java - Why use a singleton instead of static methods?

Question

I have never found good answers to these simple questions about helper/utility classes:

• Why would I create a singleton (stateless) instead of using static methods?
• Why would an object instance be needed if an object has no state?

Answer 1

Often, singletons are used to introduce some kind of global state to an application. (More often than really necessary, to be honest, but that's a topic for another time.)

However, there are a few corner cases where even a stateless singleton can be useful:

• You expect to extend it with state in the foreseeable future.
• You need an object instance for some particular technical reason.
  Example: Synchonization objects for the C# lock or the Java synchronized statement.
• You need inheritance, i.e., you want to be able to easily replace your singleton with another one using the same interface but a different implementation.
  Example: The Toolkit.getDefaultToolkit() method in Java will return a singleton whose exact type is system dependent.
• You want reference equality for a sentinel value.
  Example: DBNull.Value in C#.