android - reverse the order of an array

Question

I have an array of objects.

Is it possible to make a new array that is a copy of this array, but in reverse order? I was looking for something like this.

// my array
ArrayList<Element> mElements = new ArrayList<Element>();
// new array
ArrayList<Element> tempElements = mElements;

tempElements.reverse(); // something to reverse the order of the array

score 232 · Accepted Answer · answered Mar 23 '11 at 22:38

232

You can do this in two steps:

ArrayList<Element> tempElements = new ArrayList<Element>(mElements);
Collections.reverse(tempElements);

answered Mar 23 '11 at 22:38

Ted Hopp

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Yikes, thanks Ted. Looks like I have some java.util.* reading to do. – Matthew Mar 23 '11 at 22:41

score 3 · Answer 2 · answered May 04 '21 at 14:56

3

Kotlin

val reverseList: List<Int> = yourActualList.reversed();

Reference

answered May 04 '21 at 14:56

Kishan Solanki

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score 0 · Answer 3 · answered Jan 11 '18 at 13:42

0

Simple approach without implementing anything.

                ArrayList<YourObject> oldlist = new ArrayList<YourObject>();
                ArrayList<YourObject> newList = new ArrayList<YourObject>();
                int size = oldlist.size()-1;

                for(int i=size;i>=0;i--){
                    newList.add(oldlist.get(i));
                }

answered Jan 11 '18 at 13:42

Samir

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An improvement would be to create `newList` with an initial capacity at least equal to the size of `oldList`. That would avoid internal memory reallocations as elements are added in the loop. – Ted Hopp Jul 20 '18 at 16:04

DarkCygnus · Answer 4 · 2018-07-20T16:32:58.553

0

For Android on Kotlin, this can be done with Anko's forEachReversedByIndex{} lambda operation, like this:

val tempElements = ArrayList<Element>(mElements.size)
mElements.forEachReversedByIndex{tempElements.add(it)}

edited Jul 20 '18 at 16:32

answered May 31 '18 at 23:48

DarkCygnus

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An improvement would be to create `tempElements` with an initial capacity at least equal to the size of `mElements`. That would avoid internal memory reallocations as elements are added in the second line. – Ted Hopp Jul 20 '18 at 16:03
@TedHopp thanks for the suggestion, will see to incorporate it to the answer :) – DarkCygnus Jul 20 '18 at 16:25

score 0 · Answer 5 · answered May 24 '20 at 23:54

I reversed the Profile from ascending to descending order by

In kotlin

 // A comparator to compare points of Profile 
class ProfileComparator {
    companion object : Comparator<Profile?> {
        override fun compare(o1: Profile?, o2: Profile?): Int {
            if (o1 == null || o2 == null) {
                return 0;
            }
            return o2.points.compareTo(o1.points)
        }
    }
}

and then

var profilesList = profiles.sortedWith(ProfileComparator)

android - reverse the order of an array

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