Defining the Test doubles

There is a lot of confusion about test doubles, many programmers are not even aware of all the options.  This is a shame because each option has value.

Stub is a implementation that returns canned values. This implies that some of the results can make no sense in the real world, but for the purposes of the test they are good enough.

Fake is a full implementation, but it doesn't do everything the real world implementation needs. It might use an in memory database, it might pretend to be a serial port with a device attached, but not actually use a real serial port. 

Mock is a way to ensure functions are called in specific ways. Call a function with the wrong argument and the test fails.

Spy is a way to check what functions were called after the fact.

I hope that the above are obvious, but still it is good to have everything defined so we can have a conversation about test doubles.

Of course the next step is defining when to use each.  That is a large topic that I hope to explore over time.