With Mockito
framework, you can use AdditionalMatchers
ClassA classA = Mockito.mock(ClassA.class);
Mockito.when(classA.getString(Matchers.eq("a"))).thenReturn("something");
Mockito.when(classA.getString(AdditionalMatchers.not(Matchers.eq("a")))).thenReturn("something else");
Hope it helps.