mockito anyList of a given size

One way is to use a Captor

ArgumentCaptor<List> captor = ArgumentCaptor.forClass(List.class);
verify(mock).createButtons(captor.capture());
assertEquals(x, captor.getValue().size()); // if expecting single list
assertEquals(x, captor.getValues().size()); // if expecting multiple lists

See http://docs.mockito.googlecode.com/hg/org/mockito/Mockito.html#15 for the documentation.

You could also use a custom argument matcher. The documentation shows an example that does exactly what you want:

http://docs.mockito.googlecode.com/hg/org/mockito/ArgumentMatcher.html

 class IsListOfTwoElements extends ArgumentMatcher<List> {
     public boolean matches(Object list) {
         return ((List) list).size() == 2;
     }
 }
 
 List mock = mock(List.class);
 when(mock.addAll(argThat(new IsListOfTwoElements()))).thenReturn(true);
 mock.addAll(Arrays.asList("one", "two"));
 verify(mock).addAll(argThat(new IsListOfTwoElements()));

You could, for instance, also add a constructor so you can specify list size desired, etc.

With Mockito 3.x, and 2.1+ you can use Java 8 lambda expressions:

verify(mock).createButtons(argThat(list -> list.size() == 5));

With Mockito 2.1 and below similar:

verify(mock).createButtons(argThat(list -> ((List) list).size() == 5));

To check emptiness it is even easier:

verify(mock).createButtons(argThat(List::isEmpty));

Hamcrest (hamcrest-library dependency) provides a simpler way.

verify(mock).addAll((List) argThat(IsCollectionWithSize.hasSize(4)));

or with static import org.hamcrest.collection.IsCollectionWithSize;

verify(mock).addAll((List) argThat(hasSize(4)));