JUnit test case using Mock API

Mock API helps to create dummy object and returns back something to program when we are calling any method on that dummy object instead calling actual object.
Ex:

context.checking(new Expectations() {{
      oneOf (accountDAO).selectAccount(with("1234")); 
      will(returnValue(null));
  }});
 

Here we are asking Mock api return "null" value when I Call  
selectAccount method with 1234 value on AccountDAO object.

You can find very good example with maven integration in the below link.

http://onjavahell.blogspot.in/2009/05/good-unit-testing-with-jmock.html