Now I’m converted – it’s a very neat and fast way of stubbing out the behaviours that I wish to code.

This morning, however, I came across a slightly trickier case to mock – at least, a case that required trial and error, as it hasn’t been documented that fully.

I have a slave interface:

And the behaviour I want is that when a slave is ordered to “doSthg!”, it should answer() and doSthg() pretty sharpish. So the test looks like this:

To verify().that(slave.answer()) means to verify that slave.answer was called once.

First off, I needed to define the doSthg() and answer() methods, having created a mock slave from the interface (both return a String object):

And then I came to the tricky bit – since I’m creating this object out of an Interface definition how can I define the behaviour of slave.order()?

Well, thankfully the GenericAnswer object literally holds the answer to that question – it is instantiated with a function thus and given to the mock slave.order (if the call parameter is correct!):

So now I have created an ISlave interface with three public methods, and defined how the methods interact, and precisely what parameter in order() will trigger the correct interaction. In full it looks like this:

P.S. Obviously, the slave should not have a order() method – that’s the responsibility of the master (e.g. master.order(slave, “doSthg!”)), but it serves to make this example work, so forgive me!