/doc/source/java/basic.rst

Java
****



Hello World
===========

.. warning:: 

    Filename should be same as class-name, otherwise compilation error will be generated.


.. code-block:: java

    // HelloWorld.java

    public class HelloWorld{

        // main method is the starting point for execution
        public static void main(String[] args){
            System.out.println("Hello World"); // println = print + \n
        }
    }


.. code-block:: shell

    $ javac HelloWorld.java 
    $ java HelloWorld 
    Hello World



Variables
=========


.. code-block:: java

    // HelloWorld.java

    public class HelloWorld{
        public static void main(String[] args){
            System.out.println("Hello World");

            int i = 100;
            double j = 10.10;
            double k;

            k = i + j;

            // System.out.println("k= " + k);
            System.out.print("k= " + k + "\n");

            char c = 'c';                           // char in single quote
            System.out.println("char = " + c);

            boolean b = true;  // true/false
            System.out.println("Boolean b = " + b);
            System.out.println("3>4 :" + (3>4));

            // string is not a datatype; it's a predefined class
            String name = "Meher Krishna Patel";    // string in double quote
            System.out.println("Name = " + name);
        }
    }


.. code-block:: shell

    $ javac HelloWorld.java 
    $ java HelloWorld 
    Hello World
    k= 110.1
    char = c
    Boolean b = true
    3>4 :false
    Name = Meher Krishna Patel




Printing values in Binary, Hex, Octal and Decimal formats
=========================================================


.. code-block:: java

    // hexDec.java

    class hexDec{
        public static void main(String arg[]){
            int x = 15;
            System.out.printf("x = %d\n", x);

            int h = 0xb;
            int o = 013;
            
            System.out.printf(" hex value of x = %x\n", x); // hex
            System.out.printf(" oct value of x = %o\n", x); // octal
            System.out.printf(" decimal value of x = %d\n\n", x); // decimal

            System.out.printf(" hex value of h = %x\n", h); // hex
            System.out.printf(" oct value of h = %o\n", h); // octal
            System.out.printf(" decimal value of h = %d\n\n", h); // decimal

            System.out.printf(" hex value of o = %x\n", o); // hex
            System.out.printf(" oct value of o = %o\n", o); // octal
            System.out.printf(" decimal value of o = %d\n\n", o); // decimal


        }
    }



Define constant using 'final'
=============================

.. warning:: 

    Since method 'main' is of type **static**, therefore variables 'pi' and 'blob' should be of type '**static**', otherwise these can not be used in 'main' function. 


.. code-block:: java

    // defineConst.java

    public class defineConst{
        public static final double pi = 3.14;
        public static final String blog = "PythonDSP"; 
        
        public static void main(String[] args){
            final int radius = 2;
            double area; 

            area = pi * Math.pow(radius, 2);
            System.out.printf("Area of circle = %f\n", area);
            System.out.printf("Blog : %s\n", blog);
        }
    }

.. code-block:: shell

    $ javac defineConst.java 
    $ java defineConst 
    Area of circle = 12.560000
    Blog : PythonDSP



    

.. code-block:: shell

    $ javac hexDec.java 
    $ java hexDec 
    x = 15
     hex value of x = f
     oct value of x = 17
     decimal value of x = 15

     hex value of h = b
     oct value of h = 13
     decimal value of h = 11

     hex value of o = b
     oct value of o = 13
     decimal value of o = 11




Read input from user
====================


.. code-block:: java

    // ReadEx.java

    import java.util.Scanner; // used to read input

    public class ReadEx{
        public static void main(String[] args){
            // Create object of Scanner
            Scanner scn = new Scanner(System.in);
            
            int x, y, z; // variables

            // print
            System.out.print("Enter first number: ");
            x = scn.nextInt(); // read x

            System.out.print("Enter second number: ");
            y = scn.nextInt(); // read y

            z = x+y; // add

            // printf
            System.out.printf("Sum = %d\n", z);
        }
    }




Operators and Control statements
================================


Operators
---------

Arithmetic operators
^^^^^^^^^^^^^^^^^^^^^

.. code-block:: java

    // incrementEx.java

    public class incrementEx{
        public static void main(String[] args){
            int i = 1;
            System.out.printf("i = %d\n\n", i);
            
