After ArrayList.add() in a loop, all elements are identical. Why?

Question

If I do this [quasi-java-code]:

while (loop)
{
    localObject = getDataForObject();
    globalPublicStaticArrayList<Object>.add(localObject);
}

All the elements in globalPublicStaticArrayList are identical, equal to the last copy of localObject added. I stepped thru the loop in the debugger and saw that as soon as an Object is added, it is copied in to all the elements of the globalPublicStaticArrayList.

The workaround I found is:

while (loop)
{
    localObject = getDataForObject();
    globalPublicStaticArrayList<Object>.add(new Object(localObject.member1, localObject.member2,...));
}

Has it something to do with pass-by-reference in Java? How come the elements are identical in the first case? Thanks.

show the code for `getDataForObject()`. or perhaps your full activity code — waqaslam, Nov 09 '12 at 12:54
This question was asked before the other question. How is this a duplicate and not that one? — iceman, Nov 28 '15 at 01:16

score 5 · Accepted Answer · answered Nov 09 '12 at 12:55

5

Java uses call by value, but here those values are references to objects.

What you are adding to the list is not a copy of the object, but a copy of the reference. Your method returned the same object each time you called it. It probably should return a new object each time, then you wouldn't need this workaround.

answered Nov 09 '12 at 12:55

Mark Byers

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thanks for your insight. If you are referring to getDataForObject(), it is just a place holder in the question. I can see in the debugger/log that localObject holds different data for each iteration. The only difference is in the add() call. This is java.util.ArrayList.add(); – iceman Nov 09 '12 at 13:46
@iceman: `I can see in the debugger/log that localObject holds different data for each iteration.` That tells you nothing. It can contain different data but still be the same object. Analogy: Your house can contain different furniture each day, but it's still the same house. Changing the furniture does not make it a different house. – Mark Byers Nov 09 '12 at 13:47
Ok. I get you. My question now is, after each pass, or rather after each add() call, all elements in globalPublicStaticArrayList become the same? It's like this: Pass 1: localObject = 1 globalPublicStaticArrayList = [1]; Pass 2: localObject = 2 globalPublicStaticArrayList = [2, 2]; Pass 3: localObject = 3 globalPublicStaticArrayList = [3, 3, 3]; Pass 4: localObject = someValue globalPublicStaticArrayList = [ someValue, someValue, someValue, someValue]; Pass N: localObject = finalValue globalPublicStaticArrayList = [ finalValue * N times]; – iceman Nov 09 '12 at 13:51
@iceman: You have many references to **the same object** in your list. To create a new object use the `new` operator. Like this: `localObject = new ....;`. – Mark Byers Nov 09 '12 at 13:58
Got it. Instead of ArrayList, I get ArrayList. Thanks for your effort :) – iceman Nov 09 '12 at 14:05

score 2 · Answer 2 · answered Nov 09 '12 at 12:55

2

globalPublicStaticArrayList<Object>.add(localObject);

here you are passing the localObject reference. You you want a copy of every objects you should create a new object at every iteration

answered Nov 09 '12 at 12:55

Blackbelt

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score 0 · Answer 3 · answered Nov 09 '12 at 12:59

0

In getDataForObject() may be you are not creating "new" object of return type. Because of that all objects are pointing to same address.

answered Nov 09 '12 at 12:59

Rahul Techie

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After ArrayList.add() in a loop, all elements are identical. Why?

3 Answers3

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