Query regarding wrapper classes

[rahul lahiri]

I am new to java.I was trying with the following code:-

class MyValue {
    public int x=0;
}

class Demo {
    public void valueChanger(Integer no,MyValue mv) {
        no=100;
        mv.x=200;
    }
}

class Main {
    public static void main(String ar[]) {
        Integer no=5;
        MyValue mv=new MyValue();
        Demo d=new Demo();

        d.valueChanger(no,mv);
        System.out.println(mv.x);
        System.out.println(no);
    }
}

The output that I am getting is
200
5
Now last day, I learned that Wrapper classes are objects.So no is an object but its not behaving like one.I am new to OOP and java and I don't have a clue whats happening.Please help.

Thanks in advance

[Siddhesh Deodhar]

Please make habit of putting code in code tags.

   
MyValue mv=new MyValue();
Demo d=new Demo();

d.valueChanger(no,mv);
System.out.println(mv.x);
System.out.println(no); 

----
public void valueChanger(Integer no,MyValue mv)
{
no=100;
mv.x=200;
} 
 

I dont find any complicated stuff here. You are passing mv object to function valueChanger and setting its x variable value to 200. If you are confused between int and Interger go ahead read about Boxing (auto-Boxing)

[rahul lahiri]

Actually my question was why the value of no is not getting changed to 100 like the x variable of the mv object.

[David Newton]

It *is* being changed--inside the method that's changing it. But you're doing different things: one statement sets the reference to a new value. One sets a member of a reference to a value. Try setting "mv" itself to a new value--what happens in the caller?

[Rob Spoor]

Cal lBy Reference Vs Call By Value

[rahul lahiri]

@David
Yeah.It doesn't reflect in the main class then.But it is getting confusing.What is the rule then.If I pass an object and change the value of its member, then it would reflect outside, in the caller class and if I pass an object and change what it refers to,then it won't reflect?Is this the rule?Thank you very much for your reply.

[Siddhesh Deodhar]

Hi Rahul, I dint understand your question properly at the beginning. Have a look at following.

in the caller class and if I pass an object and change what it refers to,then it won't reflect?Is this the rule?

Yes.
Try modifying your class as follows

   
class MyValue
{
	public int x;
	public Integer x2;

	public MyValue(int intVar, Integer integerVar)
	{
		this.x = intVar;
		this.x2 = integerVar; 
	}
}

And modify method like follows

  
	public static void valueChanger(Integer no,MyValue mv)
	{
		no=100;
		mv.x=200;
		mv.x2=300;
		mv = new MyValue(30,40);	
	}	
 

In method valueChanger, you are changing content of object MyValue and you are reassigning 'no' to 100. Since you are reassigning 'no', change dint appear in calling method.
Now see in method valueChanger, I am doing mv = new MyValue(30,40); which is not at all affecting output.

[rahul lahiri]

Thank you for your reply.I did exactly this and got this output after getting the reply from David.The change in reference of the object does not reflect in the caller method.Now,like I posted before, what is the rule in java in such situations?If I change the object reference inside a method and make it refer to another(of course a compatible one), it won't reflect outside the method in which I am doing it but if I change the value of its members,then it would reflect?What if a member is an object itself?And if this is the rule, then why?I thought objects were passed by reference in java. I am really confused.Please help.

[David Newton]

What specifically is giving you a problem?

[Rob Spoor]

Have you even read the page I linked to, and the pages that page links to? I very much doubt it.

[rahul lahiri]

Yep.I didn't notice it.Yeah, it is clear now.I should have read that and then asked rest of the questions.I am very very sorry and thank you for clarifying things.

[David Newton]

No need to apologize :)

[Rob Spoor]

Indeedy