I have two lists, ListA and ListB and I want to copy ListB to ListA.
What is the difference between the following, in terms of complexity and result:
ListA = ListB;ListA = new ArrayList(ListB);
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1) copies the reference to a List 2) creates a new list with a copy of the references to the elements in the list.Peter Lawrey– Peter Lawrey2013-01-18 13:12:11 +00:00Commented Jan 18, 2013 at 13:12
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3 Answers
With listA = listB, there is only one list. So if you call listA.add(something);, it will add to listB too.
With listA = new ArrayList(listB); there are two lists (which happen to contain the same objects). So if you call listA.add(something);, it will NOT add anything to listB.
In terms of complexity, the first one is a simple assignement (i.e. runs in constant time), whereas the second runs in O(n), n being the size of listB.
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In the first case there is no copying at all. Just aliasing: listA can be viewed as a synonym, alias for ListB.
This is, naturally, due to Java's exclusive usage of reference types. No type in Java can actually hold an object as its value.
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for ListA = ListB
You are not copying the list, you are just assigning another alias (reference) to that object.
