Mockito 1.8.1 introduced anyVararg() matcher:
when(a.b(anyInt(), anyInt(), Matchers.<String>anyVararg())).thenReturn(b);
Also see history for this: https://code.google.com/archive/p/mockito/issues/62
Edit new syntax after deprecation:
when(a.b(anyInt(), anyInt(), ArgumentMatchers.<String>any())).thenReturn(b);