Fake objects
If we think of the interdependencies between objects in an application as a graph it becomes quickly apparent that there are a number of nodes that have dependencies on, not just one, but many other objects. Attempting to place an object with a lot of dependencies under test is challenging. Each of the dependent objects must be constructed and included in the test. When these dependencies interact with external resources such as the network or file system, the problem becomes intractable. Pretty soon we're testing the entire system at a time. This is a legitimate testing strategy, known as integration testing, but we're really just interested in ensuring that the functionality of a single class is correct.
Integration testing tends ...
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