Question

When is a singleton class preferred over a class that has only static methods and a private default constructor?

Please vote.

Answer 1

Use a singleton to better control when initialization occurs. With a static class, any initialization must be at class load-time, which you have little control over. For example, a simple reference to a static final MEMBER will trigger class loading. With a singleton, initialization can trivially be deferred till much later – typically, till first time of use.

Reasons to delay initialization may be:

• it’s expensive and you don’t always need it for that class
• you can’t initialize till some other resource is initialized (say, a database connection). In this case, a lazily-instantiated singleton often provides correct order of operations without any explicit control – if it’s not referenced till after the other resource is initialized, everything happens for free.

Use a singleton to improve testability. If you need to make some kind of mock object (in the broad sense) of the singleton in order to test its clients, one way to do it is to put an interface on its use, and supply a test singleton that’s of a different class but implements the same interface.

Using a singleton makes initialization testing easier as well.

Use a singleton when you might need to debug initialization. Stack traces from static initialization can be puzzling. Debugging can be puzzling too. If the class is loaded early, it may break before a breakpoint on the first line in main() is even hit.