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};
    }
}

How do I implement interfaces in Java?

By I Wayan Saryada in Basic, Core API 2 Comments

To implement an interface, a java class must use implements keyword on its class definition. For example, class A implements interface B. The class definition of class A would look like this:

class A implements B {
}

A class can implements more than one interfaces. For example, class A can implements interface B and interface C:

class A implements B, C {
}

Classes that implements an interface must implements all methods declared in that interface.

package org.kodejava.basic;

public interface Language {

    String getBirthday();
    String getGreeting();

}

The following class is an English implementation of the Language interface.

package org.kodejava.basic;

public class English implements Language {
    public String getBirthday() {
        return "Happy Birthday";
    }

    public String getGreeting() {
        return "How are you?";
    }
}

The following class is an Indonesian implementation of the Language interface.

package org.kodejava.basic;

public class Indonesian implements Language {
    public String getBirthday() {
        return "Selamat Ulang Tahun";
    }

    public String getGreeting() {
        return "Apa kabar?";
    }
}

And here is a snippet that show the interface and classes in action.

package org.kodejava.basic;

public class LanguageDemo {
    public static void main(String[] args) {
        Language language = new English();
        System.out.println(language.getBirthday());
        System.out.println(language.getGreeting());

        language = new Indonesian();
        System.out.println(language.getBirthday());
        System.out.println(language.getGreeting());
    }
}

How do I extend classes in Java?

By I Wayan Saryada in Basic, Core API 0 Comment

Inheritance is one of object-oriented programming concepts. This concept allows classes to inherit commonly used state and behavior from other classes. Inheritance is the way to put commonly used states and behaviors into one class and reuse it.

The class that inherits all the attributes from other class is called as sub class. While, the class that inherited is called as superclass. You can use the extends keyword in class definition to inherit from other classes.

When you apply the final keyword to the class declaration you will make the class final, a final class cannot be extended by other class.

For example below we have, a Truck class and a Sedan that derived from a Car class.

package org.kodejava.basic;

public class CarDemo {
    public static void main(String[] args) {
        Car car = new Car();
        car.setBrand("Honda");
        System.out.println("Brand = " + car.getBrand());

        // The setBrand() and getBrand() is inherited from the Car
        // class.
        Truck truck = new Truck();
        truck.setBrand("Ford");
        System.out.println("Brand = " + truck.getBrand());
        int loadCapacity = truck.getLoadCapacity();

        // The setBrand(), getBrand() and setNumberOfSeat methods
        // are is inherited from the Car class.
        Sedan sedan = new Sedan();
        sedan.setBrand("Hyundai");
        System.out.println("Brand = " + sedan.getBrand());
        sedan.setNumberOfSeat(2);
        int gearType = sedan.getGearType();
    }
}

Here the definition of the Car, the Truck and the Sedan classes.

package org.kodejava.basic;

public class Car {
    private String type;
    private String brand;
    private String model;
    private int numberOfSeat;

    public Car() {
    }

    public Car(String type, String brand, String model) {
        this.type = type;
        this.brand = brand;
        this.model = model;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public String getModel() {
        return model;
    }

    public void setModel(String model) {
        this.model = model;
    }

    public int getNumberOfSeat() {
        return numberOfSeat;
    }

    public void setNumberOfSeat(int numberOfSeat) {
        this.numberOfSeat = numberOfSeat;
    }

    public String getCarInfo() {
        return "Type: " + type
                + "; Brand: " + brand
                + "; Model: " + model;
    }
}

package org.kodejava.basic;

public class Truck extends Car {
private int loadCapacity;

    public Truck() {
    }

    public Truck(String type, String brand, String model) {
        super(type, brand, model);
    }

    public int getLoadCapacity() {
        return loadCapacity;
    }

    public void setLoadCapacity(int loadCapacity) {
        this.loadCapacity = loadCapacity;
    }

    @Override
    public String getCarInfo() {
        return "Type: " + getType()
                + "; Brand: " + getBrand()
                + "; Model: " + getModel()
                + "; Load capacity: " + getLoadCapacity();
    }
}

package org.kodejava.basic;

public class Sedan extends Car {
    private int gearType;

    public Sedan() {
    }

    public int getGearType() {
        return gearType;
    }

    public void setGearType(int gearType) {
        this.gearType = gearType;
    }
}

How do I create a class in Java?

