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copied!<p>A Singleton, by definition, has exactly one instance. Hence its creation is strictly controlled by the class itself. Typically it is a concrete class, not an interface, and due to its private constructor it is not subclassable. Moreover, it is found actively by its clients (by calling <code>Singleton.getInstance()</code> or an equivalent), so you can't easily use e.g. <a href="http://en.wikipedia.org/wiki/Dependency_injection" rel="noreferrer">Dependency Injection</a> to replace its "real" instance with a mock instance:</p> <pre><code>class Singleton { private static final myInstance = new Singleton(); public static Singleton getInstance () { return myInstance; } private Singleton() { ... } // public methods } class Client { public doSomething() { Singleton singleton = Singleton.getInstance(); // use the singleton } } </code></pre> <p>For mocks, you would ideally need an interface which can be freely subclassed, and whose concrete implementation is provided to its client(s) by dependency injection.</p> <p>You can relax the Singleton implementation to make it testable by </p> <ul> <li>providing an interface which can be implemented by a mock subclass as well as the "real" one</li> <li>adding a <code>setInstance</code> method to allow replacing the instance in unit tests</li> </ul> <p>Example:</p> <pre><code>interface Singleton { private static final myInstance; public static Singleton getInstance() { return myInstance; } public static void setInstance(Singleton newInstance) { myInstance = newInstance; } // public method declarations } // Used in production class RealSingleton implements Singleton { // public methods } // Used in unit tests class FakeSingleton implements Singleton { // public methods } class ClientTest { private Singleton testSingleton = new FakeSingleton(); @Test public void test() { Singleton.setSingleton(testSingleton); client.doSomething(); // ... } } </code></pre> <p>As you see, you can only make your Singleton-using code unit testable by compromising the "cleanness" of the Singleton. In the end, it is best not to use it at all if you can avoid it.</p> <p><strong>Update:</strong> And here is the obligatory reference to <a href="http://rads.stackoverflow.com/amzn/click/0131177052" rel="noreferrer">Working Effectively With Legacy Code</a> by Michael Feathers.</p>

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