Reverse iteration through ArrayList gives IndexOutOfBoundsException

Question

When I reverse iterate over an ArrayList I am getting a IndexOutOfBoundsException. I tried doing forward iteration and there is no problem. I expect and know that there are five

Answer 1

You can reverse by one line that is

Collections.reverse(list);

ArrayList arrayList = new ArrayList();

arrayList.add("A");
arrayList.add("B");

System.out.println("Before Reverse Order : " + arrayList);

Collections.reverse(arrayList);

System.out.println("After Reverse : " + arrayList);

Output

Before Reverse Order : [A, B]
After Reverse : [B, A]

Answer 2

Java arrays are zero-indexed. You will have to set j = list.size() - 1 and continue until j = 0.

Answer 3

The most elegant way is to reverse the array and then use a direct (or even implicit) iterator :

Collections.reverse(arrayList);
for (Object item : arrayList) {
    ...
}

Answer 4

I know this is an old question, but Java contains a Collections.reverse( List<T> ) method. Why wouldn't you just reverse it and do forward iteration?

Answer 5

Avoid indexes altogether? How about:

for (ListIterator iterator = list.listIterator(list.size()); iterator.hasPrevious();) {
  final Object listElement = iterator.previous();
}

Answer 6

Start the iteration at list.size() - 1 because array (or ArrayList) elements are numbered from 0 up through 1 less than the size of the list. This is a fairly standard idiom:

for (int j = list.size() - 1; j >= 0; j--) {
    // whatever
}

Note that your forward iteration works because it stops before reaching list.size().