Why do I need a functional Interface to work with lambdas?

Question

I think this question is already somewhere out there, but I wasn\'t able to find it.

I don\'t understand, why it\'s necessary to have a functional interface to work

Answer 1

You seem to be looking for anonymous classes. The following code works:

public class Test {

    public static void main(String...args) {
        TestInterface i = new TestInterface() {
            public void hans() {
                System.out.println("Hans");
            }
            public void hans(String a) {
                System.out.println(a);
            }
        };

        i.hans();
        i.hans("Hello");
    }
}

public interface TestInterface {
    public void hans();
    public void hans(String a);
}

Lambda expressions are (mostly) a shorter way to write anonymous classes with only one method. (Likewise, anonymous classes are shorthand for inner classes that you only use in one place)