As it is, it can't effectively be mocked. You need to move the local variable to a field.
ThreadMXBean mxbean = ManagementFactory.getThreadMXBean(); @Override public int compare(Long threadId1, Long threadId2) { return Long.compare(mxbean.getThreadCpuTime(threadId2), mxbean.getThreadCpuTime(threadId1)); } Now you can reassign the field to a mock in your test (I'll be using Mockito, but other frameworks exist as well):
@Test public void yourTestMethod(){ YourClass yourClass = new YourClass(); ThreadMXBean mock = Mockito.mock(ThreadMXBean.class) Mockito.when(mxbean.getThreadCpuTime(1L)).thenReturn(1); Mockito.when(mxbean.getThreadCpuTime(2L)).thenReturn(2); yourClass.mxbean = mock; assertThat(yourClass.compare(1L, 2L), is(-1)); } But it's even nicer to use the Mockito JUnit Runner and have Mockito create mocks automatically:
@RunWith(MockitoJUnitRunner.class) public class YourTest{ @InjectMocks YourClass yourClass = new YourClass(); @Mock ThreadMXBean mock; @Test public void yourTestMethodDoesntNeedToCreateMocksAnymore(){ Mockito.when(mxbean.getThreadCpuTime(1L)).thenReturn(1); Mockito.when(mxbean.getThreadCpuTime(2L)).thenReturn(2); assertThat(yourClass.compare(1L, 2L), is(-1)); } } Finally: test get more readable if you replace all Mockito.* calls with static imports, e.g.
MyFoo foo = mock(MyFoo.class); when(foo.bar()).thenReturn(baz);