Davide Lorenzo Marino

The singleton pattern is a design pattern that restricts the instantiation of a class to one object.

In software engineering, a plain old Java object (POJO) is an ordinary Java object, not bound by any special restriction.

Both definitions are taken from wikipedia.

So why you said that this is not a singleton, but a pojo? I don't understand.

There are context in wich you need to know how to build a singleton:

- when the customer like to have to total control of the code

- because introducing a framework is not compatible with performance aspects

- because somebody will ask you how to create a singleton in a job interview

- you are creating a framework and you need to expose singletons

- for personal curiosity

Thanks for the reference to the JLS specifications

In most situations the eager implementation is the best one. Also the enum solution proposed by Ondrej is a good one. But the eager one will be loaded always also when is not used only because you import the class. The process of class loading will create the singleton instance also if never used. All the lazy implementations will be loaded only on demand.

Yes enum is another possibility from java 1.5 because the instantiation is thread safe.