            // print first, then increment
            System.out.printf("i++ = %d (i.e. access first and then increment): \n", i++); // 1
            System.out.printf("i = %d\n\n", i); // 2 (i.e. i is incremented by above statement)

            // increment first, then print
            System.out.printf("++i = %d (i.e. increment first and then access):\n", ++i); // 3
            System.out.printf("i = %d\n", i); // 3
        }
    }


.. code-block:: shell

    $ java incrementEx 
    i = 1

    i++ = 1 (i.e. access first and then increment): 
    i = 2

    ++i = 3 (i.e. increment first and then access):
    i = 3


Relational operators
^^^^^^^^^^^^^^^^^^^^

Logical operators
^^^^^^^^^^^^^^^^^




Decision statements
-------------------

If-else statements
^^^^^^^^^^^^^^^^^^

.. code-block:: java

    // ElseIfEx.java

    import java.util.Scanner; // used to read input

    public class ElseIfEx{
        public static void main(String[] args){
            // Create object of Scanner
            Scanner scn = new Scanner(System.in);
            
            System.out.print("Enter Grade: ");
            String input = scn.next();      
            char grade = input.charAt(0);
            
            // if grade is A or a
            if (grade == 'A' || grade == 'a')   // logical operator
                System.out.printf("Excellent Student\n");
            else if ((grade == 'B') || (grade == 'b'))   
                System.out.printf("Very Good Student\n");
            else if (grade == 'C' || grade == 'c')
                System.out.printf("Good Student\n");
            else if (grade == 'D' || grade == 'd')
                System.out.printf("Need improvements\n");
            else
                System.out.printf("Invalid grade\n");
        }
    }


.. code-block:: shell

    $ javac ElseIfEx.java 
    $ java ElseIfEx 
    Enter Grade: b
    Very Good Student
    $ java ElseIfEx 
    Enter Grade: 4
    Invalid grade


Switch case
^^^^^^^^^^^

.. code-block:: java

    // switchCase.java

    import java.util.Scanner; // used to read input

    public class switchCase{
        public static void main(String[] args){
            // Create object of Scanner
            Scanner scn = new Scanner(System.in);
            
            System.out.print("Enter Grade: ");
            String input = scn.next();      
            char grade = input.charAt(0);
            
            switch(grade){
                case ('A'): 
                    System.out.printf("A: performance is excellent\n");
                    break;
                case 'B' :
                    System.out.printf("B: performance is good\n");
                    break;
                case 'C' :
                    System.out.printf("C: performance is not bad\n");
                    break;
                default :
                    System.out.printf("Invalid grade\n");   
            }
        }
    }


.. code-block:: shell

    $ javac switchCase.java 
    $ java switchCase 
    Enter Grade: A
    A: performance is excellent
    $ java switchCase 
    Enter Grade: a
    Invalid grade


Conditional operator (?:)
^^^^^^^^^^^^^^^^^^^^^^^^^


Loops
-----

For loop
^^^^^^^^

.. code-block:: java

    // forLoop.java

    public class forLoop{
        public static void main(String[] args){
            int i;

            // print 0 to 4
            for (i=0; i<5; i++){
                System.out.printf("%d, ", i); //0, 1, 2, 3, 4,
            }

            // value of i is 5
            System.out.printf("\n");
            System.out.printf("Value of i = %d\n", i); // Value of i = 5
        }
    }


.. code-block:: shell

    $ javac forLoop.java 
    $ java forLoop 
    0, 1, 2, 3, 4, 
    Value of i = 5



While loop
^^^^^^^^^^

.. code-block:: java

    // whileLoop.java

    public class whileLoop{
        public static void main(String[] args){
            int i=0;

            // print 0 to 4
            while(i < 5){
                System.out.printf("%d, ", i); //0, 1, 2, 3, 4,
                i = i+1;
            }

            // value of i is 5
            System.out.printf("\n");
            System.out.printf("Value of i = %d\n", i); // Value of i = 5
        }
    }

