Help in understanding how are Wrapper classes immutable

[Siphiwe Madi]

Hi Ranchers,

I read on the Core Java Vol 1, that:
Wrapper classes are immutable - you cannot change a wrapped value after the wrapper has been constructed.

I understood this piece of information to imply that once you construct a wrapped object holding a value of (e.g 3),
then you cannot change that value to (e.g. 4).

code:
----
public static void main(String[] args) {

Integer i = 3;
System.out.println("initial value: " + i); // line 1: prints 3

//change the wrapped value
n++;
System.out.println("final value: " + i); // line 2: prints 4
}
----

I expected that the output of line 2, would print 3 (as it is said that you cannot change that value of a wrapped object), but instead the output is 4.

Can anyone please help me understand?

Regards,
Siphiwe Madi

[Ulf Dittmer]

I assume you meant "i++", not "n++".

The value of the Integer object didn't change. A new object got created and assigned to the object reference named i. The value of the original object is still the same, although its value isn't accessible any longer. If you create a second reference to the object, you can check that its value didn't change. Try the following:

Integer i = 3;
Integer j = i;    // sets j to point to the same object as i
i++;
System.out.println("i="+i);
System.out.println("j="+j);

[Ali Khalfan]

the object itself is immutable// prior to java 5, this won't work

however in java5 it unboxes the value stores in a temp var increment it and then auto-box it again.

So something like this:

Integer i = 3;

i++ //i'm assuming this is i not n as you wrote

would be automatically converted to:

temp_i = i.intValue();

temp_i++;

Integer i = new Integer(temp_i);

this is autoboxing

[Siphiwe Madi]

Thanks a lot for you help - now I understand.

Regards,
Siphiwe Madi