Can @FunctionalInterfaces have default methods?

Question

Why can\'t I create a @FunctionalInterface with a default method implementation?

@FunctionalInterface
public interface MyInterface {
    default         


        

Answer 1

That's because @FunctionalInterface can have default methods, as many as you want. For example, consider the java.util.Function interface. It contains two default methods: compose and andThen. But there should be exactly one non-default method. Otherwise how compiler would know which of your default methods should be mapped to lambda?