.. code-block:: shell

    $ javac whileLoop.java 
    $ java whileLoop 
    0, 1, 2, 3, 4, 
    Value of i = 5



Do while loop
^^^^^^^^^^^^^

.. code-block:: java

    // dowhileLoop.java

    public class dowhileLoop{
        public static void main(String[] args){
            int i=0;

            // print 0 to 4
            do{
                System.out.printf("%d, ", i); //0, 1, 2, 3, 4,
                i = i+1;
            }while(i < 5);

            // value of i is 5
            System.out.printf("\n");
            System.out.printf("Value of i = %d\n", i); // Value of i = 5
        }
    }

.. code-block:: shell

    $ javac dowhileLoop.java 
    $ java dowhileLoop 
    0, 1, 2, 3, 4, 
    Value of i = 5



Continue and Break statements
-----------------------------


Continue
^^^^^^^^

.. code-block:: java

    // forContinueLoop.java

    public class forContinueLoop{
        public static void main(String[] args){
            int i;

            // print 0 to 4
            for (i=0; i<5; i++){
                if (i%2==0)
                    continue;
                System.out.printf("%d, ", i); //0, 1, 2, 3, 4,

            }

            // value of i is 5
            System.out.printf("\n");
            System.out.printf("Value of i = %d\n", i); // Value of i = 5
        }
    }


.. code-block:: shell

    $ javac forContinueLoop.java 
    $ java forContinueLoop 
    1, 3, 
    Value of i = 5


Break
^^^^^

.. code-block:: java

    // whileBreakLoop.java

    public class whileBreakLoop{
        public static void main(String[] args){
            int i=0;

            // print 0 to 6 (break at 7)
            while(i < 15){
                System.out.printf("%d, ", i);
                i = i+1;
                if (i%7==0)
                    break;
            }

            System.out.printf("\n");
            System.out.printf("Value of i = %d\n", i); 
        }
    }


.. code-block:: shell

    $ javac whileBreakLoop.java 
    $ java whileBreakLoop 
    0, 1, 2, 3, 4, 5, 6, 
    Value of i = 7


Array
=====

.. code-block:: java

    // arrayEx.java

    public class arrayEx{
        public static void main(String[] args){
            int i;
            
            int[] a; // uninitialized array of size 5 i.e. a[0]-a[4]
            a = new int[10];
            
            double[] b={2, 4.5, 6}; // initialized array of size 3
            int[] c = {3, 5};
                    
            System.out.printf("a[1] = %d\n", a[1]);  // uninitialized array has 0 value

            System.out.printf("c[0] = %d\n", c[0]); // 3


            // print all values of array b  
            // %10s create the width of 10 after 'element' 
            // and 'value' will be printed as right-aligned e.g. see 4.5 in output
            System.out.printf("%s %10s\n", "element", "value");
            for (i=0; i<3; i++){
                System.out.printf("%4d %12.1f\n", i, b[i]); // %12.1f = show minimum 12 integer & 1 decimal 
            }

            // assign values of array a
            for (i=0; i<5; i++){
              a[i] = 2*i;
            }

            // print values of array a
            System.out.printf("%s %10s\n", "element", "value");
            for (i=0; i<5; i++){
                System.out.printf("%4d %12d\n", i, a[i]);  // %12.1f = show minimum 12 integer and 1 decimal place 
            }
        }
    }


.. code-block:: shell

    $ javac arrayEx.java 
    $ java arrayEx 
    a[1] = 0
    c[0] = 3
    element      value
       0          2.0
       1          4.5
       2          6.0
    element      value
       0            0
       1            2
       2            4
       3            6
       4            8





OOPs
====



Class and object
----------------

* Class Jungle

.. code-block:: java

    // Jungle.java

    public class Jungle{

        // public : to allow access outside the class
        public void welcomeMessage(){
            System.out.println("Welcome to Jungle");
        }
    }


* Object 'j' of class Jungle

.. code-block:: java

    // JungleTest.java

    public class JungleTest{
        public static void main(String[] args){
            Jungle j = new Jungle();
            j.welcomeMessage();
        }
    }


