constructors marked private

[hari harann]

Constructors can use any access modifier, including private. (A private
constructor means only code within the class itself can instantiate an object
of that type, so if the private constructor class wants to allow an instance
of the class to be used, the class must provide a static method or variable that allows access to an instance created from within the class.)

can someone please explain the stuff given inside ( ). i really dont understand what they meant...

[Campbell Ritchie]

A similar question came up a couple of days ago: here.

[hari harann]

Thanks campbell, i have understood that. but my issue is the following sentence,

"the class must provide a static method or variable that allows access to an instance created from within the class"

please explain about the static method which they have mentioned here..

[Campbell Ritchie]

That's a factory method. You call it something like getInstance.

public final class NoInstance
{
   private final int number;
 
   private NoInstance()
   {
      this(3);//Sets number to 3.
   }
 
   private NoInstance(int number)
   {
      this.number = number;
   }
 
   public int getNumber()
   {
      return number;
   }
 
   /**
    * @param number The value to set in this object
    * @return A NoInstance.
    */
   public static NoInstance getInstance(int number)
   {
      return new NoInstance(number);
   }
 
   /**
    * @return A NoInstance with the number set to 3.
    */
   public static NoInstance getInstanceValue3()
   {
      return new NoInstance();//defaults to 3 as in no-args constructor
   }
}

Note you can give different names to the factory methods; one here says it had the value 3 which comes from the no-args constructor.

Not a class anybody would use in real life, but it shows what you can do.

[G Estes]

Or a singleton pattern where the static class gives out the one and only instance...

[fred rosenberger]

if all the constructors are private, the only thing that can call those constructors are objects of that class, which won't exist until a constructor is called...

the work around is to create a static method that can be called without creating the object. that static method, since it is part of the class, can then call the constructor.