Question

Hey all, I ran into an interesting occurrence and am looking for an explanation.

In Java 1.6:

Integer a = new Integer(5);
Integer b = new Integer(5);

System.out.println(a == b);

Integer c = 5;
Integer d = 5;

System.out.println(c == d);

I get:

false
true

In Eclipse I checked in the debugger. a and b are different objects, while c and d are the same objects (but different from a and b).

Can anyone clue me in on what’s going on under the hood? Is this JVM magic? Realizing that a Integer(5) is already on the stack?

Answer 1

Java caches Integer instances for values it deems close enough to zero if they’re constants. Manually creating an Integer using new bypasses that cache. You can call Integer.valueOf with an int to get the corresponding Integer without bypassing the cache.

You may want to search for “JVM Integer cache” on your search engine of choice for more information.