What is the Simplest Way to Reverse an ArrayList?

Question

What is the simplest way to reverse this ArrayList?

ArrayList aList = new ArrayList<>();

//Add elements to ArrayList object
aList.add(\         


        

Answer 1

Not the simplest way but if you're a fan of recursion you might be interested in the following method to reverse an ArrayList:

public ArrayList<Object> reverse(ArrayList<Object> list) {
    if(list.size() > 1) {                   
        Object value = list.remove(0);
        reverse(list);
        list.add(value);
    }
    return list;
}

Or non-recursively:

public ArrayList<Object> reverse(ArrayList<Object> list) {
    for(int i = 0, j = list.size() - 1; i < j; i++) {
        list.add(i, list.remove(j));
    }
    return list;
}

Answer 2

A little more readable :)

public static <T> ArrayList<T> reverse(ArrayList<T> list) {
    int length = list.size();
    ArrayList<T> result = new ArrayList<T>(length);

    for (int i = length - 1; i >= 0; i--) {
        result.add(list.get(i));
    }

    return result;
}

Answer 3

We can also do the same using java 8.

public static<T> List<T> reverseList(List<T> list) {
        List<T> reverse = new ArrayList<>(list.size());

        list.stream()
                .collect(Collectors.toCollection(LinkedList::new))
                .descendingIterator()
                .forEachRemaining(reverse::add);

        return reverse;
    }

Answer 4

Just in case we are using Java 8, then we can make use of Stream. The ArrayList is random access list and we can get a stream of elements in reverse order and then collect it into a new ArrayList.

public static void main(String[] args) {
        ArrayList<String> someDummyList = getDummyList();
        System.out.println(someDummyList);
        int size = someDummyList.size() - 1;
        ArrayList<String> someDummyListRev = IntStream.rangeClosed(0,size).mapToObj(i->someDummyList.get(size-i)).collect(Collectors.toCollection(ArrayList::new));
        System.out.println(someDummyListRev);
    }

    private static ArrayList<String> getDummyList() {
        ArrayList dummyList = new ArrayList();
        //Add elements to ArrayList object
        dummyList.add("A");
        dummyList.add("B");
        dummyList.add("C");
        dummyList.add("D");
        return dummyList;
    }

The above approach is not suitable for LinkedList as that is not random-access. We can also make use of instanceof to check as well.