Java Interfaces Complete Guide

Interfaces

100% abstract contracts defining method signatures.

Interface Implementation

interface Drawable {
    void draw();
    
    default void resize() { // Java 8+
        System.out.println("Resizing");
    }
}

class Rectangle implements Drawable {
    public void draw() {
        System.out.println("Drawing rectangle");
    }
}

Key Points

• implements keyword for classes.
• Multiple inheritance: class C extends A implements B, C.
• Default/static methods since Java 8.
• All methods public abstract by default.