Why same values got from ArrayList are changed to different ones?

Question

I am using Java ArrayList.

ArrayList<Integer> list = new ArrayList<Integer>();
list.add(-129);
list.add(-129);
return (list.get(0) == list.get(1));

The return value is false. I know it must be some range problem since when I use -128, it returns true. But could someone tell me the reason of it?

You're checking reference equality. Use `.equals()` instead. -128 works because it's in the range of cached values. — shmosel, Feb 23 '17 at 03:34
Possible duplicate of [Integers caching in Java](http://stackoverflow.com/questions/3131136/integers-caching-in-java) — Keith, Feb 23 '17 at 03:39

score 1 · Accepted Answer · answered Feb 23 '17 at 03:36

Java caches integers in the range -128..127 (signed bytes). It saves a lot of allocations of tiny numbers which is very common in lots of code. If you use equals() instead of ==, it will work. The == check is comparing that two int types, that have been magically boxed (i.e. converted) to Integer are the same reference - which they are not. But int in the signed byte range will be the same. The equals check will actually compare the value of the variables:

return (list.get(0).equals(list.get(1)));

score 0 · Answer 2 · answered Feb 23 '17 at 07:29

When you perform: list.add(-129); list.add(-129);

It create two separate object with separate memory location.when you perform == it return false. In Java, values from -128 to 127 are cached, so the same objects are returned and you will get true.

For more information see:http://www.geeksforgeeks.org/comparison-autoboxed-integer-objects-java/

score -1 · Answer 3 · edited Feb 23 '17 at 06:33

If two references point to two different objects, then in accordance with the "= =" to judge both is unequal, even though both have the same reference content. As we know, if two references refer to the same object, in accordance with the "= =" to judge the two are equal.

If you go to the Integer. Java classes, you will find that there is a internal private class IntegerCache. Java, it caches from all the Integer object - between 128 ~ 127.

So the thing is, all the small integers are in the internal cache. If the value is between 128 ~ 127, then it will be returned from a cache instance,point to the same object.

Why same values got from ArrayList are changed to different ones?

3 Answers3