How do I create static variables in Java?

Class variables or static variable is variable that declared with static modifier. A given class will have only one copy of each of its static variables, regardless of how many times the class has been instantiated.

If the value of a static variable is changed, the new value is available equally in all instances of the class. The final keyword could be added to indicate the value of static variable will never change. If you try to assign a new value to final variable, you will get a compile error.

package org.kodejava.example.fundametal;

public class StaticDemo {
    // static variable
    static int x = 12;

    // static variable with final value that never change
    final static int Y = 20;

    // non-static variable
    int z;

    public static void main(String[] args) {
        StaticDemo sd0 = new StaticDemo();

        System.out.println("x before update = " + StaticDemo.x);
        System.out.println("y= " + StaticDemo.Y);

        sd0.z = StaticDemo.x + StaticDemo.Y;
        System.out.println("z= " + sd0.z);

        StaticDemo.x = 15;
        System.out.println("x after update = " + StaticDemo.x);

        StaticDemo sd1 = new StaticDemo();
        StaticDemo sd2 = new StaticDemo();
        StaticDemo.x = 20;

        System.out.println("StaticDemo.x = " + StaticDemo.x);
        System.out.println("sd0 = " + sd0.getX());
        System.out.println("sd1 = " + sd1.getX());
        System.out.println("sd2 = " + sd2.getX());

        //
        // try to assign value to final variable, it will cause a
        // compile time error
        //
        // StaticDemo.Y = 30;
    }

    public int getX() {
        return StaticDemo.x;
    }
}

Here is the output printed by the program:

x before update = 12
y= 20
z= 32
x after update = 15
StaticDemo.x = 20
sd0 = 20
sd1 = 20
sd2 = 20

How do I catch multiple exceptions?

If a try block can throw several different kind of exceptions and you want to handle each exception differently, you can put several catch blocks to handle it.

package org.kodejava.example.fundametal;

public class MultipleCatchExample {
    public static void main(String[] args) {
        int[] numbers1 = {1, 2, 3, 4, 5};
        int[] numbers2 = {1, 2, 3, 4, 5, 6};

        try {
            //
            // This line throws an ArrayIndexOutOfBoundsException
            //
            MultipleCatchExample.printResult(numbers1);

            //
            // This line throws an ArithmeticException
            //
            MultipleCatchExample.printResult(numbers2);
        } catch (ArithmeticException e) {
            e.printStackTrace();
        } catch (ArrayIndexOutOfBoundsException e) {
            e.printStackTrace();
        } finally {
            System.out.println("Finally block is always executed.");
        }
    }

    /**
     * Divide the given first number by the second number.
     *
     * @param x the first number.
     * @param y the second number.
     * @return the result of division.
     */
    private static int divide(int x, int y) {
        return x / y;
    }

    /**
     * Print the output result of divide operation by calling the
     * divide() method.
     *
     * @param numbers integer arrays of the divided number
     * @throws ArrayIndexOutOfBoundsException when an exception
     * occurs.
     */
    private static void printResult(int[] numbers) {
        int x, z, y = 1;
        for (int i = 0; i < 6; i++) {
            x = numbers[i];
            if (i == 5) {
                y = 0;
            }
            z = MultipleCatchExample.divide(x, y);
            System.out.println("z = " + z);
        }
    }
}

How do I use the super keyword?

When a class extends from other class, the class or usually called as subclass inherits all the accessible members and methods of the superclass. If the subclass overrides a method provided by its superclass, a way to access the method defined in the superclass is through the super keyword.

package org.kodejava.example.fundametal;

public class Bike {
    public void moveForward() {
        System.out.println("Bike: Move Forward.");
    }
}

In the ThreeWheelsBike's moveForward() method we call the overridden method using the super.moveForward() which will print the message from the Bike class.

package org.kodejava.example.fundametal;

public class ThreeWheelsBike extends Bike {
    @Override
    public void moveForward() {
        super.moveForward();
        System.out.println("Three Wheels Bike: Move Forward.");
    }

    public static void main(String[] args) {
        Bike bike = new ThreeWheelsBike();
        bike.moveForward();
    }
}