java: how to mock Calendar.getInstance()?

Question

In my code I have something like this:

private void doSomething() {    Calendar today = Calendar.getInstance();    .... } 

How can I "mock" it in my junit test to return a specific date?

Answer 1

You can mock it using PowerMock in combination with Mockito:

On top of your class:

@RunWith(PowerMockRunner.class) @PrepareForTest({ClassThatCallsTheCalendar.class}) 

The key to success is that you have to put the class where you use Calendar in PrepareForTest instead of Calendar itself because it is a system class. (I personally had to search a lot before I found this)

Then the mocking itself:

mockStatic(Calendar.class); when(Calendar.getInstance()).thenReturn(calendar); 

Answer 2

As far as I see it you have three sensible options:

1. Inject the Calendar instance in whatever method/class you set that day in.

   private void method(final Calendar cal) { Date today = cal.getTime(); }

2. Use JodaTime instead of Calendar. This is less an option and more a case of a suggestion as JodaTime will make your life a lot easier. You will still need to inject this time in to the method.

   DateTime dt = new DateTime();

   Date jdkDate = dt.toDate();

3. Wrap Calendar inside some interface that allows you to fetch the time. You then just mock that interface and get it to return a constant Date.

   Date today = calendarInterfaceInstance.getCurrentDate()