How do I swap two arraylist in JAVA?

Question

There are two ArrayList e.g.list1 = {1,2,3} and list2 = {7,6,4,2} How to can swap these two list. So the result will be list1 = {7,6,4,2} and list2 = {1,2,3} Can I implement like this:

public void swapList(ArrayList<Integer> list1, ArrayList<Integer> list2){
    ArrayList<Integer> tmpList = list1;
    list1 = list2;
    list2 = tmpList;
}

+1 You asked below the follow-up question "Is there any less memory cost way to solve this problem? Since I may have much element in my list." Consider asking that as a separate StackOverflow question. Show how you're using this method and ask that question. I think it's a really good one. — Erick Robertson, Feb 29 '12 at 00:06

Andreas Dolk · Accepted Answer · 2012-02-28T13:40:50.213

14

No you can't implement it like that. Same with arrays. Pass-reference-by-value problem, like the others explained already.

If you want the lists to swap their content, then you have to clear and copy:

public static void swapList(List<Integer> list1, List<Integer> list2){
    List<Integer> tmpList = new ArrayList<Integer>(list1);
    list1.clear();
    list1.addAll(list2);
    list2.clear();
    list2.addAll(tmpList);
}

Some additional thoughts:

List<Integer> list1 = getList1Magic();
List<Integer> list2 = getList2Magic();

if (isSwapReferences()) {
  // this does not affect the actual lists
  List<Integer> temp = list2;
  list2 = list1;
  list1 = temp;
} else if (isSwapListContent()) {
  // this modifies the lists
  swapList(list1, list2);  // method from above
}

The swap strategy depends on your requirements. First block has a local effect, second block a global one.

edited Feb 28 '12 at 13:40

answered Feb 28 '12 at 12:19

Andreas Dolk

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This is precise solution, customization can also be made for null values of the arguments (list1 and list2) – Goran Nastov Feb 28 '12 at 12:23
Keep in mind that this is much slower. If you can, you should swap just the references. – Piotr Praszmo Feb 28 '12 at 12:24
It makes my head hurt to think about all the memory and CPU being wasted here. – Erick Robertson Feb 28 '12 at 12:38
2

This is *Part 1: How To Do It*. *Part 2: How To Avoid It* is sold separately ;) – Andreas Dolk Feb 28 '12 at 12:42
Is there any less memory cost way to solve this problem? Since I may have much element in my list. – Renjun Zou Feb 28 '12 at 13:24
1

Sure: don't use a method to swap. Do it, where you have defined your variables (local variables or members) and swap the references there. If you want to swap in a method, than it gets expensive. *And*: the swapped content effects *all* other variables, that hold references to the lists. They see the swapped content too. So it depends on your requirement. – Andreas Dolk Feb 28 '12 at 13:35

score 4 · Answer 2 · edited Sep 27 '18 at 07:39

4

The problem with your solution is that Java is a pass-by-value language. So any change to list1 and list2 variables will not have an effect outside the method, but if this is not in a method, then it's ok.

Two more things:

You probably meant list2 = tmpList;, not list2 = list1;.
You cannot use generic with primitives, should be List<Integer>.

edited Sep 27 '18 at 07:39

Jianxin Gao

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answered Feb 28 '12 at 12:08

MByD

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Not entirely, see the other two answers. Still you are closest to the correct answer. – Boris Strandjev Feb 28 '12 at 12:09
6

The problem is that Java is *not* pass-by-reference. – Thilo Feb 28 '12 at 12:09

score 2 · Answer 3 · edited May 23 '17 at 12:00

You can't do it in a function because of Java is pass by value only. And you can't create a List of int, you can create a List of Integer.

You can try this to verify that it does not work:

public static void main(String... args) throws Exception {
    List<Integer> list1 = Arrays.asList(1, 2, 3);
    List<Integer> list2 = Arrays.asList(4, 5, 6);
    System.out.println(list1); //prints 1, 2, 3
    swapList(list1, list2);
    System.out.println(list1); //prints 1, 2, 3
}

public static void swapList(List<Integer> list1, List<Integer> list2){
    List<Integer> tmpList = list1;
    list1 = list2;
    list2 = tmpList;
}

Kuldeep Jain · Answer 4 · 2012-02-28T12:15:20.303

0

Minor correction in your approach:

    ArrayList<Integer> tmpList = list1;
    list1 = list2;
    list2 = tmpList;

But you can not do this in a method the way you are doing.

edited Feb 28 '12 at 12:15

answered Feb 28 '12 at 12:07

Kuldeep Jain

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score -3 · Answer 5 · edited Oct 01 '12 at 08:09

-3

You can use the swap method from Collections:

Collections.swap(Integer, list1, list2);

edited Oct 01 '12 at 08:09

Tiago Almeida

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answered Oct 01 '12 at 08:02

John

1

[this is now how swap works](http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html) – oers Oct 01 '12 at 10:02

How do I swap two arraylist in JAVA?

5 Answers5