What is the purpose of mock objects?

Since you say you are new to unit testing and asked for mock objects in "layman's terms", I'll try a layman's example.

Unit Testing

Imagine unit testing for this system:

cook <- waiter <- customer

Its generally easy to envision testing a low-level component like the cook:

cook <- test driver

The test driver simply orders different dishes and verifies the cook returns the correct dish for each order.

Its harder to test a middle component, like the waiter, that utilizes the behavior of other components. A naive tester might test the waiter component the same way we tested the cook component:

cook <- waiter <- test driver

The test driver would order different dishes and ensure the waiter returns the correct dish. Unfortunately, that means that this test of the waiter component may be dependent on the correct behavior of the cook component. This dependency is even worse if the cook component has any test-unfriendly characteristics, like non-deterministic behavior (the menu includes chef's surprise as an dish), lots of dependencies (cook won't cook without his entire staff), or lot of resources (some dishes require expensive ingredients or take an hour to cook).

Since this a waiter test, ideally, we want to test just the waiter, not the cook. Specifically, we want to make sure the waiter conveys the customer's order to the cook correctly and delivers the cook's food to the customer correctly.

Unit testing means testing units independently, so a better approach would be to isolate the component under test (the waiter) using what Fowler calls test doubles (dummies, stubs, fakes, mocks).

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test cook <- waiter <- test driver

Here, the test cook is "in cahoots" with the test driver. Ideally, the system under test is designed so that the test cook can be easily substituted (injected) to work with the waiter without changing production code (e.g. without changing the waiter code).

Mock Objects

Now, the test cook (test double) could be implemented different ways:

• a fake cook - a someone pretending to be a cook by using frozen dinners and a microwave,
• a stub cook - a hot dog vendor that always gives you hot dogs no matter what you order, or
• a mock cook - an undercover cop following a script pretending to be a cook in a sting operation.

See Fowler's article for the more specifics about fakes vs stubs vs mocks vs dummies, but for now, let's focus on a mock cook.

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mock cook <- waiter <- test driver

A big part of unit testing the waiter component focuses on how the waiter interacts with the cook component . A mock-based approach focuses on fully specifying what the correct interaction is and detecting when it goes awry.

The mock object knows in advance what is supposed to happen during the test (e.g. which of its methods calls will be invoked, etc.) and the mock object knows how it is supposed to react (e.g. what return value to provide). The mock will indicate whether what really happens differs from what is supposed to happen. A custom mock object could be coded for the expected behavior of each test case, but a mocking framework strives to allow such a behavior specification to be clearly and easily indicated directly in the test case.

The conversation surrounding a mock-based test might look like this:

test driver to mock cook: expect a hot dog order and give him this dummy hot dog in response

test driver (posing as customer) to waiter: I would like a hot dog please
waiter to mock cook: 1 hot dog please
mock cook to waiter: order up: 1 hot dog ready (gives dummy hot dog to waiter)
waiter to test driver: here is your hot dog (gives dummy hot dog to test driver)

test driver: TEST SUCCEEDED!

But since our waiter is new, this is what could happen:

test driver to mock cook: expect a hot dog order and give him this dummy hot dog in response

test driver (posing as customer) to waiter: I would like a hot dog please
waiter to mock cook: 1 hamburger please
mock cook stops the test: I was told to expect a hot dog order!

test driver notes the problem: TEST FAILED! - the waiter changed the order

or

test driver to mock cook: expect a hot dog order and give him this dummy hot dog in response

test driver (posing as customer) to waiter: I would like a hot dog please
waiter to mock cook: 1 hot dog please
mock cook to waiter: order up: 1 hot dog ready (gives dummy hot dog to waiter)
waiter to test driver: here is your french fries (gives french fries from some other order to test driver)

test driver notes the unexpected french fries: TEST FAILED! the waiter gave back wrong dish

It may be hard to clearly see the difference between mock objects and stubs without a contrasting stub-based example to go with this, but this answer is way too long already :-)

Also note that this is a pretty simplistic example and that mocking frameworks allow for some pretty sophisticated specifications of expected behavior from components to support comprehensive tests. There's plenty of material on mock objects and mocking frameworks for more information.