice implements Payable { private String partNumber; private String partDescription; private int quantity; private double pricePerItem; // four-argument constructor public Invoice( String part, String description, int count, double price ) { partNumber = part; partDescription = description; setQuantity( count ); // validate and store quantity setPricePerItem( price ); // validate and store price } public void setPartNumber( String part )
- 110. Object Oriented Programming OOP 110 { partNumber = part; } // end method setPartNumber public String getPartNumber() { return partNumber; } // end method getPartNumber public void setPartDescription( String description ) { partDescription = description; } // end method setPartDescription public String getPartDescription() { return partDescription; } // end method getPartDescription public void setQuantity( int count ) { quantity = ( count < 0 ) ? 0 : count; // quantity } // end method setQuantity // get quantity public int getQuantity() { return quantity; } // end method getQuantity // set price per item public void setPricePerItem( double price ) { pricePerItem = ( price < 0.0 ) ? 0.0 : price; // validate price } // end method setPricePerItem // get price per item public double getPricePerItem() { return pricePerItem; } // end method getPricePerItem
- 111. Object Oriented Programming OOP 111 public String toString() { return String.format( "%s: n%s: %s (%s) n%s: %d n%s: $%,.2f", "invoice", "part number", getPartNumber(), getPartDescription(), "quantity", getQuantity(), "price per item", getPricePerItem() ); } // end method toString // method required to carry out contract with interface Payable public double getPaymentAmount() { return getQuantity() * getPricePerItem(); // calculate } } // end class Invoice 3. Create Class Employee to Implement Interface Payable We now Create class Employee that it implements interface Payable, we declared method earnings as abstract, and as a result class Employee had to be declared abstract. This forced each Employee subclass to override earnings with a concrete implementation. public abstract class Employee implements Payable { private String firstName; private String lastName; private String socialSecurityNumber; // three-argument constructor public Employee( String first, String last, String ssn ) { firstName = first; lastName = last; socialSecurityNumber = ssn; } // end three-argument Employee constructor
- 112. Object Oriented Programming OOP 112 public void setFirstName( String first ) { firstName = first; } // end method setFirstName // return first name public String getFirstName() { return firstName; } // end method getFirstName // set last name public void setLastName( String last ) { lastName = last; } // end method setLastName // return last name public String getLastName() { return lastName; } // end method getLastName // set social security number public void setSocialSecurityNumber( String ssn ) { socialSecurityNumber = ssn; // should validate } // end method setSocialSecurityNumber // return social security number public String getSocialSecurityNumber() { return socialSecurityNumber; } // end method getSocialSecurityNumber // return String representation of Employee object public String toString() { return String.format( "%s %snsocial security number: %s", getFirstName(), getLastName(), getSocialSecurityNumber() ); } // end method toString
- 113. Object Oriented Programming OOP 113 4. Create Class SalariedEmployee for Use in the Payable Hierarchy and derived from Employee Create class SalariedEmployee that extends Employee and fulfills superclass Employee's contract to implement method getPaymentAmount of interface Payable. public class SalariedEmployee extends Employee { private double weeklySalary; // four-argument constructor public SalariedEmployee( String first, String last, String ssn, double salary ) { super( first, last, ssn ); // pass to Employee constructor setWeeklySalary( salary ); // validate and store salary } // end four-argument SalariedEmployee constructor public void setWeeklySalary( double salary ) { weeklySalary = salary < 0.0 ? 0.0 : salary; } // end method setWeeklySalary // return salary public double getWeeklySalary() { return weeklySalary; } // end method getWeeklySalary public double getPaymentAmount() { return getWeeklySalary(); } // end method getPaymentAmount public String toString() { return String.format( "salaried employee: %sn%s: $%,.2f", super.toString(), "weekly salary", getWeeklySalary() ); } // end method toString } // end class SalariedEmployee
- 114. Object Oriented Programming OOP 114 When a class implements an interface, the same is-a relationship provided by inheritance applies. For example, class Employee implements Payable, so we can say that an Employee is a Payable. In fact, objects of any classes that extend Employee are also Payable objects. SalariedEmployee objects, for instance, are Payable objects. As with inheritance relationships, an object of a class that implements an interface may be thought of as an object of the interface type. Objects of any subclasses of the class that implements the interface can also be thought of as objects of the interface type. Thus, just as we can assign the reference of a SalariedEmployee object to a superclass Employee variable, we can assign the reference of a SalariedEmployee object to an interface Payable variable. Invoice implements Payable, so an Invoice object also is a Payable object, and we can assign the reference of an Invoice object to a Payable variable. 5. All methods of class Object can be called by using a reference of an interface type. A reference refers to an object, and all objects inherit the methods of class Object. public class PayableInterfaceTest { public static void main( String args[] ) { // create four-element Payable array Payable payableObjects[] = new Payable[ 4 ]; // populate array with objects that implement Payable payableObjects[ 0 ] = new Invoice( "01234", "seat", 2, 375.00 ); payableObjects[ 1 ] = new Invoice( "56789", "tire", 4, 79.95 ); payableObjects[ 2 ] = new SalariedEmployee( "John", "Smith", "111-11- 1111", 800.00 );
