Interfaces

Interfaces are reference types in Java. As such, they define the skeleton of classes and objects but without including the actual functionality of methods. Classes implement interfaces but do not extend them. Let's look at an example of a simple interface, further developing the idea of building classes to represent different types of computers.

interface Computer {

    public String getDeviceType();

    public String getSpeed();  

}

class Tablet implements Computer {

    public String getDeviceType() {

        return "it is a tablet";

    }

    public String getSpeed() {

        return "1GHz";

    }

}

class Example15 {

    public static void main(String[] args) {

        Tablet myTab = new Tablet();

        System.out.println( myTab.getDeviceType() );

        System.out.println( myTab.getSpeed() );

    }

}

As you might have guessed, the output for this example is:

it is a tablet

1GHz

Process finished with exit code 0

Some relevant notes on interfaces follow:

• Interfaces can extend other interfaces.
• Unlike classes, which can only extend one class at a time, interfaces can extend multiple interfaces at once. You do so by adding the different interfaces separated by commas.
• Interfaces have no constructors.