removeAll Java Issue

Question

I have an issue where I am creating an arraylist and adding multiple arraylists to this one. At some points in the program I need to remove these lists from the one central list. I have been using removeAll(); but this removes all instances of one element. For example the arraylist can contain (1,2,3,4,5) and one can add the list (1,2,3) to it. Yet when you go to remove this list the resultant list now contains (4,5) while it is desired for it to contain (1,2,3,4,5). How can that be accomplished? Thanks for any help.

Answer 1

Sounds like you should just use remove instead of removeAll. You can put it in a loop to remove all the elements from a collection:

ArrayList<Integer> bigList = new ArrayList<Integer>();

// Put multiple smaller lists into big list
bigList.addAll(list1);
bigList.addAll(list2);
bigList.addAll(list3);

// Remove list2's elements from bigList
for (Integer i : list2) {
  bigList.remove(i);
}

---

Update:

Runnable version:

import java.util.*;

public class RemoveTest {
  public static void main(String[] args) {
    List<Integer> list1 = Arrays.asList(1, 2, 3);
    List<Integer> list2 = Arrays.asList(1, 2, 3, 4, 5);
    List<Integer> list3 = Arrays.asList(9);

    ArrayList<Integer> bigList = new ArrayList<Integer>();

    // Put multiple smaller lists into big list
    bigList.addAll(list1);
    bigList.addAll(list2);
    bigList.addAll(list3);

    // Remove list2's elements from bigList
    for (Integer i : list2) {
      bigList.remove(i);
    }

    System.out.println(bigList);
    // Result:
    // [1, 2, 3, 9]
  }
}