Originally posted by Pandu Rangam: if the constructor is private, then is it a singleton. I mean Math,Runtime, System etc are following Singleton design pattern?
No. Singleton implies private constructor, but private constructor does not imply Singleton.
A private constructor may be used to help to forbid any instantiation of a utility class. It may also be used for instantiable classes whose public construction interface is a factory method, rather than a constructor. Type-safe enums, pre Java 5, are another use. There are probably more.
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