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copied!<p>In order to replace the actual B with your own using Mockito (or any other mocking framework) you'll <em>have</em> to be able to inject B either manually by creating a constructor</p> <pre><code>public A(B b) { localB = b; } </code></pre> <p>by creating a method</p> <pre><code>public void setB(B b) { localB = b; } </code></pre> <p>or by using some automatic injection framework.</p> <p>However, If B is completely encapsulated and hidden and that cannot be changed then you can write tests against B. to show that given known inputs B returns the expected outputs</p> <p>And then tests against A i.e. given known inputs does A.calculate() give the expected outputs.</p> <p>If you can show that B behaves as expected and A behaves as expected then you don't need to show that A calls B correctly explicitly since you're testing it implicitly.</p> <hr> <p>EDITED because @david wallace has less coffee or more sleep than me!</p> <p>You don't <em>have</em> to inject a <strong>mock</strong> B. If it helps you can inject the actual B but first <a href="http://mockito.googlecode.com/hg-history/1.5/javadoc/org/mockito/Mockito.html" rel="nofollow">create a spy</a> <code>a.getB();</code> </p> <pre><code>B spyB = spy(new B()); //maybe stub the method return here A a = new A(spyB); a.calculate(); verify(spyB).complicatedThingForCalculate("a", "lot", "of", "specified", "parameter"); </code></pre> <p>Further edit: As pointed out in the comments you're much better off using a Mock than a spy. From the documentation:</p> <blockquote> <p>Real spies should be used carefully and occasionally, for example when dealing with legacy code.</p> </blockquote> <p>So I included it here as you say you don't want / have the freedom to change A and B.</p>

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