Java Extreme Programming Cookbook

Chapter 6. Mock Objects

Introduction

Writing tests for simple, standalone Java classes is easy. Just create an instance of your class and run tests against its methods. But testing gets a whole lot more interesting for classes with complex dependencies on other parts of your application. When testing becomes difficult, consider refactoring your code in order to minimize dependencies. But there will always be cases where classes cannot be tested in isolation.

Suppose that a class named Automobile only runs in the context of a class named Engine. In order to test all aspects of the Automobile class, you find that you have to create many different kinds of Engines. Rather than create real Engine objects, which may require a lot of setup logic, you can write a dummy Engine implementation. This dummy implementation is known as a mock object and it provides a simple way to set up fake testing data.

The mock Engine can even include assertions to ensure that instances of Automobile use the Engine correctly. For example, the Engine may verify that Automobile only calls the Engine.startup( ) method one time. This ability to verify that objects use their environment correctly is a key advantage of mock objects when compared to other testing techniques that only check to see if classes react to method calls correctly.

A mock object is a “fake” implementation of a class or interface, almost always written for the specific purpose of supporting unit tests. When writing JDBC unit tests, you might ...