Question

I have 2 ArrayList, one containing Strings, the other Integers. The list2 contains indices of elements of list1.

Now I need to remove all the elements from list1 whose index is in list2. Any ideas?

ArrayList<String> list1 = new ArrayList<String>(); 
list1.add("a");
list1.add("b");
list1.add("c");
list1.add("d");
list1.add("e");
list1.add("f");

ArrayList<Integer> list2 = new ArrayList<Integer>();
list2.add(1);
list2.add(4);
list2.add(2);

The problem here is, you cannot remove from original list1 as the index will keep changing. I tried creating a temp HashMap to store the array index and String relation.

I iterate over list2 and map. When I found a matching key=index, I skipped that. Else I put String element in new list.

Any better suggestions?

Answer 1

This solution assumes that null is not a valid value in list1.

You could iterate through list2, and for each index you get to, set the corresponding value in list1 to null. Then remove all the nulls from list1 at the end. This will work even if there are duplicate elements in list2, which jlordo’s solution would have difficulty with.

for(Integer index : list2){
    list1.set(index,null);
}
list1.removeAll(Collections.singleton(null));