How do I remove sequential elements from a Java ArrayList?

Question

I'm a relatively new Java programmer and I'm having difficuly removing more than one element from an ArrayList. Ideally I'd like to do something like this:

ArrayList ar1 = new ArrayList();
ar1.add(...)
ar1.add(...)
ar1.add(...)
ar1.add(...)

for (int i = 0; i < 2; i++){
     ar1.remove(i);
}

I think iterator might help, but I can't find an example that matches close enough to what I'm trying to do. Any help would be appreciated. Thanks.

Answer 1

Here's what you want to do:

ar1.subList(0, 2).clear();

This creates a sublist view of the first 2 elements of the list and then clears that sublist, removing them from the original list. The subList method exists primarily for this sort of thing... doing operations on a specific range of the list.

Answer 2

You can certainly do that

    ArrayList ar1 = new ArrayList();
    ar1.add("a");
    ar1.add("b");
    ar1.add("c");
    ar1.add("d");

    for (int i = 0; i < 2; i++) {
        ar1.remove(i);
    }
    System.out.println(ar1);

Only pay attention that after you remove first element, other elements shift. Thus, calling

ar1.remove(0);
ar1.remove(1);

will effectively remove first and third elements from the list. This will delete first two elements, though:

ar1.remove(0);
ar1.remove(0);

Answer 3

For indexed removals from a list, you need to count backwards:

 for (int i = 1; i >= 0; i--)

otherwise, your first removal shifts the items "above" it in the collection and you don't wind up removing the items you think you are removing.

Answer 4

You can use Collection.removeAll(toRemove) if you have a separate list of objects to remove.

http://download.oracle.com/javase/6/docs/api/java/util/Collection.html

If your collection is indexed based, like ArrayList is, you can call

remove(index)

to remove the element at the index. You can do that in a loop, but beware that removing shifts all the indexes as another answer points out.

If all you want to do is remove the first two elements from the list, then

   list.remove(0);
   list.remove(0);

should do it.

Answer 5

If you know the indexes of the items you want to remove, you can remove them in reverse order, without worrying about shifting indexes:

    ArrayList ar1 = new ArrayList();
    ar1.add("a");
    ar1.add("b");
    ar1.add("c");
    ar1.add("d");

    int[] indexesToRemove = {0,2,3};
    Arrays.sort(indexesToRemove);
    for (int i=indexesToRemove.length-1; i>=0; i--) {
        ar1.remove(indexesToRemove[i]);
    }

Answer 6

You could try this:

List<Whatever> l = new ArrayList<Whatever>();
l.add(someStuff);
Iterator<Whatever> it = l.iterator();
int i = 0;
while (i < 2 && it.hasNext()) {
    it.next();
    it.remove();
    i++;
}

Or, more generally:

List<Whatever> l = new ArrayList<Whatever>();
l.add(someStuff);
Iterator<Whatever> it = l.iterator();
while (it.hasNext()) {
    Whatever next = it.next();
    if (shouldRemove(next)) {
        it.remove();
    }
}

EDIT: I guess it depends if you are trying to remove particular indices or particular objects. It also depends on how much logic you need to decide if something should be removed. If you know the indices then remove them in reverse order. If you have a set of Objects to be removed, then use removeAll. If you want to iterate over the list and remove objects that match a predicate then use the above code.