We all know about stunt doubles in movies. A stunt double or dummy is a trained replacement used for dangerous action sequences in movies, such as a fight sequence on the top of a burning train, jumping from an airplane, and so on, mainly fight scenes. Stunt doubles are used to protect the real actors, are used when the actor is not available, or when the actor has a contract to not get involved in stunts.
Similarly, sometimes it is not possible to unit test the code because of the unavailability of the collaborator objects, or the cost of interaction and instantiation of collaborators. For instance, when the code is dependent on database access, it is not possible to unit test the code unless the database is available, or when a piece of code needs to send information to a printer and the machine is not connected to a LAN. The primary reason for using doubles is to isolate the unit you are testing from the external dependencies.
Gerard Meszaros coined the term test doubles in his book xUNIT TEST PATTERNS, Addison-Wesley—this book explores the various test doubles and sets the foundation for Mockito.
Test doubles can be created to impersonate collaborators and can be categorized into the types, as shown in the following diagram: