Default and static interface methods

Definition

Since Java 8 an interface can contain methods with a body. A default method gives implementing classes a ready-made behaviour they may override, and a static method belongs to the interface itself. This let Java add new methods to old interfaces without breaking existing classes.

Example code

An interface with a default method and a static helper

interface Greeter {
    String name();

    default String greet() {                      // implementing classes get this for free
        return "Hello, " + name() + "!";
    }

    static Greeter of(String name) {              // a helper that lives on the interface
        return () -> name;
    }
}

class Polite implements Greeter {
    public String name() { return "Asha"; }

    @Override
    public String greet() { return "Good morning, " + name(); }   // can still override it
}

class Demo {
    public static void main(String[] args) {
        System.out.println(Greeter.of("Ravi").greet());
        System.out.println(new Polite().greet());
    }
}

// Output:
// Hello, Ravi!
// Good morning, Asha