Interface vs abstract class

Definition

An interface describes what a type can do and holds no state. An abstract class can also hold shared fields and partly finished code. A class can implement many interfaces but extend only one class.

Example code

An abstract base class plus an interface

interface Flyable {
    void fly();
    default String speed() { return "slow"; }       // an interface can offer a default method
}

abstract class Animal {
    protected final String name;                    // state can live in an abstract class
    Animal(String name) { this.name = name; }
    abstract String sound();                        // each animal must define this
    void introduce() { System.out.println(name + " says " + sound()); }
}

class Bird extends Animal implements Flyable {      // one class, many interfaces
    Bird(String name) { super(name); }
    String sound() { return "tweet"; }
    public void fly() { System.out.println(name + " is flying, speed " + speed()); }
}

class Demo {
    public static void main(String[] args) {
        Bird bird = new Bird("Robin");
        bird.introduce();
        bird.fly();
    }
}

// Output:
// Robin says tweet
// Robin is flying, speed slow