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.

Answer 1

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.