Two implementation examples of the Design Pattern Singleton. The Logs class is a Singleton, the Fib class is also a Singleton its client is the MainFib class.
The MyTestSupport utility class is used for tests, the TestLogs subclass is the test for the Logs class.
Note the static method getInstance(), the static field, and the private constructor needed to avoid creating additional instances from the outside.
The Pagamenti class implements a Multiton, its client is the ClientPagam class.