Test Doubles explained

Dummy objects are passed around but never actually used.

Fakes - Working implementation, but much more simple than the original

• An in-memory list of username/password pairs that provide the authentication.
• A database interface where data stored in memory only, maybe in a hash or an in-memory SQLite DB.

Mocks - Mocks are objects that register calls they receive, but do not execute the real system behind.

Stubs - Stub is an object that holds predefined data and uses it to answer calls during tests.

• A list of "random values".
• Responses given to prompt.

Spies usually record some information based on how they were called and then call the real method. (or not)