Java Wrapper equality test

Question

  public class WrapperTest {      public static void main(String[] args) {          Integer i = 100;         Integer j = 100;          if(i == j)             System.out.println("same");         else             System.out.println("not same");     }     } 

The above code gives the output of same when run, however if we change the value of i and j to 1000 the output changes to not same. As I'm preparing for SCJP, need to get the concept behind this clear. Can someone explain this behavior.Thanks.

Answer 1

In Java, Integers between -128 and 127 (inclusive) are generally represented by the same Integer object instance. This is handled by the use of a inner class called IntegerCache (contained inside the Integer class, and used e.g. when Integer.valueOf() is called, or during autoboxing):

private static class IntegerCache {     private IntegerCache(){}      static final Integer cache[] = new Integer[-(-128) + 127 + 1];      static {         for(int i = 0; i < cache.length; i++)             cache[i] = new Integer(i - 128);     } } 

See also: http://www.owasp.org/index.php/Java_gotchas

Answer 2

Basically Integers between -127 and 127 are 'cached' in such a way that when you use those numbers you always refer to the same number in memory, which is why your == works.

Any Integer outside of that range are not cached, thus the references are not the same.