What's the difference between faking, mocking, and stubbing?

Question

I know how I use these terms, but I'm wondering if there are accepted definitions for faking, mocking, and stubbing for unit tests? How do you define these for your tests? Describe situations where you might use each.

Here is how I use them:

Fake: a class that implements an interface but contains fixed data and no logic. Simply returns "good" or "bad" data depending on the implementation.

Mock: a class that implements an interface and allows the ability to dynamically set the values to return/exceptions to throw from particular methods and provides the ability to check if particular methods have been called/not called.

Stub: Like a mock class, except that it doesn't provide the ability to verify that methods have been called/not called.

Mocks and stubs can be hand generated or generated by a mocking framework. Fake classes are generated by hand. I use mocks primarily to verify interactions between my class and dependent classes. I use stubs once I have verified the interactions and am testing alternate paths through my code. I use fake classes primarily to abstract out data dependencies or when mocks/stubs are too tedious to set up each time.

Answer 1

You can get some information :

From Martin Fowler about Mock and Stub

Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production

Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it 'sent', or maybe only how many messages it 'sent'.

Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.

From xunitpattern:

Fake: We acquire or build a very lightweight implementation of the same functionality as provided by a component that the SUT depends on and instruct the SUT to use it instead of the real.

Stub : This implementation is configured to respond to calls from the SUT with the values (or exceptions) that will exercise the Untested Code (see Production Bugs on page X) within the SUT. A key indication for using a Test Stub is having Untested Code caused by the inability to control the indirect inputs of the SUT

Mock Object that implements the same interface as an object on which the SUT (System Under Test) depends. We can use a Mock Object as an observation point when we need to do Behavior Verification to avoid having an Untested Requirement (see Production Bugs on page X) caused by an inability to observe side-effects of invoking methods on the SUT.

Personally

I try to simplify by using : Mock and Stub. I use Mock when it's an object that returns a value that is set to the tested class. I use Stub to mimic an Interface or Abstract class to be tested. In fact, it doesn't really matter what you call it, they are all classes that aren't used in production, and are used as utility classes for testing.

Answer 2

Stub - an object that provides predefined answers to method calls.

Mock - an object on which you set expectations.

Fake - an object with limited capabilities (for the purposes of testing), e.g. a fake web service.

Test Double is the general term for stubs, mocks and fakes. But informally, you'll often hear people simply call them mocks.