MutableInt { public void inc() { value++; } MutableInt();  MutableInt(int value); void set(int value); int get(); void inc(); @Override int hashCode(); @Override boolean equals(Object obj);  }
@Test public void inc() { assertThat(integer.get()).isEqualTo(0); integer.inc(); assertThat(integer.get()).isEqualTo(1); integer.inc(); assertThat(integer.get()).isEqualTo(2); }