A java interface is a blueprint for a class. Defines the methods that the class must implement. Interfaces cannot be instantiated, but can be implemented by classes.

Implementing an interface in Java, requires using the implements keyword. For example, the following code shows how the Runnable interface implemented:

Java

public class MyThread implements Runnable
{
    public void run()
    {
        // Do something.
    }
}

The Runnable interface defines a single method called run(). The MyThread class implements the run() method by providing its own implementation.

Here's another example of how to implementing an interface in Java:

Java

public interface Shape
{
    void draw();
}

public class Circle implements Shape
{
    public void draw()
    {
        // Draw a circle.
    }
}

The Shape interface defines a single method called draw(). The Circle class implements the draw() method by providing its own implementation.

If a class implements an interface, it must provide implementations for all the methods defined in that interface. A class is said to be an abstract class if it foes not provide implementations for all the methods defined in the interface, then the class is said to be abstract.