Default and Static Methods

Methods with a Body (Java 8+)

Kind	Keyword	Inherited by implementers?	Called as
Abstract	(none)	must be implemented	`obj.m()`
Default	`default`	yes, can be overridden	`obj.m()`
Static	`static`	no	`InterfaceName.m()`
Private (Java 9+)	`private`	no, helper for default methods	inside the interface only

Default methods let library authors add new methods to an interface without breaking every class that already implements it. That is how List.sort() and Collection.stream() were added in Java 8.

Example

public class Main {
    public static void main(String[] args) {
        Discount festive = new FestiveDiscount();
        Discount none = new NoDiscount();
        System.out.println(festive.apply(1000));
        System.out.println(none.apply(1000));
        System.out.println(Discount.isValidPrice(-5));
    }
}

interface Discount {
    double percent();

    default String apply(double price) {          // shared behaviour
        double finalPrice = price - price * percent() / 100;
        return label() + " -> " + finalPrice;
    }

    private String label() {                      // helper, hidden from implementers
        return percent() + "% off";
    }

    static boolean isValidPrice(double price) {   // utility on the interface itself
        return price > 0;
    }
}

class FestiveDiscount implements Discount {
    public double percent() { return 20; }
}

class NoDiscount implements Discount {
    public double percent() { return 0; }
    @Override
    public String apply(double price) {           // overriding a default method
        return "No discount -> " + price;
    }
}

Output

20.0% off -> 800.0
No discount -> 1000.0
false