Unexpected result during Long comparison

December 22, 2022

I’ve been learning Java for quite a while and recently stumbled upon a strange result, during Long comparison.

Here is a piece of code:

public class Test {
    public static void main(String[] args) {
        Long a = 100L;
        Long b = 100L;
        
        Long c = 150L;
        Long d = 150L;

        System.out.println(a == b);
        System.out.println(c == d);
    }
}

true
false

I know that while Java doesn’t support your general comparison between reference types with ==, it usually caches all constant values that you hardcode and then smart compare them.

And I honestly believed that, until my code crashed. After some tedious debugging I found the bug, and even reproduced it in few lines.

What is going on here?

>Solution :

Java caches values from 0-127. So, == on Uppercase Long will be true for those values. But all others, false.

But that is not behaviour you should depend on. Just use Long::equals or Objects::equals.

Here is a code example.


   private class SOQ_20221222_1
   {
   
      public SOQ_20221222_1()
      {
      
         Long a = 100L;
         Long b = 100L;
        
         Long c = 101L;
         Long d = 101L;
        
         Long e = 102L;
         Long f = 102L;
        
         Long g = 126L;
         Long h = 126L;
        
         Long i = 127L;
         Long j = 127L;
        
         Long k = 128L;
         Long l = 128L;
      
         Long m = 129L;
         Long n = 129L;
      
         System.out.println(a == b);
         System.out.println(c == d);
      
         System.out.println(e == f);
         System.out.println(g == h);
      
         System.out.println(i == j);
         System.out.println(k == l);
      
         System.out.println(m == n);
         
      }
   
   }