What does an interface include in object-oriented programming?

In object-oriented programming, an interface is an abstract type that defines a set of methods (and sometimes properties) that a class must implement.

It does not contain implementations, only method signatures. This allows for creating flexible and extensible architectures where different classes can have a common contract but different implementations.

Example of an interface in Java:

public interface Drawable {
    void draw();
}

public class Circle implements Drawable {
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
}

Here, Drawable defines the draw() method, which must be implemented in any class that wants to be "drawable".