Java Tutorial

Java Tutorial
BY AKASH PANDEY
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Java - General
• Java is:
• platform independent programming language and Technology.
• similar to C++ in syntax
• Developed by James Gosling of Sun Microsystem now acquired
by Oracle Corp.

Java - General
• Java has some interesting features:
• automatic type checking,
• automatic garbage collection,
• simplifies pointers; no directly accessible pointer to memory,
• simplified network access,
• multi-threading!

Compile-time EnvironmentCompile-time
Environment
Java
Bytecodes
move locally
or through
network
Java
Source
(.java)
Java
Compiler
Java
Bytecod
e
(.class )
Java
Interprete
r
Just in
Time
Compiler
Runtime System
Class
Loader
Bytecod
e
Verifier
Java
Class
Libraries
Operating System
Java
Virtual
machine
How it works…!

How it works…!
• Only depends on the Java Virtual Machine (JVM)
• Code is compiled to bytecode, which is interpreted
by the resident JVM
• JIT (just in time) compilers attempt to increase
speed.

Java - Security
• Pointer denial - reduces chances of virulent programs
corrupting host.
• Applets even more restricted.

Object-Oriented
• Java supports OOP
• Polymorphism
• Inheritance
• Encapsulation
• Java programs contain nothing but definitions and
instantiations of classes
• Everything is encapsulated in a class!

Java Advantages
• Portable - Write Once, Run Anywhere
• Security has been well thought through
• Robust memory management
• Designed for network programming
• Multi-threaded (multiple simultaneous tasks)

Primitive Types and Variables
• boolean, char, byte, short, int, long, float, double etc.
• These basic (or primitive) types are the only types that are
not objects (due to performance issues).
• This means that you don’t use the new operator to create
a primitive variable.
• Declaring primitive variables:
float initVal;
int retVal, index = 2;// declaration with initialization
boolean valueOk = false;
But in Android:Image name must contain only a-z & 0-9 & _ (Capitals
not allowed)

Initialisation
• Java sets primitive variables to default values
zero or false or null if not initialized.
• All object references are initially set to null
• An array of anything is an object
• Set to null on declaration
• Elements to zero false or null on creation

Declarations
int index = 1.2; // compiler error
boolean retOk = 1; // compiler error
double fiveFourths = 5 / 4; // no error!
float ratio = 5.8; // incorrect
double huh = 0 / 0; // AE- DBZ
But for floating point:
S.o.pln(-5.4/0+ “ “ + 0.0/0); // -Infinity NaN
• 1.2f is a float value accurate to 7 decimal places.
• 1.2 is a double value accurate to 15 decimal places.

Basic Mathematical Operators
• * / % + - are the mathematical operators
• * / % have a higher precedence than + or -
double myVal = a + b % d – c * d / b;
• Is the same as:
double myVal = (a + (b % d)) –
((c * d) / b);

Statements & Blocks
• A simple statement is a command terminated by a
semi-colon:
name = “Fred”;
• A block is a compound statement enclosed in curly
brackets:
{
name1 = “Fred”; name2 = “Bill”;
}
• Blocks may contain other blocks

Flow of Control
• Java executes one statement after the other in
the order they are written
• Many Java statements are flow control
statements:
Alternation: if, if else, switch
Looping: for, while, do while
Escapes: break, continue, return

If – The Conditional Statement
• The if statement evaluates an expression and if that
evaluation is true then the specified action is taken
if ( x < 10 ) x = 10;
• If the value of x is less than 10, make x equal to 10
• It could have been written:
if ( x < 10 )
x = 10;
• Or, alternatively:
if ( x < 10 ) { x = 10; }

Relational Operators
== Equal (careful)
!= Not equal
>= Greater than or equal
<= Less than or equal
> Greater than
< Less than

If… else
• The if … else statement evaluates an expression and
performs one action if that evaluation is true or a
different action if it is false.
if (x != oldx) {
System.out.print(“x was changed”);
}
else {
System.out.print(“x is unchanged”);
}

Nested if … else
if ( myVal > 100 ) {
if ( remainderOn == true) {
myVal = mVal % 100;
}
else {
myVal = myVal / 100.0;
}
}
else
{
System.out.print(“myVal is in range”);
}

else if
• Useful for choosing between alternatives:
if ( n == 1 ) {
// execute code block #1
}
else if ( j == 2 ) {
}
else {
// if all previous tests have failed, execute
code block #3
}

The switch Statementswitch ( n ) {
default:
// if all previous tests fail then
//execute code block #4
break;
case 1:
break;
case 2:
break;
}

The for loop
• Loop n times
for ( i = 0; i < n; n++ ) {
// this code body will execute n times
// ifrom 0 to n-1
}
• Nested for loop:
for ( j = 0; j < 10; j++ ) {
for ( i = 0; i < 20; i++ ){
// this code body will execute 200 times
}
}

while loops
while(response == 1) {
System.out.print( “ID =” + userID[n]);
n++;
response = readInt( “Enter “);
}
What is the minimum number of times the loop
is executed?
What is the maximum number of times?

do {… } while loops
do {
System.out.print( “ID =” + userID[n] );
n++;
response = readInt( “Enter ” );
}while (response == 1);
What is the minimum number of times the loop
is executed?
What is the maximum number of times?

