Method reference to private interface method

Question

Consider the following code:

public class A {
    public static void main(String[] args) {
        Runnable test1 = ((I)(new I() {}))::test;  // compiles OK
             


        

Answer 1

This is counter-intuitive. First let's simplify this a bit:

static interface Inter {
    private void test() {
        System.out.println("test");
    }
}


public static void main(String[] args) {
    ((Inter) new Inter() {
    }).hashCode();
}

This makes sense as you are calling the public hashCode method, here is the (important part only) byte code for it:

public static void main(java.lang.String[]);
Code:
   0: new           #2   // class com/test/DeleteMe$1
   3: dup
   4: invokespecial #3   // Method com/test/DeleteMe$1."":()V
   7: invokevirtual #4   // Method java/lang/Object.hashCode:()I
  10: pop
  11: return

Looks very sane to me. Now let's change that to calling test():

public static void main(String[] args) {
    ((Inter) new Inter() {
    }).test();
}

The byte code for this:

 invokestatic  #4  // InterfaceMethod com/test/DeleteMe$Inter.access$000:(Lcom/test/DeleteMe$Inter;)V

Since private methods are not inherited, you are actually "going" to that method via the access$n static synthetic method.