java: basics, user input, data type, constructor

BASIC STRUCTURE – Fundamentals to Understand
Class
Attributes
Primitive data types in Java
Arrays – How to declare them
Methods in Java
Main method - It provides the control of program flow.
Simple methods – We need object to access such methods
Static methods - We DON’T need object to access such methods
Constructors
Object – An instance of a class
How to instantiate? Calling of method/attribute.
//Creating an object of Student class with name as “ram”
Student ram = new Student();
//Accessing the method/attribute of ram
ram.course=“M.C.A”;
System.out.println(“New course of ram is: ”+ ram.course)

Single line comments: These comments start with //
e.g.: // this is comment line
Multi line comments: These comments start with /* and end with */
e.g.: /* this is comment line*/
Since Java is purely an Object Oriented Programming language –
• We must have at least one class.
• We must create an object of a class, to access SIMPLE methods/attributes of
a class.
Static methods are the methods, which can be called and executed without
creating objects. e.g. main() method.

BASIC STRUCTURE – Flow of Execution
JVM contains the Java interpreter, which in turn calls the main () method using its Classname.main
() at the time of running the program.
main() method must contain String array as argument.
Since, main () method should be available to the JVM, it should be
declared as public.
Write the main method. Execution starts from main() method.
[Plz note, we do not need an object to access/execute main() method becuase it is static.]
Write the first class containing the main() method.
Name of the class and the name of the file should be EXACLTY same.
Import the required packages. By default import java.lang.*
A package is a kind of directory that contains a group of related classes and interfaces. A class or interface contains methods.

//This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
PROGRAM EXECUTION – STEP I
Enter main method.
The main method provides the control of program flow. The Java interpreter
executes the application by invoking the main method.

//This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
PROGRAM EXECUTION – STEP II
Execute Statement 1

ESCAPE SEQUENCE
Java supports all escape sequence which is supported by C/ C++.
t Insert a tab in the text at this point.
b Insert a backspace in the text at this point.
n Insert a newline in the text at this point.
r Insert a carriage return in the text at this point.
f Insert a form feed in the text at this point.
' Insert a single quote character in the text at this point.
" Insert a double quote character in the text at this point.
Insert a backslash character in the text at this point.

USER INPUT
There are three ways to take input from a user:
1. User input as Command Line Argument
2. User input using Scanner class object
3. User input using BufferedReader class object

USER INPUT - Command Line Argument - Example
class Student{
String course;
String address;
Integer semester;
}
import java.lang.*;
class Test{
public static void main(String[] args){
Student ramesh = new Student();
//Set the values to attributes
ramesh.course="B.Tech";
ramesh.address=“Delhi";
}
}

class Student{
String course;
String address;
Integer semester;
}
import java.lang.*;
class Test{
ramesh.course=args[0];
ramesh.address=args[1];
}
}
javac Test.java
java Test B.Tech Delhi

class Student{
String course;
String address;
Integer semester;
}
import java.lang.*;
class Test{
ramesh.course=args[0];
ramesh.address=args[1];
int i = Integer.parseInt(args[2]);
ramesh.semester=i
}
}
javac Test.java
java Test B.Tech Delhi 5

USER INPUT - using Scanner class object
import java.util.Scanner;
class Test{
Scanner ob=new Scanner(System.in);
System.out.println("Please enter the course");
String courseTemp=ob.nextLine();
System.out.println("Please enter sem no. ");
int semesterTemp=ob.nextInt();
ramesh.course=courseTemp;
ramesh.semester=semesterTemp;
}
}
For more: http://www.tutorialspoint.com/java/util/java_util_scanner.htm

USER INPUT - using BufferedReader class object
import java.io.BufferedReader;
import java.io.Exception;
import java.io.InputStreamReader;
public class Test{
public static void main(String[] args) throws Exception {
BufferedReader br=new BufferedReader(new
InputStreamReader(System.in));
System.out.print("Enter String");
String s = br.readLine();
System.out.print("Enter Integer:");
try{
int i = Integer.parseInt(br.readLine());
}catch(Exception e){
System.err.println("Invalid Format!");
System.out.println(e);
}
}
}
For more: http://stackoverflow.com/questions/2231369/scanner-vs-bufferedreader

Class
Attributes
Primitive data types in Java
Arrays – How to declare them
Methods in Java
Main method - It provides the control of program flow.
Simple methods – We need object to access such methods
Static methods - We DON’T need object to access such methods
Constructors
Object – An instance of a class
How to instantiate? Calling of method/attribute.