By I Wayan Saryada in Basic, Core API 0 Comment

A class is a specification or blueprint from which individual objects are created. A class contains fields that represent the object’s states and methods that defines the operations that are possible on the objects of the class.

The file name that contains the definition of a class is always the same as the public class name and the extension is .java to identify that the file contains a Java source code.

A class has constructors, a special method that is used to create an instance or object of the class. When no constructor define a default constructor will be used. The constructor method have the same name with the class name without a return value. The constructors can have parameters that will be used to initialize object’s states.

Here is a Person.java file that defines the Person class.

package org.kodejava.example.fundamental;

public class Person {
    private String name;
    private String title;
    private String address;

    /**
     * Constructor to create Person object
     */
    public Person() {

    }

    /**
     * Constructor with parameter
     *
     * @param name
     */
    public Person(String name) {
        this.name = name;
    }

    /**
     * Method to get the name of person
     *
     * @return name
     */
    public String getName() {
        return name;
    }

    /**
     * Method to set the name of person
     *
     * @param name
     */
    public void setName(String name) {
        this.name = name;
    }

    /**
     * Method to get the title of person
     *
     * @return title
     */
    public String getTitle() {
        return title;
    }

    /**
     * Method to set the title of person
     *
     * @param title
     */
    public void setTitle(String title) {
        this.title = title;
    }

    /**
     * Method to get address of person
     *
     * @return address
     */
    public String getAddress() {
        return address;
    }

    /**
     * Method to set the address of person
     *
     * @param address
     */
    public void setAddress(String address) {
        this.address = address;
    }

    /**
     * Method to get name with title of person
     *
     * @return nameTitle
     */
    public String getNameWithTitle() {
        String nameTitle;
        if (title != null) {
            nameTitle = name + ", " + title;
        } else {
            nameTitle = name;
        }
        return nameTitle;
    }

    /**
     * Method used to print the information of person
     */
    @Override
    public String toString() {
        return "Info [" +
                "name='" + name + ''' +
                ", title='" + title + ''' +
                ", address='" + address + ''' +
                ']';
    }
}

Here is a ClassExample.java file that defines the ClassExample class that use the Person class.

package org.kodejava.example.fundamental;

public class ClassExample {
    public static void main(String[] args) {
        Person person = new Person();
        person.setName("Andy");
        person.setTitle("MBA");
        person.setAddress("NY City");
        System.out.println(person);

        String nameTitle1 = person.getNameWithTitle();
        System.out.println("Name with title: " + nameTitle1);

        Person person2 = new Person("Sarah");
        String nameTitle2 = person2.getNameWithTitle();
        System.out.println("Name with title 2: " + nameTitle2);
    }
}

How do I create custom exception class?

By I Wayan Saryada in Basic, Core API 0 Comment

You can define your own exception class for your application specific purposes. The exception class is created by extending the java.lang.Exception class for checked exception or java.lang.RuntimeException for unchecked exception. By creating your own Exception classes, you could identify the problem more precisely.

package org.kodejava.basic;

public class CustomExceptionExample {
    public static void main(String[] args) {
        int x = 1, y = 0;

        try {
            int z = CustomExceptionExample.divide(x, y);
            System.out.println("z = " + z);
        } catch (DivideByZeroException e) {
            e.printStackTrace();
        }
    }

    public static int divide(int x, int y) throws DivideByZeroException {
        try {
            return (x / y);
        } catch (ArithmeticException e) {
            String m = x + " / " + y + ", trying to divide by zero";
            throw new DivideByZeroException(m, e);
        }
    }
}

package org.kodejava.basic;

class DivideByZeroException extends Exception {
    DivideByZeroException() {
        super();
    }

    DivideByZeroException(String message) {
        super(message);
    }

    DivideByZeroException(String message, Throwable cause) {
        super(message, cause);
    }
}