Casting a lambda to this @FunctionalInterface can cause a behavior change from casting to a functional superinterface, which is surprising to users. Prefer decorator methods to this surprising behavior.


The problem

There are only two correct ways to have one @FunctionalInterface extend another @FunctionalInterface—the way where you leave the abstract method alone, and the way where you make a different abstract method but a default method for the original abstract method simply delegates to the new name.

If you do anything else, you create a situation where what looks like a “cast” actually changes behavior. That is really quite bad for understandability.

For example, if the method bar() changes the behaviour of qux(), then the same lambda cast to A or B could have completely different behaviour.

interface A {
  Foo bar();

interface B extends A {
  Foo qux();
  default void bar() {
    // anything here but exactly `return qux();` or perhaps `return (SomeType) qux();`

If you need to change the behaviour of an existing lambda, prefer an explicit decorator method, e.g.:

static Runnable crashTerminating(Runnable r) {
  return () -> { ...wrapping behavior goes here... }


Suppress false positives by adding an @SuppressWarnings("FunctionalInterfaceMethodChanged") annotation to the enclosing element.