Wrappers question

[Alex Khvatov]

Consider the following code:

  public class Wrapper2 {
 
    
    
    public void go(){
        Integer x1 = 1000;
        Integer x2 = 1000;
        
        
        if(x1!=x2){
            System.out.println("Objects not same.");
        }
        System.out.println("x1="+x1 + " x2="+ x2);
        if(x1==x2){
            System.out.println("Object are the same.");
        }
    }
    public static void main(String[] args) {
        new Wrapper2().go();
    }
 
}

When compiled and run produces:
Objects not same.
x1=1000 x2=1000

but when
lines :
Integer x1 = 1000;
Integer x2 = 1000;
changed to
Integer x1 = 100;
Integer x2 = 100;

Produces:
x1=100 x2=100
Object are the same.

Why?
[ November 05, 2006: Message edited by: Alex Khvatov ]

[Keith Lynn]

The answer depends on the VM implementation. The Java Language Specification only requires that ints between -128 and 127 be autoboxed into the same Integer object.

This is the discussion from the Java Language Specification 5.1.7

Ideally, boxing a given primitive value p, would always yield an identical reference. In practice, this may not be feasible using existing implementation techniques. The rules above are a pragmatic compromise. The final clause above requires that certain common values always be boxed into indistinguishable objects. The implementation may cache these, lazily
or eagerly.

For other values, this formulation disallows any assumptions about the identity of the boxed values on the programmer's part. This would allow (but not require) sharing of some or all of these references.

This ensures that in most common cases, the behavior will be the desired one, without imposing an undue performance penalty, especially on small devices. Less memory-limited implementations might, for example, cache all characters and shorts, as well as integers and longs in the range of -32K - +32K.

[Joe San]

Thank you Keith,

I was looking for the range. So between -127 and 128, two similar Integer objects are given the same refernce.

Thank you very much.

[Sanjeev Singh]

Alex,
Similar disscussion is in this thread.