Wrapper Object Equality Test

[Jan tenpas]

I'm currently studying for the SCJP exam (or OCP JSEP now maybe?) and am having issues with the objective that deals with Wrappers/Boxing. I understand that Java sees Wrapper Objects as two separate objects and also as meaningful equal.

My question is why does it consider the Objects not equal if the value is large, but equal if the value is small? Shouldn't it see both as not equal since they are different objects? See below for the code I used to test (this was done in TextPad).

Integer l4 = 3;
Integer l5 = 3;

Integer i3 = 1000;
Integer i4 = 1000;

void longtest()
{
if(l4 != l5) System.out.println("different objects @ 3"); //no output, makes sense

if(i3 != i4) System.out.println("different objects @ 1000"); //output?!?
}

[Rikesh Desai]

one more observation:

if the value of 1000 is replaced with 127, it doesnt show them as different objects.
but for 128, it still shows that they are different..

it seems to be somehow related to the value of Integer wrapper objects being limited from -128 to 127.
not very sure of the exact answer though.
waiting for some expert's reply.

[Henry Wong]

Just search JavaRanch for "integer cache"... this topic comes up a lot.

Henry

[Rikesh Desai]

thanks Henry!

[Kushan Athukorala]

This is well explained in Kathy and Bert ' s SCJP book at the page 246 as well.

Thanks,
Kushan

[Jan tenpas]

I didn't see the response on page 246. Can you post the quote that references integer caching on page 246? Maybe I just have an older version or something...

Either way, looks like Henry nailed. Thanks!