Re: wrong behaviour when checking the equality of two elements of an ArrayList<Integer> using the operator "=="

Hey Joshua,

next, try the following:

int myPrimitiveInt = 400;

ArrayList<Integer> list = new ArrayList<Integer>();

list.add(myPrimitiveInt);

list.add(myPrimitiveInt);

and the answer will be "false".

This is "expected behaviour", believe it or not :)  Autoboxing caches integer objects for reuse.  4 is in the cache, 400 is not.

Regards

Heinz

-- 

Dr Heinz M. Kabutz (PhD CompSci)

Author of "The Java(tm) Specialists' Newsletter"

Sun Java Champion

http://www.javaspecialists.eu

I hope this will not be considered as a misuse of the java-dev mailing list. The issue I am reporting is probably not relevant for most developers but I came across it and I was very surprised.

Imagine creating an ArrayList of Integers

ArrayList<Integer> list = new ArrayList<Integer>();

and adding elements to it. You can do this is two ways (at least):

1) you may have some primitive of type int and you may add that directly to your ArrayList

list.add(myPrimitiveInt);

2) or you can create the Integer object from the primitive

list.add(new Integer(myPrimitiveInt));

Now imagine to do the following:

int myPrimitiveInt = 4;

ArrayList<Integer> list = new ArrayList<Integer>();

list.add(myPrimitiveInt);

list.add(myPrimitiveInt);

What really surprises me is that now you will get true from the condition (list.get(0) == list.get(1)) !

This is really unexpected as the method ArrayList.get(int) returns an Integer object not a primitive and the == operator should check if the two objects are the same and not if their value is the same.

In fact if you do the following:

int myPrimitiveInt = 4;

ArrayList<Integer> list = new ArrayList<Integer>();

list.add(new Integer(myPrimitiveInt));

list.add(new Integer(myPrimitiveInt));

the condition (list.get(0) == list.get(1)) will return false as expected.

I checked with several other JVMs on different platform and none of them gave this weird result (i.e. all of them gave false in both cases).

Has anybody else come across this before? Do Apple has a place where you can post bugs of this type? Is there some obvious thing I am missing?

Many thanks. Cheers,

Francesca

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