* Code execution

.. code-block:: shell

    $ javac Jungle.java 
    $ javac JungleTest.java 
    $ java JungleTest 
    Welcome to Jungle


.. note::

    We can compile multiple file as below, 

    .. code-block:: shell
    
        $ javac Jungle.java JungleTest.java

        or 

        $ javac *.java



Method with parameters
----------------------


.. code-block:: java

    // Jungle.java

    public class Jungle{

        // public : to allow access outside the class
        public void welcomeMessage(String name){
            System.out.printf("Hello %s! Welcome to Jungle\n", name);
        }
    }


.. code-block:: java

    // JungleTest.java

    public class JungleTest{
        public static void main(String[] args){
            Jungle j = new Jungle();
            j.welcomeMessage("Meher");
        }
    }



.. code-block:: shell

    $ javac *.java
    $ java JungleTest 
    Hello Meher! Welcome to Jungle


set and get method
------------------

.. code-block:: java

    // Jungle.java

    public class Jungle{

        private String visitorName;

        // set (save) visitor name
        public void setVisitorName(String name){
            visitorName = name;
        }

        // get visitor name
        public String getVisitorName(){
            return visitorName;
        }

        // print message 
        public void welcomeMessage(){
            System.out.printf("Hello %s! Welcome to Jungle\n", getVisitorName());
        }
    }


.. code-block:: java

    // JungleTest.java

    public class JungleTest{
        public static void main(String[] args){
            Jungle j = new Jungle();

            // save visitor name
            j.setVisitorName("Krishna");

            // print message
            j.welcomeMessage();
        }
    }


.. code-block:: shell

    $ javac *.java
    $ java JungleTest 
    Hello Krishna! Welcome to Jungle


Constructor
-----------

* Initialize object using constructor
  
.. code-block:: java

    // Jungle.java

    public class Jungle{

        private String visitorName;

        // Constructor : name should be same as class
        public Jungle(String name){ // constructor with one argument
            visitorName = name;
        }

        // set (save) visitor name
        public void setVisitorName(String name){
            visitorName = name;
        }

        // get visitor name
        public String getVisitorName(){
            return visitorName;
        }

        // print message 
        public void welcomeMessage(){
            System.out.printf("Hello %s! Welcome to Jungle\n", getVisitorName());
        }
    }


.. code-block:: java

    // JungleTest.java

    public class JungleTest{
        public static void main(String[] args){
            Jungle j = new Jungle("Patel");

            // print message
            j.welcomeMessage();
        }
    }


.. code-block:: shell

    $ javac *.java
    $ java JungleTest 
    Hello Patel! Welcome to Jungle




Inheritance
-----------

.. code-block:: java

    // RateJungle.java

    public class RateJungle extends Jungle{
        private int feedback;
        private String name;

        public RateJungle(String name){
            super(name); // call the base class constructor
            feedback = 0; // set feedback to 0
        }

        public void setFeedback(int val){
            feedback = val;
        }

        public void printRating(){
            System.out.printf("Thanks %s\n", getVisitorName());
            System.out.printf("Your feedback is set as : %d\n", feedback);
        }
    }
                
         
.. code-block:: java

    // RateJungleTest.java

    public class RateJungleTest{
        public static void main(String[] args){
            RateJungle m = new RateJungle("Meher");
            RateJungle k = new RateJungle("Krishna");

            m.printRating();
            m.setFeedback(2);
            m.printRating();
            
            k.printRating();
        }
    }



.. code-block:: shell

    $ javac *.java
    $ java RateJungleTest 
    Thanks Meher
    Your feedback is set as : 0
    Thanks Meher
    Your feedback is set as : 2
    Thanks Krishna
    Your feedback is set as : 0



multiple constructor
--------------------

.. code-block:: java

    // RateJungle.java

    public class RateJungle extends Jungle{
        private int feedback;
        private String name;

        // constructor 1
        public RateJungle(String name){
            super(name); // call the base class constructor
            feedback = 0; // set feedback to 0
        }

        // constructor 2
        public RateJungle(String name, int val){
            super(name);
            feedback = val;
        }

        // constructor 3 : empty constructor
        public RateJungle(){
        }

        public void setFeedback(int val){
            feedback = val;
        }

        public void printRating(){
            System.out.printf("Thanks %s\n", getVisitorName());
            System.out.printf("Your feedback is set as : %d\n", feedback);
        }
    }
                
         
* Since child class has empty constructor, therefore we need to add empty constructor in parent class as well. 

