mockito callbacks and getting argument values

I've never used Mockito, but want to learn, so here goes. If someone less clueless than me answers, try their answer first!

Mockito.when(reader.document(anyInt())).thenAnswer(new Answer() {
 public Object answer(InvocationOnMock invocation) {
     Object[] args = invocation.getArguments();
     Object mock = invocation.getMock();
     return document(fakeIndex((int)(Integer)args[0]));
     }
 });

Check out ArgumentCaptors:

https://site.mockito.org/javadoc/current/org/mockito/ArgumentCaptor.html

ArgumentCaptor<Integer> argument = ArgumentCaptor.forClass(Integer.class);
Mockito.when(reader.document(argument.capture())).thenAnswer(
  new Answer() {
    Object answer(InvocationOnMock invocation) {
      return document(argument.getValue());
    }
  });

You might want to use verify() in combination with the ArgumentCaptor to assure execution in the test and the ArgumentCaptor to evaluate the arguments:

ArgumentCaptor<Document> argument = ArgumentCaptor.forClass(Document.class);
verify(reader).document(argument.capture());
assertEquals(*expected value here*, argument.getValue());

The argument's value is obviously accessible via the argument.getValue() for further manipulation / checking or whatever you wish to do.

With Java 8, this could be something like this:

Mockito.when(reader.document(anyInt())).thenAnswer(
  (InvocationOnMock invocation) -> document(invocation.getArguments()[0]));

I am assuming that document is a map.