As mock objects verify whether they are used correctly, they allow us to obtain a finer understanding of how the object under test interacts with the collaborating objects.
Dependency Injection suggests that a class explicitly declares the interfaces of its collaborators (for example, in the constructor or as parameters in a method) but leaves the responsibility for the creation of their concrete implementation to the container. Because the class is not in control of the creation of its collaborators anymore, this principle is also known as Inversion of Control.
Service Locator means that a class internally locates its concrete collaborators through the dependency on another object (the locator). A simple example of a locator could be a factory object that uses a configuration file to load the required collaborators dynamically.
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