CODEWITHSUNDEEP
javaarraylist
Renjun Zou
Renjun Zou

Reputation: 101

How do I swap two arraylist in JAVA?

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;
}

Upvotes: 4

Views: 10124

Answers (5)

MByD
MByD

Reputation: 137442

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:

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

Upvotes: 4

John
John

Reputation: 1

You can use the swap method from Collections: 

Collections.swap(Integer, list1, list2);

Upvotes: -3

Andreas Dolk
Andreas Dolk

Reputation: 114847

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.

Upvotes: 14

Kuldeep Jain
Kuldeep Jain

Reputation: 8598

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.

Upvotes: 0

assylias
assylias

Reputation: 328923

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;
}

Upvotes: 2

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