Uniqueness of static variable

[Balakrishna Thati]

Pragramatically how to show that there is only one instance of Static variable irrespective of number of objects...

I thought of doing it by
1. showing their values being equal.. But the counter argument is that the "having the same value doesn't sufficient to say they are from the same variable"
2. equating the static variable's hash code accessing it from different objects. But Hashcode can't guarantee uniqueness.

is there any other way to prove that irrespective of number of objects there is only one static variable exists for a Class.

-- KVT

[Campbell Ritchie]

You mean apart from reading the Java™ Language Specification?

Create an Object instance which is static, and access this from each instance of your class, and use its equals() method which reads something like this:

public boolean equals(Object o)
{
   return o == this;
}

[Balakrishna Thati]

Thanks Ritchie,

Here is my code

   
public class ProveIt {

          public static void main(String[] args){

                   Abc obj = new Abc();
                  if(obj.count == Abc.count)
                  {
			System.out.println("We have only one instance");
                  }         
          }

}

class Abc{

public static int count;

}


 

[Jonas Isberg]

Balakrishna Thati wrote:System.out.println("We have only one instance");

Yes and no I would say. The only instance you have is of the type Abc. count is a primitive
type that is compared by value (and gets a default value of 0).

What if the code had been like below, would that prove anything?

public class DoesThisProveIt {  
    public static void main(String[] args){  
        Abc obj = new Abc();  
        if(obj.count == (new Abc()).count) {  
            System.out.println("We have only one instance");  
        }           
    }  
}  
  
class Abc {  
    public int count;  
}

[Balakrishna Thati]

if the comparision is between the primitives, it again fall back to my option 1, that i stated earlier.

To conclude it finally, i changed the code little bit.

    
public class ProveIt {

          public static void main(String[] args){

                   Abc obj = new Abc();
                  if(obj.count.equals(new Abc().count))
                  {
			System.out.println("We have only one instance");
                  }         
          }

}

class Abc{

public static Integer count=new Integer(10);

}

[Jonas Isberg]

public class Test {
    public static void main(String[] args) {
        System.out.println(new Integer(10).equals(new Integer(10)));
    }
}

class [surl='http://www.javaranch.com/unit-testing.jsp' class='api' title='article: evil unit testing wrote:Test[/surl]]true

So, have we proven anything?
If I wanted to prove to myself that static actually works, I would change the static member
and see if it is really changed when accessing from another instance.

[Campbell Ritchie]

I don't think you are doing anything useful with that count. You can prove two objects are the same object with the == operator. On the object references. Since you have no way of increasing the count variable, it will always be th same for all instances of that class. If you did increment it, then all instances would have the same increment and th same value. So your test is no use to you.

[Balakrishna Thati]

How about this ?

    
if(obj.count == new Abc().count)
{
      System.out.println("We have only one instance");
}  

[Jonas Isberg]

If that code gives different outputs when changing from static to not static,
then I would say you have proven something.

[Balakrishna Thati]

Is this ok ? or any thing else missed ??

    
public class ProveIt {

          public static void main(String[] args){

                   Abc obj1 = new Abc();
                   Abc obj2 = new Abc();

  	  if(obj1.sobj == obj2.sobj)
                  {
		System.out.println("We have only one instance");
                  }         
	else
	{
		System.out.println("They are different");			
	}
          }

}

class Abc{

public static Sclass sobj = new Sclass();
//public Sclass sobj = new Sclass();
	public Abc()
	{
		sobj.count++;
		System.out.println("count= "+sobj.count);
	}

}

class Sclass
{
	public int count;
}

[Jonas Isberg]

Now you both check so everything actually references the same instance
and you use it (for counting the instances) in way that would have failed
if it was not static. Well done!

(Don't forget that indentation helps readability. )

[Balakrishna Thati]

Thanks Isberg and Ritchie... we done it flawlessly atlast

[Campbell Ritchie]

You're welcome