CONSTRUCTOR
A class contains constructors that are invoked to create objects.
There are basically two rules defined for the constructor:
• Constructor name must be same as its class name
• Constructor must have no explicit return type
Class Bicycle{
int gear, cadence, speed;
public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
}
To create a new Bicycle object called myBike, a constructor is called by the new operator:
Bicycle myBike = new Bicycle(30, 0, 8);
new Bicycle(30, 0, 8) creates space in memory for the object and initializes its fields.

CONSTRUCTOR
Although Bicycle only has one constructor, it could have others, including a no-argument
constructor:
//Inside Bicycle class
public Bicycle() {
gear = 1;
cadence = 10;
speed = 0;
}
//Outside Bicycyle class – e.g. Inside main method
Bicycle yourBike = new Bicycle();
above statement invokes the no-argument constructor to create a new Bicycle
object called yourBike.
• Both (or more) constructors could have been declared in same class - Bicycle.
• Constructors are differentiated by Java Interpreter based on the SIGNATURE.
• SIGNATURE = No. of arguments + Data type of arguments + Order of arguments

CONSTRUCTOR
class Student{
String course;
String address;
int semester;
//Default Constructor – NO ARGUMENT
public Student(){
//Creates space in memory for the object and initializes its fields to defualt.
}
//Parameterized Constructor – ALL ARGUMENTS
public Student(String s1, String s2, int i1){
this.course=s1;
this.course=s2;
this.semester=i1;
}
//Parameterized Constructor – FEW ARGUMENTS
public Student(String s1){
this.course=s1;
}
}
Rule: If there is no constructor in a class, compiler automatically creates a default constructor.

CONSTRUCTOR
import java.lang.*;
class Test{
//Get the values in to TEMPORARY variables
String temp1 = args[0];
String temp2 = args[1];
int temp3 = args[2];
Student ramesh = new Student(temp1,temp2,temp3);
System.out.println(ramesh.course);
System.out.println(ramesh.address);
System.out.println(ramesh.semester);
}
}
javac Test.java
java Test B.Tech Delhi 5

PRIMITIVE DATA TYPE in JAVA
Type Contains Default Size Range
byte Signed integer 0 8 bits -128 to 127
short Signed integer 0 16 bits -32768 to 32767
int Signed integer 0 32 bits -2147483648 to 2147483647
float
IEEE 754 floating
point
0.0f 32 bits ±1.4E-45 to ±3.4028235E+38
long Signed integer 0L 64 bits
-9223372036854775808 to
9223372036854775807
double
IEEE 754 floating
point
0.0d 64 bits ±4.9E-324 to ±1.7976931348623157E+308
boolean true or false FALSE 1 bit NA
char Unicode character 'u0000' 16 bits u0000 to uFFFF

FEATURES of JAVA
• Simple: Learning and practicing java is easy because of resemblance with C and C++.
• Object Oriented Programming Language: Unlike C++, Java is purely OOP.
• Distributed: Java is designed for use on network; it has an extensive library which works
in agreement with TCP/IP.
• Secure: Java is designed for use on Internet. Java enables the construction of virus-free,
tamper free systems.
• Robust (Strong enough to withstand intellectual challenge): Java programs will not
crash because of its exception handling and its memory management features.
• Interpreted: Java programs are compiled to generate the byte code. This byte code can
be downloaded and interpreted by the interpreter. .class file will have byte code
instructions and JVM which contains an interpreter will execute the byte code.

FEATURES of JAVA
• Portable: Java does not have implementation dependent aspects and it yields or gives
same result on any machine.
• Architectural Neutral Language: Java byte code is not machine dependent, it can run
on any machine with any processor and with any OS.
• High Performance: Along with interpreter there will be JIT (Just In Time) compiler which
enhances the speed of execution.
• Multithreaded: Executing different parts of program simultaneously is called
multithreading. This is an essential feature to design server side programs.
• Dynamic: We can develop programs in Java which dynamically change on Internet (e.g.:
Applets).
P l e a s e G o o g l e / Re a d m o re fo r R E D h i g h l i g h t e d c o l o r ke y w o rd s
F O R M O R E I N F O / O F F I C I A L L I N K - JAVA
h t t p : / / d o c s . o ra c l e . c o m / j a v a s e / t u t o r i a l / j a v a / j a v a O O / i n d e x . h t m l

java: basics, user input, data type, constructor

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