private constructor

[mohammad shaid]

From K&B 5
page 129

Rules For Constructors :
If the private Constructors class wants to allow an instance of class to be used,the class must provide a static method or variable that allows access to an instance created from within the class

when i try to make a private constructor of a class and run normally,it works well.. what i mean to say here is i dint understand this particular statement;;

can someone elaborate please..

[Jim Hoglund]

If your constructor is private then 'new' will not work (try it). It follows that without an
object, any instance methods cannot be invoked. For example, aa.getName() cannot
work if aa is null. What's the solution - a static method. It works fine without 'new'
because no object is required. Here is a common way to create a MyClass object.
The private constructor is called from within the getInstance() method and the object
created is returned.

MyClass aa = MyClass.getInstance();

Jim ... ...

[mohammad shaid]

   class Drag {
	int a;String n;
	private Drag() { System.out.print("ddd"); }
	Drag(int a,String n) {
		this.a=a;this.n=n;
	}
	void sh() {
		System.out.println(a+" its a method "+n);
	}
	public static void main(String[] args) {
		Drag n=new Drag();
		System.out.println(n.a);
		n.sh();
//		new Drag(4,"shah");
	}
}
 

This code is working and gives the output:

ddd0
0 its a method null

so where is the problem??

[Henry Wong]

You do know that your main method is part of the class right? And has full access to the class, including the private constructors.

Try to do it from a main() method from a different class.

Henry

[Seetharaman Venkatasamy]

If the private Constructors class wants to allow an instance of class to be used ,the class must provide a static method

i changed this bit : If the private Constructors class wants to allow an instance of class to be used in another class ,the class must provide a static method

variable that allows access to an instance created from within the class

this is what you did

hth

<edit>Oops! Henry beaten me ! </edit>

[Prasad Kharkar]

did you mean this?
I tried some thing like this
is this correct?
the program is correct and is running
but is the logic and concept correct?

 
/** A program to demonstrate private constructor
 *  Developer: Prasad Kharkar
 *  Date	 : 03-06-2010
 */

class PrivateConstructor{
	
	private PrivateConstructor(){
	}

	private PrivateConstructor(String s){
		System.out.println(s);
	}
	// the method returns the PrivateConstructor object 
	static PrivateConstructor method(){
		System.out.println("static method and the object rerurned from here");
		return new PrivateConstructor();
	}
}

class TestPrivateConstructor {
	public static void main(String [] args){
		// using new does not work as we are have written the private constructor in the class
		//PrivateConstructor pc = new PrivateConstructor();
		PrivateConstructor p = PrivateConstructor.method();
		//done here
	}
}

   

[mohammad shaid]

Thats what i wanted to clear.. its from Another class.. its not mentioned here in K&B 5 clearly about this.. but i got the concept now...

And prasad thats the code i believe

[Jim Hoglund]

Mohammad : I see that you have been quite active since you joined JavaRanch in May.
It's good to see that you are a regular visitor.

Jim ... ...

[harmeet saini]

Jim Hoglund wrote:If your constructor is private then 'new' will not work (try it). It follows that without an
object, any instance methods cannot be invoked. For example, aa.getName() cannot
work if aa is null. What's the solution - a static method. It works fine without 'new'
because no object is required. Here is a common way to create a MyClass object.
The private constructor is called from within the getInstance() method and the object
created is returned.

MyClass aa = MyClass.getInstance();

Jim ... ...

I have another doubt related to this topic

following is a question from self test answers from K&B

What is the result?

   
class Alpha {
	static String s = " ";
	protected Alpha() { s += "alpha ";}
}
class SubAlpha extends Alpha {
	private SubAlpha() { s+= "sub";}
}
public class SubSubAlpha extends Alpha {
	private SubSubAlpha() { s += "subsub "; }
	public static void main(String[] args) {
		new SubSubAlpha();
		System.out.println(s);
	}
}
 

Answer:- alpha subsub
Reason :-SubSubAlpha extends Alpha! Since the code doesn't attempt to make a SubAlpha, the private constructor in SubAlpha is okay

as per the explanation the constuctor "private SubSubAlpha()" is fine bit what i am not able to understand that once this"private SubSubAlpha()" is invoked isn't it that a default call i.e super(); invokes private constructor "private SubAlpha()" so what does the line mean "Since the code doesn't attempt to make a SubAlpha, the private constructor in SubAlpha is okay" ? as per my understanding the answer should be "alpha sub subsub". please correct me if i am wrong

Thanks in advance

[Frits Walraven]

Hi Harmeet,

what i am not able to understand that once this"private SubSubAlpha()" is invoked isn't it that a default call i.e super(); invokes private constructor "private SubAlpha()"

The call to super() invokes the constructor of the parent of SubSubAlpha and that is Alpha (SubSubAlpha doesn't extend SubAlpha)

Regards,
Frits

[Prabhakar Reddy Bokka]

Yeah. Thats right.

  public class SubSubAlpha extends Alpha {     

SubAlpha is never called.

[harmeet saini]

Thanks Frits
i think i just overlooked that code statement