Abstract Classes & Interfaces

Abstract classes and interfaces define contracts that other classes must follow. Abstract classes can have both abstract and concrete methods. Interfaces define only method signatures (prior to Java 8). They're essential for designing flexible, modular code.

45 min•By Priygop Team•Last updated: Feb 2026

Abstract Classes & Interfaces

Example

// Abstract class — cannot be instantiated directly
abstract class Animal {
    String name;
    
    Animal(String name) {
        this.name = name;
    }
    
    // Abstract method — subclasses MUST implement
    abstract String sound();
    
    // Concrete method — shared by all subclasses
    void display() {
        System.out.println(name + " says: " + sound());
    }
}

// interface — a contract of methods
interface Trainable {
    void train(String trick);
    boolean isTrainable();
}

interface Swimmable {
    void swim();
}

// Implementing abstract class + interfaces
class Dog extends Animal implements Trainable, Swimmable {
    Dog(String name) {
        super(name);
    }
    
    @Override
    String sound() { return "Woof!"; }
    
    @Override
    public void train(String trick) {
        System.out.println(name + " learned: " + trick);
    }
    
    @Override
    public boolean isTrainable() { return true; }
    
    @Override
    public void swim() {
        System.out.println(name + " is swimming!");
    }
}

class Cat extends Animal {
    Cat(String name) {
        super(name);
    }
    
    @Override
    String sound() { return "Meow!"; }
}

public class AbstractDemo {
    public static void main(String[] args) {
        Dog dog = new Dog("Rex");
        Cat cat = new Cat("Luna");
        
        dog.display();
        dog.train("shake");
        dog.train("roll over");
        dog.swim();
        System.out.println("Trainable: " + dog.isTrainable());
        
        System.out.println();
        cat.display();
    }
}

Try It Yourself: Media Player interface

Try It Yourself: Media Player interfaceJava

Java Editor

✓ ValidTab = 2 spaces

interface Playable {
    void play();
    void pause();
    void stop();
    String getTitle();
}

interface Downloadable {
    void download();
    double getFileSize();
}

class Song implements Playable, Downloadable {
    private String title;
    private String artist;
    private double sizeMB;
    private boolean isPlaying;
    
    Song(String title, String artist, double sizeMB) {
        this.title = title;
        this.artist = artist;
        this.sizeMB = sizeMB;
        this.isPlaying = false;
    }
    
    public void play() {
        isPlaying = true;
        System.out.println("Playing: " + title + " by " + artist);
    }
    
    public void pause() {
        isPlaying = false;
        System.out.println("Paused: " + title);
    }
    
    public void stop() {
        isPlaying = false;
        System.out.println("Stopped: " + title);
    }
    
    public String getTitle() { return title; }
    
    public void download() {
        System.out.println("Downloading: " + title + " (" + sizeMB + " MB)");
    }
    
    public double getFileSize() { return sizeMB; }
}

public class MediaPlayerDemo {
    public static void main(String[] args) {
        Song[] playlist = {
            new Song("Bohemian Rhapsody", "Queen", 6.5),
            new Song("Imagine", "John Lennon", 4.2),
            new Song("Hotel California", "Eagles", 7.1)
        };
        
        System.out.println("=== My Playlist ===");
        for (Song song : playlist) {
            song.play();
            song.download();
            System.out.println();
        }
    }
}

interface Playable {
    void play();
    void pause();
    void stop();
    String getTitle();
}

interface Downloadable {
    void download();
    double getFileSize();
}

class Song implements Playable, Downloadable {
    private String title;
    private String artist;
    private double sizeMB;
    private boolean isPlaying;
    
    Song(String title, String artist, double sizeMB) {
        this.title = title;
        this.artist = artist;
        this.sizeMB = sizeMB;
        this.isPlaying = false;
    }
    
    public void play() {
        isPlaying = true;
        System.out.println("Playing: " + title + " by " + artist);
    }
    
    public void pause() {
        isPlaying = false;
        System.out.println("Paused: " + title);
    }
    
    public void stop() {
        isPlaying = false;
        System.out.println("Stopped: " + title);
    }
    
    public String getTitle() { return title; }
    
    public void download() {
        System.out.println("Downloading: " + title + " (" + sizeMB + " MB)");
    }
    
    public double getFileSize() { return sizeMB; }
}

public class MediaPlayerDemo {
    public static void main(String[] args) {
        Song[] playlist = {
            new Song("Bohemian Rhapsody", "Queen", 6.5),
            new Song("Imagine", "John Lennon", 4.2),
            new Song("Hotel California", "Eagles", 7.1)
        };
        
        System.out.println("=== My Playlist ===");
        for (Song song : playlist) {
            song.play();
            song.download();
            System.out.println();
        }
    }
}

Output

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Java|65 lines|1583 chars|✓ Valid syntax

UTF-8

Quick Quiz: Abstract & Interfaces

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