How do I swap two arraylist in JAVA?

Question

There are two ArrayList eg 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;
}

Answer 1

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.

Answer 2

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> .

Answer 3

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;
}

Answer 4

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.

Answer 5

您可以使用Collections中的swap方法：

Collections.swap(Integer, list1, list2);