Break
• A break statement causes an exit from the
innermost containing while, do, for or switch
statement.
for ( int i = 0; i < maxID, i++ ) {
while ( userID[i] == targetID ) {
index = i;
break;
}
}// program jumps here after break

Continue
• Can only be used with while, do or for.
• The continue statement causes the innermost loop to
start the next iteration immediately
for ( int i = 0; i < maxID; i++ ) {
if ( userID[i] != -1 ) continue;
System.out.print( “UserID ” + i + “ :” +
userID);
}

Arrays
• Am array is a list of similar things sharing a common
name in between in a contagious fashion.
• An Array is an object, in fact everything in java!
• An array has a fixed:
• name
• type
• length
• These must be declared when the array is created.
• Arrays sizes cannot be changed during the execution of
the code

myArray has room for 8 elements
 the elements are accessed by their index
 array indices start at 0
3 6 3 1 6 3 4 1myArray =
[0] [1] 2 3 4 5 6 [7]
int myArray[] = {3, 6, 3, 1, 6, 3, 4, 1};

Declaring Arrays
int arr[]; or int []arr; or int[] arr;
declares arr to be an array of integers
arr = new int[8];// array definition
sets up 8 integer-sized spaces in memory, labelled arr[0] to arr[7]
int arr[] = new int[8];
combines the two statements in one line

Assigning Values
• refer to the array elements by index to store values in
them.
myArray[0] = 3;
myArray[1] = 6;
myArray[2] = 3; ...
• can create and initialise in one step:
int myArray[] = {3, 6, 3, 1, 6, 3, 4, 1};

Iterating Through Arrays
• for loops are useful when dealing with arrays:
for (int i = 0; i < arr.length; i++) {
arr[i] = getsomevalue();
}
// diff b/w length & length()

Arrays of Objects
• So far we have looked at an array of primitive types.
• integers
• could also use doubles, floats, characters…
• Often want to have an array of objects
• Students, Books, Loans ……
• Need to follow 3 steps.

Declaring the Array
1. Declare the array
private Student studentList[];
• this declares studentList
2 .Create the array
studentList = new Student[10];
• this sets up 10 spaces in memory that can hold
references to Student objects
3. Create Student objects and add them to the array:
studentList[0] = new Student("Cathy",
"Computing");

Classes ARE Object Definitions
• OOPS - object oriented programming
Structure, ie code built from objects
• Class is the template or blueprint having related data and
relevant methods & Object is the instance of that class.
• Name of the file must match the public class name
containing main().

The three principles of OOP
• Encapsulation
• Objects hide their functions
(methods) and data
(instance variables)
• Inheritance
• Each subclass inherits all
variables of its superclass
• Polymorphism
• Interface same despite
different data types
car
auto-
matic
manual
Super class
Subclasses
draw(3args) draw(4args)
abstraction

Simple Class and Method
Class Fruit{
float grams;
float cals_per_gram;
float total_calories() {
return(grams*cals_per_gram);
}
}

Methods
• A method is a named sequence of code with parenthesis
at the end that can be invoked by other Java code.
• A method takes some parameters, performs some
computations and then optionally returns a value (or
object).
• Methods can be used as part of an expression statement.
public float convertCelsius(float tempC) {
return( ((tempC * 9.0f) / 5.0f) + 32.0 );
}

Method Signature & Prototype
• A method signature specifies:
• The name of the method.
• The type and name of each parameter.
Prototype= Signature + return type.
• The type of the value (or object) returned by the method.
• The checked exceptions thrown by the method. modifiers type
name ( parameter list ) [throws exceptions ]
public boolean setUserInfo ( int i, int j, String name ) throws
IndexOutOfBoundsException {}

Access Specifiers:
• Methods/data may be declared public or
private or protected meaning they may or
may not be accessed by code in other
classes … java has a default (package or
friendly ) access as well.
• Good practice:
• keep data private
• keep methods public

Using objects
• Here, code in one class creates an instance of
another class and does something with it.
Fruit plum=new Fruit();
int cals;
cals = plum.total_calories();
• Dot operator allows you to access (public)
data/methods inside Fruit class

Constructors
• The line
plum = new Fruit();
• invokes a constructor with which you can set the
initial data of an object
• You may choose several different type of
constructor with different argument lists
eg Fruit(), Fruit(a) ...

Overloading
• Can have several versions of a method in class with different
types, numbers or order of arguments
Fruit() {int grams=50;}
Fruit(int a,int b) { grams=a; cals_per_gram=b;}
• By looking at arguments Java decides which one to use

Overriding
• Prototype is same but the method is in the subclass (child
class).
Eg:
class Water{p v weight() {grams=50;}}
class Milk extends Water{p v weight() {grams=5;}}

Package:• Collection of classes, interfaces, enumerations, and sub
packages.
Eg: package pack1;
it must be the first statement of your java program if package is
there.
(Android package must contain at least one sub package ex:
package abc.def;
Don’t write your packageName as package)

Package
• When you create a class in a package, the generated class file
must reside in a subdirectory of a directory listed in CLASSPATH
or in the same subdirectory of a JAR (or ZIP) file that resides in
a directory named in the CLASSPATH.

Importing
• To import and use the predefined classes, interfaces.
Default:
Import java.lang.*;

Inner class:
• A (nested) class within another class.
• Types:
• Static Inner Classes
• Local Inner Classes
• Anonymous Inner Classes
• A non final variable cant be referred (called) inside a different
method of inner class.

differencebetweenconstructionofanewobject,and
constructionofanewinnerclassextendingaclass:
• Person queen=new Person(“Mary”); //Person Object
created.
//An object of an inner class extending Person
Person demon= new Person(“Dracula”){ //class code
here};

Java Tutorial

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