I have a method, say m1 in class, say C1. Method m1 calls a method, say m2 from another service class, say C2. So I mock m2 in junit test-cases for m1. However, m2 itself calls a third method, say m3 from a third class, say C3. When I run the test-cases for m1, they fail with the exception:

junit.framework.AssertionFailedError: null

On debugging my code, I found that reference to C3 in the mock for C2 was null. I tried mocking both C2 and C3. Yet the same problem occurred. Please help me understand how to correct this situation and run my test-cases successfully.