How do I use the final keyword in Java?

By I Wayan Saryada in Basic, Core API 0 Comment

The final modifier is used to mark a class final so that it cannot be extended, to prevent a method being overridden, and to prevent changing the value of a variable. Arguments of a method if declared as final is also can not be modified within the method.

package org.kodejava.basic;

public class FinalExample {
    // breed is declared final.
    // can't change the value assigned to breed
    public final String breed = "pig";
    private int count = 0;

    public static void main(String[] args) {
        FinalExample fe = new FinalExample();
        // assign a value to breed variable will cause a
        // compile-time error
        //
        // fe.breed = "dog";

        int number = fe.count(20);
    }

    // sound() method is declared final, so it can't be overridden
    public final void sound() {
        System.out.println("oink oink");
    }

    // number parameter is declared final. can't change the value
    // assigned to number
    public int count(final int number) {
        // assign a value to number variable will cause a
        // compile-time error
        //
        // number = 1;

        count = +number;
        return count;
    }
}

final class SubFinalExample extends FinalExample {

    // try to override sound() method of superclass will cause a
    // compile-time error
    //
    // public void sound() {
    //     System.out.println("oink");
    // }
}

// try to inherit a class that declared final will cause a
// compile-time error
//
class OtherFinalExample extends SubFinalExample {
}

How do I create constructors for a class?

By I Wayan Saryada in Basic, Core API 0 Comment

Every class in Java has a constructor. constructor is a method that is used to create an instance or object of the class. Every time you create an instance, you must invoke a constructor.

If you do not create a constructor method of your class, the compiler will build a default one. A default constructors is a constructor that accept no argument.

Things to be noted when you declare a constructor:

  • Constructor must have the same name as the class in which they are declared.
  • Constructor can’t have a return type.
  • Constructor can have access modifier.
  • Constructor can take arguments.
  • Constructor can’t be marked static.
  • Constructor can’t be marked final or abstract.
package org.kodejava.basic;

public class ConstructorDemo {
    private String arg;
    private int x;
    private int y;

    public ConstructorDemo() {
    }

    public ConstructorDemo(String arg) {
        this.arg = arg;
    }

    public ConstructorDemo(int x) {
        this.x = x;
    }

    public ConstructorDemo(int x, int y) {
        this.y = y;
    }
}

class RunConstructor {
    public static void main(String[] args) {
        // Change the default constructor from private to public in
        // the ConstructorDemo class above then call the statement 
        // below. It will create an instance object cons0 without 
        // any error.
        ConstructorDemo cons0 = new ConstructorDemo();

        // Change the default constructor back to private, then call 
        // the statement below. ConstructorDemo() is not visible 
        // because it declared as private.
        ConstructorDemo cons1 = new ConstructorDemo();

        // invoke Constructor(String arg)
        ConstructorDemo cons2 = new ConstructorDemo("constructor");

        // invoke public Constructor(int x)
        ConstructorDemo cons3 = new ConstructorDemo(1);

        //invoke Constructor(int x, int y)
        ConstructorDemo cons4 = new ConstructorDemo(1, 2);
    }

}

How do I overload methods in Java?

By I Wayan Saryada in Basic, Core API 0 Comment

Method overloading allows a method to use the same name or identifier as the method name as long as the argument list is different. Java can differentiate each method by their method signatures. For example to print some value you can create a print method that accept different kind of objects or values as its parameters.

Overloaded method is differentiated by the number and the type of argument they accept. The print(String string) and print(int number) are distinct and unique due to their argument type.

The compiler does not count a return type as a method differentiator. So it is not legal to create a method with the same name, the same number, the same type of argument but with a different return type.

package org.kodejava.basic;

public class OverloadedExample {
    public void print(Object object) {
        System.out.println("object = " + object);
    }

    public void print(String string) {
        System.out.println("string = " + string);
    }

    public void print(int number) {
        System.out.println("number = " + number);
    }

    public void print(float number) {
        System.out.println("number = " + number);
    }

    public void print(double number) {
        System.out.println("number = " + number);
    }
}

How do I use the super keyword?

By I Wayan Saryada in Basic, Core API 0 Comment

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.basic;

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.basic;

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

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

How do I create an interface in Java?

By I Wayan Saryada in Basic, Core API 0 Comment

Interface only contains methods declaration and all its methods are abstract methods. In its most common form, an interface is a group of related methods with empty bodies. To create an interface, use interface keyword in class definition. The file name of interface always the same with the interface name in class definition and the extension is .java.

The RemoteControl interface defines four move methods and a getPosition() methods. These methods have no bodies. If a class implements an interface, the class should implement all the contracts / methods defined by the interface.

package org.kodejava.basic;

public interface RemoteController {
    void moveUp(int n);

    void moveRight(int n);

    void moveDown(int n);

    void moveLeft(int n);

    int[] getPosition();
}

The following snippet show you how to create a class that implements the RemoteController interface.

package org.kodejava.basic;

public class DummyRemoteControl implements RemoteController {
    private int x;
    private int y;

    public DummyRemoteControl(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public static void main(String[] args) {
        RemoteController control = new DummyRemoteControl(0, 0);
        control.moveDown(10);
        control.moveLeft(5);
        System.out.println("X = " + control.getPosition()[0]);
        System.out.println("Y = " + control.getPosition()[1]);
    }

    public void moveUp(int n) {
        x = x + n;
    }

    public void moveRight(int n) {
        y = y + n;
    }

    public void moveDown(int n) {
        x = x - n;
    }

    public void moveLeft(int n) {
        y = y - n;
    }

    public int[] getPosition() {
        return new int[]{x, y};
    }
}