When a class declares only private instance constructors, it is not possible for classes outside the program to derive from the class or to directly create instances of it. (Except Nested classes)
Make a constructor private if:
1) You want it to be available only to the class itself. For example, you might have a special constructor used only in the implementation of your class' Clone method.
2) You do not want instances of your component to be created. For example, you may have a class containing nothing but Shared utility functions, and no instance data. Creating instances of the class would waste memory.