.. code-block:: java

    // Jungle.java

    public class Jungle{

        private String visitorName;

        // Constructor : name should be same as class
        public Jungle(String name){ // constructor with one argument
            visitorName = name;
        }

        // empty constructor : as child class has an empty constructor
        public Jungle(){
        }

        // set (save) visitor name
        public void setVisitorName(String name){
            visitorName = name;
        }

        // get visitor name
        public String getVisitorName(){
            return visitorName;
        }

        // print message 
        public void welcomeMessage(){
            System.out.printf("Hello %s! Welcome to Jungle\n", getVisitorName());
        }
    }


.. code-block:: java

    // RateJungleTest.java

    public class RateJungleTest{
        public static void main(String[] args){
            RateJungle m = new RateJungle("Meher");
            RateJungle k = new RateJungle("Krishna", 5);
            RateJungle p = new RateJungle();

            p.setVisitorName("Patel");
            p.setFeedback(2);

            m.printRating();
            k.printRating();
            p.printRating();
        }
    }


.. code-block:: shell

    $ javac *.java
    $ java RateJungleTest 
    Thanks Meher
    Your feedback is set as : 0
    Thanks Krishna
    Your feedback is set as : 5
    Thanks Patel
    Your feedback is set as : 2



Polymorphism
------------


.. code-block:: java

    // Animal.java

    public class Animal{
        public void scarySound(){
            System.out.println("Animals are running away due to scary sound.");
        }
    }


.. code-block:: java

    // Bird.java

    public class Bird{
        public void scarySound(){
            System.out.println("Birds are flying away due to scary sound.");
        }
    }


.. code-block:: java

    // Insect.java

    // empty class
    public class Insect{
    }


.. code-block:: java

    // PolymorphismTest.java

    public class PolymorphismTest{
        public static void main(String[] args){
            Animal a = new Animal();
            Bird b = new Bird();

            a.scarySound();
            b.scarySound();
        }
    }

.. code-block:: shell

    $ javac *.java
    $ java PolymorphismTest 
    Animals are running away due to scary sound.
    Birds are flying away due to scary sound.


Abstract class
--------------


.. code-block:: java

    // Abstract_Jungle.java

    // abstract class
    public abstract class Abstract_Jungle{

        private String visitorName;

        // set (save) visitor name
        public void setVisitorName(String name){
            visitorName = name;
        }

        // get visitor name
        public String getVisitorName(){
            return visitorName;
        }

        // print message 
        public void welcomeMessage(){
            System.out.printf("Hello %s! Welcome to Jungle\n", getVisitorName());
        }

        // abstract method
        public abstract void scarySound();
    }


.. code-block:: java

    // Animal.java

    public class Animal extends Abstract_Jungle{
        public void scarySound(){
            System.out.println("Animals are running away due to scary sound.");
        }
    }


.. code-block:: java

    // Bird.java

    public class Bird extends Abstract_Jungle{
        public void scarySound(){
            System.out.println("Birds are flying away due to scary sound.");
        }
    }


.. code-block:: java

    // Abstract_JungleTest.java

    public class Abstract_JungleTest{
        public static void main(String[] args){
            Animal a = new Animal();
            Bird b = new Bird();
            Insect i = new Insect();

            a.scarySound();
            b.scarySound();
            i.scarySound();
        }
    }


.. code-block:: shell

    $ javac *.java
    $ java Abstract_JungleTest 
    Animals are running away due to scary sound.
    Birds are flying away due to scary sound.
    Insects do not care about scary sound.


.. error:: 

    If we do not define the 'scarySound()' in class Insect, then following error will be generated. 

    .. code-block:: shell

        $ javac *.java

        Insect.java:4: error: Insect is not abstract and does not override abstract method scarySound() in Abstract_Jungle
        public class Insect extends Abstract_Jungle{
               ^
        1 error