Why This Code Shows Error...

[Thangaraj Selvamani]

Why This Code Shows Error...

 
public class test13{
public static void main(String[] str){
Integer in = 1000;
Integer in1 = 1000;
System.out.println(in == in1);
System.out.println(in != in1);
}
}

C:\SCJP>java -version
java version "1.6.0_07"
Java(TM) SE Runtime Environment (build 1.6.0_07-b06)
Java HotSpot(TM) Client VM (build 10.0-b23, mixed mode, sharing)

[Steven Landers]

== and != can only be used with the Integer wrapper when the number is between -127 and 127. Otherwise, the == and != will answer the question "are the references pointing to the same object?" correctly as "false"

This is from K&B's SCJP 6.0 book - I'd highly recommend purchasing it -

...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.
....
[ November 10, 2008: Message edited by: Steven Landers ]

[Ankit Garg]

Well Thangaraj is that code giving ERROR or the output is not what you expected. It runs fine on my PC. The output has been explained by Steven...

[Maleen Abeydeera]

as an extension to this problem, is there an Integer pool like a Srting pool?

I mean, when I say

Integer i = 23;
Integer j =23;
System.out.println(i==j);

it prints "true" , but

Integer i = 23;
Integer j =new Integer(23);
System.out.println(i==j);

prints "false"

can somebody explain why?

[joseph gonzales]

Hi Maleen,

for my understanding i==j does not actually comparing their values but instead it compares the address of where the value 23 was stored in the memory.

sample 1:
Integer i = 23; // reference i points to the address where 23 was stored
Integer j =23; // since 23 already exist in the memory, make reference j point to that same address in memory.

thats why (i==j) equals to true because i and j points to the same address


sample 2:
Integer i = 23; // reference i points the the address where 23 was stored
Integer j =new Integer(23);// this time a new Object was created and stored in different address in memory, then make reference j points to that address

so i==j will result to false.

so to get the result as expected you can use the .equals method.
[ November 10, 2008: Message edited by: joseph gonzales ]

[Rekha Srinath]

Maleen & Joseph,

The answer to your questions is there in Steven's post...

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:
.........
.........
Short and Integer from -128 to 127

So, in case of
Integer i1 = 127;
Integer i2 = 127;
These are Integer variables created through boxing (ie. without using new operator)...So, as per the rule given above, an == operation on i1 and i2 will be true..

If you change the value of i1 and i2 to 128 or greater, == will return false
[ November 10, 2008: Message edited by: Rekha Srinath ]

[Maleen Abeydeera]

Thanks, Joseph and Rekha.

So my understanding is that like the String pool, an Integer pool also exists, and behaves exactly like the String pool. Am I correct?

[Rekha Srinath]

I don't think you can call it an Integer pool... And I don't know if something like that exists...

Also, note that this specific == rule for Integer created through boxing is valid only for integer values till 127 (and other data types, which Joseph had quoted from K&B). But, in case of String literals, there is no such bound.