Can we use lambda expression inside an interface as a default or static implementation inside interfaces?

Question

I have this strange doubt now, As far as I know, in order to use lambda expressions it should be a functional interface having single abstract method. Now the question is can we provide its implementation in another interface as a static or default implementations? Thank you.

Answer 1

You can define a lambda expression that would represent an A instance regardless of the context you are in. It can be a static interface method, it could be a default interface method.

@FunctionalInterface
interface A {
    void printMessage();
}
interface B {
    default void printMessage() {
        A a = () -> System.out.println("A implementation (1)");
        a.printMessage();
    }
}
interface C {
    static void printMessage() {
        A a = () -> System.out.println("A implementation (2)");
        a.printMessage();
    }
}

As you may have noticed, it's not particularly useful because both methods could be simply rewritten to

System.out.println("...");

If there is a relationship between interfaces, a default method would provide an implementation for the method defined in the super interface. A static method would cause a compilation error since they can't override instance methods.

@FunctionalInterface
interface A {
    void printMessage();
}
interface B extends A {
    @Override
    default void printMessage() {
        A a = () -> System.out.println("A implementation (1)");
        a.printMessage();
    }
}
interface C extends A {
    // compilation error
    static void printMessage() {
        A a = () -> System.out.println("A implementation (2)");
        a.printMessage();
    }
}