Question

I’ve needed to get a value back from an anonymous inner class. Inner classes can only close over final variables, of course, which leads to this horrible workaround:

public String sampleMethod(){
  final String[] output = new String[1];
  findResult(new SampleOperation(){
    @Override
    private void perform(){
      output[0] = "result";
    }
  });
  return output[0];
}

private void findResult(SampleOperation op){
  op.perform();
}

private static interface SampleOperation {
  void perform();
}

Obviously a simplified example; here the class is easily removable, but the principal of the problem is there. If there is a dependency further down (inside findResult(), such as a latch that needs triggered) then unwrapping such a class becomes impractical.

Wrapping the final array means it’s accessible, but this has one of the worst smells I’ve ever come across.

Is there a sane way to get a return type from such a delegate? (i.e. Not use this?)

Answer 1

The problem here is that SampleOperation.perform returns void. Just make it return String (or be generic) and it’s fine:

public String sampleMethod(){
  return findResult(new SampleOperation(){
    @Override
    private String perform() {
      return "result";
    }
  });
}

private String findResult(SampleOperation op){
  return op.perform();
}

private static interface SampleOperation {
  String perform();
}

Ultimately, whenever you’re thinking about “I need to get a value back” you should be thinking about a value being returned from a method.