CODEWITHSUNDEEP
javaandroidarraylistcollections
Ishu
Ishu

Reputation: 5457

What is the Simplest Way to Reverse an ArrayList?

What is the simplest way to reverse this ArrayList?

ArrayList<Integer> aList = new ArrayList<>();

//Add elements to ArrayList object
aList.add("1");
aList.add("2");
aList.add("3");
aList.add("4");
aList.add("5");

while (aList.listIterator().hasPrevious())
  Log.d("reverse", "" + aList.listIterator().previous());

Upvotes: 381

Views: 430354

Answers (13)

ΓDΛ
ΓDΛ

Reputation: 11060

2023 Edition for Kotlin

asReversed

Returns a reversed read-only view of the original List. All changes made in the original list will be reflected in the reversed one.

val original = mutableListOf('a', 'b', 'c', 'd', 'e')
val originalReadOnly = original as List<Char>
    val reversed = originalReadOnly.asReversed()

println(original) // [a, b, c, d, e]
println(reversed) // [e, d, c, b, a]

// changing the original list affects its reversed view
original.add('f')
println(original) // [a, b, c, d, e, f]
println(reversed) // [f, e, d, c, b, a]

original[original.lastIndex] = 'z'
println(original) // [a, b, c, d, e, z]
println(reversed) // [z, e, d, c, b, a]

reversed

Returns a list with elements in reversed order.

val myList = listOf(1, 2, 3)
println(myList.reversed())    // [3, 2, 1]

Upvotes: -1

rmuller
rmuller

Reputation: 12849

As of Java 21 you can use ArrayList#reversed(). Where ArrayList implements SequencedCollection.

This is part of Sequenced Collections added to the Collections framework.

Upvotes: 6

Kishan Solanki
Kishan Solanki

Reputation: 14618

Kotlin Users

val reverse: List<Int> = list.reversed();

Happy Coding!

Reference

Upvotes: 1

vijayraj34
vijayraj34

Reputation: 2415

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;
    }

Upvotes: 3

contrapost
contrapost

Reputation: 693

Solution without using extra ArrayList or combination of add() and remove() methods. Both can have negative impact if you have to reverse a huge list. 

 public ArrayList<Object> reverse(ArrayList<Object> list) {

   for (int i = 0; i < list.size() / 2; i++) {
     Object temp = list.get(i);
     list.set(i, list.get(list.size() - i - 1));
     list.set(list.size() - i - 1, temp);
   }

   return list;
 }

Upvotes: 6

Joby Wilson Mathews
Joby Wilson Mathews

Reputation: 11116

Reversing a ArrayList in a recursive way and without creating a new list for adding elements :

   public class ListUtil {

    public static void main(String[] args) {
        ArrayList<String> arrayList = new ArrayList<String>();
        arrayList.add("1");
        arrayList.add("2");
        arrayList.add("3");
        arrayList.add("4");
        arrayList.add("5");
        System.out.println("Reverse Order: " + reverse(arrayList));

    }

    public static <T> List<T> reverse(List<T> arrayList) {
        return reverse(arrayList,0,arrayList.size()-1);
    }
    public static <T> List<T> reverse(List<T> arrayList,int startIndex,int lastIndex) {

        if(startIndex<lastIndex) {
            T t=arrayList.get(lastIndex);
            arrayList.set(lastIndex,arrayList.get(startIndex));
            arrayList.set(startIndex,t);
            startIndex++;
            lastIndex--;
            reverse(arrayList,startIndex,lastIndex);
        }
        return arrayList;
    }

}

Upvotes: 2

Tolunay Guney
Tolunay Guney

Reputation: 71

ArrayList<Integer> myArray = new ArrayList<Integer>();

myArray.add(1);
myArray.add(2);
myArray.add(3);

int reverseArrayCounter = myArray.size() - 1;

for (int i = reverseArrayCounter; i >= 0; i--) {
    System.out.println(myArray.get(i));
}

Upvotes: 7

naomimyselfandi
naomimyselfandi

Reputation: 355

The trick here is defining "reverse". One can modify the list in place, create a copy in reverse order, or create a view in reversed order.

The simplest way, intuitively speaking, is Collections.reverse:

Collections.reverse(myList);

This method modifies the list in place. That is, Collections.reverse takes the list and overwrites its elements, leaving no unreversed copy behind. This is suitable for some use cases, but not for others; furthermore, it assumes the list is modifiable. If this is acceptable, we're good.

If not, one could create a copy in reverse order:

static <T> List<T> reverse(final List<T> list) {
    final List<T> result = new ArrayList<>(list);
    Collections.reverse(result);
    return result;
}

This approach works, but requires iterating over the list twice. The copy constructor (new ArrayList<>(list)) iterates over the list, and so does Collections.reverse. We can rewrite this method to iterate only once, if we're so inclined:

static <T> List<T> reverse(final List<T> list) {
    final int size = list.size();
    final int last = size - 1;

    // create a new list, with exactly enough initial capacity to hold the (reversed) list
    final List<T> result = new ArrayList<>(size);

    // iterate through the list in reverse order and append to the result
    for (int i = last; i >= 0; --i) {
        final T element = list.get(i);
        result.add(element);
    }

    // result now holds a reversed copy of the original list
    return result;
}

This is more efficient, but also more verbose.

Alternatively, we can rewrite the above to use Java 8's stream API, which some people find more concise and legible than the above:

static <T> List<T> reverse(final List<T> list) {
    final int last = list.size() - 1;
    return IntStream.rangeClosed(0, last) // a stream of all valid indexes into the list
        .map(i -> (last - i))             // reverse order
        .mapToObj(list::get)              // map each index to a list element
        .collect(Collectors.toList());    // wrap them up in a list
}

nb. that Collectors.toList() makes very few guarantees about the result list. If you want to ensure the result comes back as an ArrayList, use Collectors.toCollection(ArrayList::new) instead.

The third option is to create a view in reversed order. This is a more complicated solution, and worthy of further reading/its own question. Guava's Lists#reverse method is a viable starting point.

Choosing a "simplest" implementation is left as an exercise for the reader.

Upvotes: 25

akhil_mittal
akhil_mittal

Reputation: 24157

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.

Upvotes: 1

Shankar Agarwal
Shankar Agarwal

Reputation: 34765

Collections.reverse(aList);

Example (Reference):

ArrayList aList = new ArrayList();
//Add elements to ArrayList object
aList.add("1");
aList.add("2");
aList.add("3");
aList.add("4");
aList.add("5");
Collections.reverse(aList);
System.out.println("After Reverse Order, ArrayList Contains : " + aList);

Upvotes: 895

dt94
dt94

Reputation: 117

Another recursive solution

 public static String reverse(ArrayList<Float> list) {
   if (list.size() == 1) {
       return " " +list.get(0);
   }
   else {
       return " "+ list.remove(list.size() - 1) + reverse(list);
   } 
 }

Upvotes: -1

Yas
Yas

Reputation: 5471

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;
}

Upvotes: 0

todd
todd

Reputation: 1286

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;
}

Upvotes: 22

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