Implementing interfaces (implements)

A class implements an interface using the implements keyword. It must provide concrete implementations for all abstract methods of the interface (unless the class is abstract).

Syntax:

class ClassName implements Interface1, Interface2, ... {
    // implementations of all abstract methods
}

Example:

interface Drawable {
    void draw();
    void resize(int factor);
}

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

    @Override
    public void resize(int factor) {
        System.out.println("Resizing circle by factor " + factor);
    }
}

Usage:

Circle c = new Circle();
c.draw();
c.resize(2);

// Polymorphic assignment
Drawable d = new Circle();
d.draw();

Important: Implementing methods must be public (because interface methods are implicitly public). Access modifier cannot be reduced.

Multiple interfaces:

interface Flyable {
    void fly();
}
interface Swimmable {
    void swim();
}
class Duck implements Flyable, Swimmable {
    public void fly() { System.out.println("Duck flying"); }
    public void swim() { System.out.println("Duck swimming"); }
}