Question

I would like to write the following code:

boolean found = false;
search(new SearchCallback() {
  @Override void onFound(Object o) { found = true; }
});

Obviously this is not allowed, since found needs to be final. I can’t make found a member field for thread-safety reasons. What is the best alternative? One workaround is to define

final class MutableReference<T> {
  private T value;
  MutableReference(T value) { this.value = value; }
  T get() { return value; }
  void set(T value) { this.value = value; }
}

but this ends up taking a lot of space when formatted properly, and I’d rather not reinvent the wheel if at all possible. I could use a List<Boolean> with a single element (either mutating that element, or else emptying the list) or even a Boolean[1]. But everything seems to smell funny, since none of the options are being used as they were intended.

What is a reasonable way to do this?

Answer 1

If you really cant use a field, Michael answers seems right.

Anyway. I dont know what signatures you can touch, but it seems to me that that callback is intented to do something (when the search succeeds) with/to the found object. You, instead, are intending to notify the caller of the search method that it found something. It would seems much more natural if your seach() method were made to return a boolean (the method will surely call s.onFound() somewhere if the search succeeds, then set an internal found flag there and return it).