Immutable wrapper classes

[Sumit David]

import java.util.*;
public class amap
{public static void main(String arg[])
{
AbstractMap ap=new HashMap();
ap.put(new Integer("2"),null);//line1
System.out.println(ap.containsKey(new Integer("2")));//line 2
}
}
Line 1 above returns true. Does that mean new Integer("2") in line 2 points to the same object as in line 1. Does it prove that primitive wrapper classes are immutable.

[Marlene Miller]

Hi Sumit David,

Line 1 above returns true. Does that mean new Integer("2") in line 2 points to the same object as in line 1?

No. The object created at line 1 and the object created at line 2 are TWO different objects. Their references are not equal.
However the equals method of Integer returns true, because the *contents* of these two distinct objects is the same. Object.equals() compares references. Integer overrides equals(). Integer.equals() compares contents.
new Integer(�2�) == new Integer(�2�) is false.
new Integer(�2�).equals(new Integer(�2)) is true.

class Test {
   public static void main(String[] args) {
      Integer i1 = new Integer("2");
      Integer i2 = new Integer("2");
      System.out.println(i1 == i2); // false
      System.out.println(i1.equals(i2)); // true
   }
}
false
true

Does it prove that primitive wrapper classes are immutable?

containsKey returns true because the objects are equals() according to Integer.equals(). The Map interface specifies that containsKey uses the equals() method.
I am not sure how to prove the wrapper classes are immutable.
[ February 06, 2004: Message edited by: Marlene Miller ]

[Sumit David]

Thanks
It cleared all my doubt. In fact, it just slipped from my mind that the keyword new creates a new object.
Thanks again Sumit

[Marlene Miller]

I wasn't sure whether I had answered the question you were asking. Thank you for letting me know that your doubts are cleared.
Yes, you are right - new returns a different reference each time.
[ February 08, 2004: Message edited by: Marlene Miller ]

[Marlene Miller]

We are told that the wrapper classes are immutable. But how do we know that?
I looked at the 1.4 source code. The Integer class is final. So it cannot be extended.
The field that hold the integer value is private but Not final!

public final class Integer extends Number implements Comparable {
...
private int value;
public Integer(int value) {
   this.value = value;
}