Subtracting one arrayList from another arrayList

Try to use subtract method of org.apache.commons.collections.CollectionUtils class.

Returns a new Collection containing a - b. The cardinality of each element e in the returned Collection will be the cardinality of e in a minus the cardinality of e in b, or zero, whichever is greater.

CollectionUtils.subtract(java.util.Collection a, java.util.Collection b) 

From Apache Commons Collections

Is there some reason you can't simply use List.removeAll(List)?

    List<Integer> one = new ArrayList<Integer>();
    one.add(1);
    one.add(2);
    one.add(3);
    List<Integer> two = new ArrayList<Integer>();
    two.add(0);
    two.add(2);
    two.add(4);
    one.removeAll(two);
    System.out.println(one);

    result: "[1, 3]"

Java 8

You can also use streams:

List<Integer> list1 =  Arrays.asList(1, 2, 3);
List<Integer> list2 =  Arrays.asList(1, 2, 4, 5);
List<Integer> diff = list1.stream()
                          .filter(e -> !list2.contains(e))
                          .collect (Collectors.toList()); // (3)

This answer does not manipulate the original list. If intention is to modify the original list then we can use remove. Also we can use forEach (default method in Iterator) or stream with filter.

Using ListUtils

Another option is to use ListUtils if we are using Apache common:

ListUtils.subtract(list, list2)

This subtracts all elements in the second list from the first list, placing the results in a new list. This differs from List.removeAll(Collection) in that cardinality is respected; if list1 contains two occurrences of null and list2 only contains one occurrence, then the returned list will still contain one occurrence.