- 115. Object Oriented Programming OOP 115 payableObjects[ 3 ] = new SalariedEmployee( "Lisa", "Barnes", "888-88- 8888",1200.00 ); System.out.println( "Invoices and Employees processed OOP:n" ); for ( Payable currentPayable : payableObjects ) { // output currentPayable and its appropriate payment amount System.out.printf( "%s n%s: $%,.2fnn", currentPayable.toString(), "payment due", currentPayable.getPaymentAmount() ); } // end for } // end main } // end class PayableInterfaceTest OUTPUT Invoices and Employees processed OOP: invoice: part number: 01234 (seat) quantity: 2 price per item: $375.00 payment due: $750.00 invoice: part number: 56789 (tire) quantity: 4 price per item: $79.95 payment due: $319.80 salaried employee: John Smith social security number: 111-11-1111 weekly salary: $800.00 payment due: $800.00 salaried employee: Lisa Barnes social security number: 888-88-8888 weekly salary: $1,200.00 payment due: $1,200.00
- 116. Object Oriented Programming OOP 116 Exercises 1) Draw a class diagram for system Training courses that contain many courses like (Java, C#, PHP). System consist of two classes Student and Course, you can adding another classes if necessary .Please write all attributes and method for every Class and discuss relationship between classes. 2) Suppose you have a class called Movie. Write a constructor for the class that initializes the id ,title and director instance variablesbased on parameters passed to the constructor. 3) Write program for show employer info, the program consists of two classes, first class for Main and second class for Employer. Please print 2 employers and the output must be like screen below. Ahamed Ali 1000 $ Saif Kassim 400 $ 4) Write program for show Customers info, the program consists of two classes, first class for Main and second class for Customer. Please print 2 customers and the output must be like screen below. 1 Ahamed 0781555552 Karada62 2 Saif 0771989898 Monsor22 5) Write a class called Laptop that contains instance data for the laptop model, make, purchaser, and purchase year. Define the Laptop constructor to accept and initialize this data. Include setter and getter methods for all instance data. Include a toString method that returns a nicely formatted, multi-line description of the laptop purchase.
- 117. Object Oriented Programming OOP 117 6) Write a class called Course that represents a course offered to students. It should contain instance data that represents the course title, course code, credits and course Instructor’s name. Define the Course constructor to accept and initialize all instance data. Include getter and setter methods for all instance data. Include a toString method that returns a one-line description of the course. Create a driver class called CourseDetails whose main method instantiates and updates several Course objects. 7) Drawing Class diagram and Create classes with all code necessary in dependency to package below 8) Draw Class diagram for system Company. System consists of two classes Person and Main. Person has two types Employer and Lecture. For print information you can using Interface PrintInfo. Please write all classes, attributes and method. 9) Write Class and Attributes depending on the Class diagram below?Knowing that attribute numberofStudentPhone is fixed. classTestCode{ public static void main(String args[]){ Person b1=new Employer(); System.out.println("Info Emp: "+b1.getInfo()); Personb2=new Servicer(); System.out.println(" Info Servicer: "+b2.getInfo()); } 1Student StudentPhone n
- 118. Object Oriented Programming OOP 118 10) Draw Class diagram and Write program for system Company. System consists of two classes Person and Main. Person has two types Employer and Lecture. For print information you can using abstract method PrintInfo. Please write all classes, attributes and method and using Array for Employer a ArrayList for Lecture. The output program must like below: . Good luck!! Information Employer Id : 1 Full name : Saif Kassim Birthday : 29.04.1984 Salary : 2400 Department : Telecom Id : 2 Full name : Ahmed Ali Birthday : 29.04.1989 Salary : 2000 Department : IT Good luck!! Dr Saif Qassim Mohammed Personal email: [email protected] Work email: [email protected] Facebook: https://www.facebook.com/Dr.saif.Kassim Channel: https://www.youtube.com/user/MrITLogic/playlists Twitter: https://twitter.com/drsaifkassim Information Lecture Id :1 Full name : Ahmed Ali Birthday : 12.09.1988 Title : PhD Specialization : Computer Science Id :2 Full name : Nor Ali Birthday : 18.09.1995 Title : MS Specialization : IT