Mockito

It will let programmers write tests with a clean & simple API.

Features:

• Mocks concrete classes as well as interfaces
• Little annotation syntax sugar - @Mock
• Verification errors are clean - click on stack trace to see failed verification in test; click on exception's cause to navigate to actual interaction in code. Stack trace is always clean.
• Allows flexible verification in order (e.g: verify in order what you want, not every single interaction)
• Supports exact-number-of-times and at-least-once verification
• Flexible verification or stubbing using argument matchers (anyObject(), anyString() or refEq() for reflection-based equality matching)
• Allows creating custom argument matchers or using existing hamcrest matchers