What is the difference between Singleton Class and Static Class? Which one to use and when?

Almost both singleton class and static class serve the same purpose which creates confusion among developers and architects about which one to use. When to use singleton class and when to use static class. In this article I have tried to differentiate between the two. Following is the list of differences between a singleton class and a static class:

1. Singleton classes are more inclined to the object-oriented concepts as compared to the static classes. With Singleton classes, you can use inheritance and polymorphism to extend a base class, implement an interface and capable of providing different implementations. While a static class cannot inherit their instance members. So, Singleton classes are more flexible than static classes.

2. Singleton classes can be passed as an object to other methods while a static class allows only static methods and you cannot pass static class as parameter.

3. If your requirement is to maintain the state, then singleton class is the better choice than static class, because maintaining the state with static classes is very cumbersome and leads to subtle bugs.

4. Singleton classes can be lazy loaded if it’s a heavy object, but static class doesn't have such advantages and always eagerly loaded. Static class is generally initialized when it is first loaded and it will lead to potential class loader issues.

5. Static classes are hard to mock and consequently hard to test than singletons, which are easy to mock and thus easy to test. It’s easier to write unit tests for singleton than static classes, because you can pass mock object whenever singleton is expected, e.g. into constructor or as method arguments.

6. Singleton classes provide more flexibility. Consider the possibility that your application has a global shared object and tomorrow you decide that you must make more than one instance of the class. If you use a singleton class then all you should do is make the constructor public. This is not so simple with static classes.

7. Many Dependency Injection framework manages singleton quite well e.g. Spring, which makes using them very easy.

8. Singleton objects are stored in Heap, but static objects are stored in Stack.

9. Static class provides better performance than singleton class, because static methods are bonded on compile time.

10. Singleton classes maintain single object across the application life cycle, so the scope is at application level. The static does not have any object pointer, so the scope is at App Domain level. Moreover, both should be implemented to be thread-safe.