private constructor

[Prashanth Lingala]

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 somebody explain this, if feasible demonstrate with a simple example...

Source: Sun certified programmer and developer exam

Thanking in advance

Regards
Prashanth Lingala

[Mehul Sanghvi]

Hi,

This is what Factory Pattern is all about.

Where in for some reason you as the writer of the class do not want the user classes to instantiate objects of your class directly. Hence you ask the users of your class to call a static method defined within your class which in turn may do some validation and then may return a new object of your class or may return a reference to singleton.

Example code for plain factory pattern:

 
public class Factory{
  private Factory(){
  }
  public static Factory getFactory() throws Exception{
      //Validation
      if(validated){
        return new Factory();
      } else {
        throw new Exception();
      }
  }
}
 

Example code for singleton factory pattern:

 
public class Factory{
  private Factory singleton = new Factory();
  private Factory(){
  }
  public static Factory getFactory() throws Exception{
      //Validation
      if(validated){
        return singleton;
      } else {
        throw new Exception();
      }
  }
}
 

Factory pattern can be taken further to achieve the following:

 
public class Factory{
  private Factory(){
  }
  public static Factory getFactory(int factoryType) throws Exception{
      //Validation
      if(validated && ){
        if(factoryType==0){ 
          return new SubFactoryOne();
        } else {
          return new SubFactoryTwo();
        }
      } else {
        throw new Exception();
      }
  }
  public class SubFactoryOne extends Factory{
    private SubFactoryOne(){}
  }
  public class SubFactoryTwo extends Factory{
    private SubFactoryTwo(){}
  }
}
 

[Georgy Bolyuba]

puclic class Singleton{
  private static Singleton instance;
  private Singleton() {}
 
  public static Singleton getInstance()
  {
     if (instance == null)
     {
        instance = new Singelton();
     }
     return instance;
  }
}

[ February 21, 2005: Message edited by: George Bolyuba ]

[Prashanth Lingala]

I will go through it, this topic is new to me, let me burn it in...

Thanks for all who chipped in...

Regards
Prashanth Lingala