@Test
public void onHalfOpen_calls_delegate_and_returns_self() {

    // given
    Runnable listener = mock(Runnable.class);
    // when
    CircuitBreaker<Integer> result = wrapper.onHalfOpen(listener);
    // then
    verify(delegateMock).onHalfOpen(listener);
    assertThat(result).isSameAs(wrapper);
}