How to access another class having private constructor using it's static variavble ?

[Nitesh Nandwana]

LOGIC PLEASE!!! Thank you in advance.

[raghava dv]

class ClassA{
static int a=3;
private ClassA(){}
}

public class ClassB{
static int b = 0;
public static void main(String arg[]){
b = ClassA.a;
System.out.println(b);
}

}

[Nitesh Nandwana]

raghava dv wrote:class ClassA{
static int a=3;
private ClassA(){}
}

public class ClassB{
static int b = 0;
public static void main(String arg[]){
b = ClassA.a;
System.out.println(b);
}

}

if there is a method in ClassA then how can i access it in ClassB using this static instance variable means i want to use this static instance variable to create object of ClasssA .

[Eli Wood]

Nitesh, I believe the situation you are asking for is often seen in the Singleton pattern.

In my example, SimpleSingleton has a private contructor, and an instance (the only instance of the class) is held in a static variable. An accessor method, getInstance(), allows other classes to retrieve this instance, although you could avoid using a method and just set the static variable (theInstance) to public so it could be retrieved directly if you really wanted (I wouldn't recommend that though).

package sample;

public class SingletonDriver {
	public static void main(String[] args) {
		SimpleSingleton s1 = SimpleSingleton.getInstance();
		SimpleSingleton s2 = SimpleSingleton.getInstance();
		
		s1.printItOut();
		s2.printItOut();
	}
}

class SimpleSingleton {
	
	private SimpleSingleton() {}
	
	// Only one instance is created for this static variable.
	private static SimpleSingleton theInstance = new SimpleSingleton();
	
	public static SimpleSingleton getInstance() {
		return theInstance;
	}
	
	public void printItOut() {
		System.out.println("Hello from an instance method! Object is " + this.toString() + ".");
	}
	
}

output:
Hello from an instance method! Object is sample.SimpleSingleton@82ba41.
Hello from an instance method! Object is sample.SimpleSingleton@82ba41.

[dennis deems]

What Eli said.

[Nitesh Nandwana]

Dennis Deems wrote:What Eli said.

Yes I got that but i want to use static instance variable to create that class object not static reference variable, how ???

[Nitesh Nandwana]

Eli Wood wrote:Nitesh, I believe the situation you are asking for is often seen in the Singleton pattern.

In my example, SimpleSingleton has a private contructor, and an instance (the only instance of the class) is held in a static variable. An accessor method, getInstance(), allows other classes to retrieve this instance, although you could avoid using a method and just set the static variable (theInstance) to public so it could be retrieved directly if you really wanted (I wouldn't recommend that though).

package sample;

public class SingletonDriver {
	public static void main(String[] args) {
		SimpleSingleton s1 = SimpleSingleton.getInstance();
		SimpleSingleton s2 = SimpleSingleton.getInstance();
		
		s1.printItOut();
		s2.printItOut();
	}
}

class SimpleSingleton {
	
	private SimpleSingleton() {}
	
	// Only one instance is created for this static variable.
	private static SimpleSingleton theInstance = new SimpleSingleton();
	
	public static SimpleSingleton getInstance() {
		return theInstance;
	}
	
	public void printItOut() {
		System.out.println("Hello from an instance method! Object is " + this.toString() + ".");
	}
	
}

output:
Hello from an instance method! Object is sample.SimpleSingleton@82ba41.
Hello from an instance method! Object is sample.SimpleSingleton@82ba41.

from k&B
Rules for Constructors

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.)

in above example you're using static method that allows to an instance creation that is correct but how using static variable as written above ??

[Eli Wood]

The code below uses a public static variable instead of a public static method. I believe it shows how this is possible to do, and why I would consider it to be a bad idea.

package sample;

public class SingletonDriver {
	public static void main(String[] args) {
		//It is possible to directly access a static variable to grab an instance.
		SimpleSingleton s1 = SimpleSingleton.theInstance;
		SimpleSingleton s2 = SimpleSingleton.theInstance;
		
		s1.printItOut();
		s2.printItOut();
		
		
		//But I wouldn't recommend it because you might run into this problem...
		SimpleSingleton.theInstance = null;
		SimpleSingleton s3 = SimpleSingleton.theInstance;
		System.out.println("This is going to be ugly....");
		s3.printItOut();
	}
}

class SimpleSingleton {
	
	private SimpleSingleton() {}
	
	// Only one instance is created for this static variable.
	public static SimpleSingleton theInstance = new SimpleSingleton();
	
	public static SimpleSingleton getInstance() {
		return theInstance;
	}
	
	public void printItOut() {
		System.out.println("Hello from an instance method! Object is " + this.toString() + ".");
	}
	
}

Output:
Hello from an instance method! Object is sample.SimpleSingleton@82ba41.
Hello from an instance method! Object is sample.SimpleSingleton@82ba41.
This is going to be ugly....
Exception in thread "main" java.lang.NullPointerException
at sample.SingletonDriver.main(SingletonDriver.java:17)

[Riza Aktunc]

Actually if you make a little bit change in Eli's code you can achieve what you want to see. You can see the necessary changes below:

private static SimpleSingleton theInstance = new SimpleSingleton(); to public static SimpleSingleton theInstance = new SimpleSingleton();
SimpleSingleton s1 = SimpleSingleton.getInstance(); to SimpleSingleton s1 = SimpleSingleton.theInstance;

This is what you want to see; but not a good implementation regarding encapsulation issues!

I was writing this post but obviously Eli made it quicker than me

[Nitesh Nandwana]

thank you Eli & Riza