The term 'Mock Objects' has become a popular one to describe special case objects that mimic real objects for testing. Most language environments now have frameworks that make it easy to create mock objects. What's often not realized, however, is that mock objects are but one form of special case test object, one that enables a different style of testing.
jMock - An Expressive Mock Object Library for Java
Mock object In object-oriented programming, mock objects are simulated objects that mimic the behavior of real objects in controlled ways. A programmer typically creates a mock object to test the behavior of some other object, in much the same way that a car designer uses a crash test dummy to simulate the dynamic behavior of a human in vehicle impacts. Reasons for use In a unit test, mock objects can simulate the behavior of complex, real objects and are therefore useful when a real object is impractical or impossible to incorporate into a unit test. If an object has any of the following characteristics, it may be useful to use a mock object in its place:
Unit testing with mock objects Unit testing has become widely accepted as a "best practice" for software development. When you write an object, you must also provide an automated test class containing methods that put the object through its paces, calling its various public methods with various parameters and making sure that the values returned are appropriate. When you're dealing with simple data or service objects, writing unit tests is straightforward. However, many objects rely on other objects or layers of infrastructure. When it comes to testing these objects, it is often expensive, impractical, or inefficient to instantiate these collaborators. For example, to unit test an object that uses a database, it may be burdensome to install, configure, and seed a local copy of the database, run your tests, then tear the local database down again.
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