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

private methods are not inherited (Closest I found so far is: JLS6.6-5: "[A private class member] is not inherited by subclasses"). That means that you can not access a private method, from a subtype (because it simply does not 'have' that method). For instance:

public static void main(String[] args) {
    I1 i1 = null;
    I2 i2 = null;

    i1.test(); // works
    i2.test(); // method test is undefined
}

interface I1 {
    private void test() {}
}

interface I2 extends I1 {}

That also means that you can not directly access the test method through the type of an anonymous subclass. The type of the expression:

(new I() {})

Is not I, but actually the non-denotable type of the anonymous subclass, so you can't access test through it.

However, the type of the expression:

((I) (new I() {}))

is I (as you explicitly cast it to I), so you can access the test method through it. (just like you can do ((I1) i2).test(); in my above example)

Similar rules apply to static methods, as they are also not inherited.