When to use a constructor in java?

Private constructors generally have one of two uses.

The first is for implementing the singleton design pattern, in which you only ever want one instance of the class in existence. So you don't want new instances of the class being generated over and over. You do something like:

class Singleton {

// All classes can access this instance of the class and no other.

public static instance = new Singleton();

private Singleton() {

}

}

The second is for limiting the creation of classes which contain only static methods or constants.

Of course, private constructors are useful any time you want to limit the way other classes/packages/programmers interact with your class.