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

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 someDummyList = getDummyList();
        System.out.println(someDummyList);
        int size = someDummyList.size() - 1;
        ArrayList someDummyListRev = IntStream.rangeClosed(0,size).mapToObj(i->someDummyList.get(size-i)).collect(Collectors.toCollection(ArrayList::new));
        System.out.println(someDummyListRev);
    }

    private static ArrayList 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.