Java Interfaces: Pure Abstraction, Multiple Inheritance & Modern Methods

An Interface in Java is a formal reference type that defines a strict contract of operational capabilities. It specifies what an implementing class must do, but provides no instance state fields.

Interface Properties in Modern Java (Java 8, 9 & 21)

• Fields: All variables are implicitly public static final (constants).
• Abstract Methods: All standard method declarations are implicitly public abstract.
• Default Methods (Java 8+): Methods with the default keyword providing default fallback implementations.
• Static Methods (Java 8+): Utility methods bound to the interface itself (called via InterfaceName.method()).
• Private Methods (Java 9+): Helper methods accessible only within default methods inside the interface.

Demonstrating Multiple Interface Inheritance

interface AudioPlayer {
    void playAudio(String filename);
}

interface VideoPlayer {
    void playVideo(String filename);
}

// MediaCenter implements MULTIPLE interfaces
public class ModernMediaCenter implements AudioPlayer, VideoPlayer {
    @Override
    public void playAudio(String filename) {
        System.out.println("Playing MP3 Audio: " + filename);
    }

    @Override
    public void playVideo(String filename) {
        System.out.println("Playing 4K Video: " + filename);
    }

    public static void main(String[] args) {
        ModernMediaCenter mc = new ModernMediaCenter();
        mc.playAudio("song.mp3");
        mc.playVideo("movie.mp4");
    }
}

Abstract Class vs. Interface (Direct Comparison)