Stubs can’t fail tests. Mocks can do (or they’d be called Stubs). Stubs simply replace an object so that we can test the object under test without problems. The easiest way to tell we are dealing with a Stub is to notice that the Stub can never ever fail the test. The asserts the test uses are always against the class under test. When using Mocks, on the other hand, The test will use the Mock object to verify if the test failed or not. Again, when using a Mock, the Mock object is the object that we use to see if the test failed or not.

Mock is normally used in the internal working of a void method, which has no value return.