Protected constructor in singleton pattern

[Suraj Behera]

While implementing singleton pattern we have two options.
1. Private constructor. - Can not be instatiated.
2. Protected constructor.- can be instatiated only by subclass.


Give me practical example for the second case i.e what is the use of Protected constructor in sigleton pattern.

[Avishkar Nikale]

Constructors are not inherited so its just the visibility which matters.

It means your sublcass & other classes in your package can create objects of your Singleton class.

Ranchers,

Please correct me if I am wrong.

[Shanky Sohar]

Welcome to JavaRanch.
have a look at this.
http://faq.javaranch.com/java/SingletonPattern

[Rob Spoor]

Avishkar Nikale wrote:Constructors are not inherited so its just the visibility which matters.

It means your sublcass & other classes in your package can create objects of your Singleton class.

Ranchers,

Please correct me if I am wrong.

Sub classes cannot create instances*. It cannot use protected constructors to create instances of the super class. Nor can it call protected methods on instances of the super class. The protected means that it can access the constructor in its own constructor (super(...)) and can call the protected methods on its own instances.

* Unless located in the same package.

[Shanky Sohar]

If the superclass constructor is made private then you cannot even extend the class
see below code

class superclass{
	private superclass(){
		System.out.print("superclass");
	}
}
public class Subclass extends superclass{


}

this will give compile time error.saying superclass constructor is not visible to subclass.

[Seetharaman Venkatasamy]

Suraj Behera wrote:Give me practical example for the second case i.e what is the use of Protected constructor in sigleton pattern.

I dont see any use of this, unless you allow someone to subclass; typically singleton has private constructor.

[Avishkar Nikale]

Rob Prime wrote:
Sub classes cannot create instances*. It cannot use protected constructors to create instances of the super class. Nor can it call protected methods on instances of the super class. The protected means that it can access the constructor in its own constructor (super(...)) and can call the protected methods on its own instances.

* Unless located in the same package.

Thanks for the clarification Rob

[Rob Spoor]

You're welcome.