Java Generics Interface casting

Question

I stumbled across a Java casting situation involving Generics and Interfaces that I do not understand.

Please consider the following code where I create a List<Interface1>. And then get() an element and cast it to Interface2 without compiler error although these two interfaces are completely unrelated.

import java.util.*;

public class Main {
  public static void main(String ... args) {
    List<Interface1> list = new ArrayList<>();
    list.add(new Interface1() {});

    Interface1 ok = list.get(0);
    Interface2 why = (Interface2)list.get(0);
  }
}

interface Interface1 {
}

interface Interface2 {
}

Can anyone explain why there is not compiler error for the cast at the second get(0)?

Two side notes: Executing the class throws a ClassCastException (as expected). And using two classes instead of interfaces does actually generate compile errors.

Answer 1

This behaviour is unrelated to generics: you can cast any interface to any other without compile errors.

You cannot do this with classes because Java can check at compile time whether one class can be casted to another.

With interfaces however the cast might succeed or fail, depending on the class that is actually implementing the interface. This can be discovered only at runtime though.