Default Methods

Java 8 also introduced default methods: non-static, concrete methods defined in interfaces.

• A default method is declared with the keyword default and must have a method body.
• A default method is inherently public (like all interface methods).

public interface Fillable {
  double LITERS_PER_GALLON = 3.78541; // public static final field
  void fill(int amount);

  static double litersFromGallons(double gallons) {
    return gallons * LITERS_PER_GALLON;
  }

  default int getCapacityLiters() {
    return 0;
  }
}

Default methods allow the author of an interface to add new methods without breaking the contract of existing implementor classes and sub-interfaces.

Drill

Which of these are valid declarations in an interface?

1. [ ] default void doStuff1();
2. [ ] public void doStuff2();
3. [ ] static void doStuff3();
4. [ ] default static void doStuff4() { System.out.println("Doing stuff."); }
5. [ ] static void doStuff5() { System.out.println("Doing stuff"); }

(Solution: InterfaceMethodDrill.java)

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