1. Java

1.1. Hello World

Warning

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

// 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
    }
}

$ javac HelloWorld.java
$ java HelloWorld
Hello World

1.2. Variables

// 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);
    }
}

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

1.3. Printing values in Binary, Hex, Octal and Decimal formats

// 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


    }
}

1.4. 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.

// 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);
    }
}

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

$ 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

1.5. Read input from user

// 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);
    }
}

1.6. Operators and Control statements

1.6.1. Operators

1.6.1.1. Arithmetic operators

// 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
    }
}

$ 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

1.6.1.2. Relational operators

1.6.1.3. Logical operators

1.6.2. Decision statements

1.6.2.1. If-else statements

// 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");
    }
}

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

1.6.2.2. Switch case

// 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");
        }
    }
}

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

1.6.2.3. Conditional operator (?:)

1.6.3. Loops

1.6.3.1. For loop

// 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
    }
}

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

1.6.3.2. While loop

// 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
    }
}

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

1.6.3.3. Do while loop

// 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
    }
}

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

1.6.4. Continue and Break statements

1.6.4.1. Continue

// 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
    }
}

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

1.6.4.2. Break

// 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);
    }
}

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

1.7. Array

// 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
        }
    }
}

$ 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

1.8. OOPs

1.8.1. Class and object

• Class Jungle

// 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

// JungleTest.java

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

• Code execution

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

Note

We can compile multiple file as below,

$ javac Jungle.java JungleTest.java

or

$ javac *.java

1.8.2. Method with parameters

// 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);
    }
}

// JungleTest.java

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

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

1.8.3. set and get method

// 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());
    }
}

// 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();
    }
}

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

1.8.4. Constructor

• Initialize object using constructor

// 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());
    }
}

// JungleTest.java

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

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

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

1.8.5. Inheritance

// 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);
    }
}

// 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();
    }
}

$ 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

1.8.6. multiple constructor

// 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.

// 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());
    }
}

// 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();
    }
}

$ 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

1.8.7. Polymorphism

// Animal.java

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

// Bird.java

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

// Insect.java

// empty class
public class Insect{
}

// PolymorphismTest.java

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

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

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

1.8.8. Abstract class

// 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();
}

// Animal.java

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

// Bird.java

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

// 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();
    }
}

$ 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.

$ 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