wrapper classes

[amit mandal]

  public class Wrapper
{
	public static void main(String args[]) {
		
  Integer i1=1000;
  Integer i2=1000;
  if(i1==i2)
  	System.out.println("equal");
  	if(i1.equals(i2))
  				System.out.println("really equal");
   	if(i1!=i2)
   			System.out.println("not equal");

    
}
}  

output:really equal
not equal


when i use i1=10 and i2=10
output: equal
really equal

when am using 1000 as the value of i1 and i2, the == test fails but for 10,it pass. Can someone explain me this unexplained behaviour? am really confused

Reference: k&b pg-246

[Ankit Garg]

The JVM uses a cache for integer wrappers in the range -128 to 127. So when you create two wrapper objects in that range using autoboxing (or calling the valueOf method), then the object from the cache is used. Thus if you create two Integer with the same value in that range, both will be the same object in the heap thus the == comparison will return true...

[Prasad Kharkar]

Actually there are some rules for immutability applied to the Wrapper classes
• Two instances of the wrapper objects Boolean, Byte, Character from \u0000 to \u007f and short and integer from -128 to 127 will always be == when their primitive values are same

hence
it gives true for value 10 and false for the value 1000

hope this helps
happy preparation

[amit mandal]

ohh! such a simple solution i now understand it properly
thanks ;)

[Imad Aydarooos]

but as you know if you created the Integer like the below code, you will get a result for the 10 like for 1000 :

public class Wrapper   
{   
   public static void main(String args[]) {   
           
     Integer i1=new Integer(10);   
     Integer i2=new Integer(10);   
     if(i1==i2)   
       System.out.println("equal");   
     if(i1.equals(i2))   
       System.out.println("really equal");   
     if(i1!=i2)   
        System.out.println("not equal");   
       
  }   
}    

[swaraj gupta]

Is there exist a separate pool for all the immutable instances on the heap? Because the behavior shown by Wrapper class instances here is also shown by the String literals as well.

[Imad Aydarooos]

I get this from K&B Page 246
{ In order to save memory, two instances of the
following wrapper objects (created through boxing), will always be == when their
primitive values are the same:
■ Boolean
■ Byte
■ Character from \u0000 to \u007f (7f is 127 in decimal)
■ Short and Integer from -128 to 127
Note: When == is used to compare a primitive to a wrapper, the wrapper will be
unwrapped and the comparison will be primitive to primitive.}

[Saibabaa Pragada]

Ideally when you write this code, Line 1 and 2 should be read as

int i1 = 1000;// (through autounboxing Integer i1=1000; )
int i2 = 1000; //(through autounboxing Integer i2=1000; )know??
if(i1 == i2)
  System.out.println("equal");//prints "equal"..Why the same is not working for
                                // "Integer" wrapper classes  even though Integer and int are same??

[Ankit Garg]

even though Integer and int are same??

What made you jump to that conclusion? int is a primitive, Integer is a reference type. How are they the same??