What is the Simplest Way to Reverse an ArrayList

Question

What is the simplest way to reverse this ArrayList   ArrayList lt Integer gt  aList   new ArrayList lt  gt        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

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A little more readable     public static  lt T gt  ArrayList lt T gt  reverse ArrayList lt T gt  list        int length   list size        ArrayList lt T gt  result   new ArrayList lt T gt  length        for  int i   length - 1  i  gt   0  i--            result add list get i               return result

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We can also do the same using java 8   public static lt T gt  List lt T gt  reverseList List lt T gt  list            List lt T gt  reverse   new ArrayList lt  gt  list size              list stream                    collect Collectors toCollection LinkedList  new                    descendingIterator                    forEachRemaining reverse  add            return reverse

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Another recursive solution   public static String reverse ArrayList lt Float gt  list       if  list size      1           return      list get 0           else          return      list remove list size   - 1    reverse list

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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  lt T gt  List lt T gt  reverse final List lt T gt  list        final List lt T gt  result   new ArrayList lt  gt  list       Collections reverse result       return result      This approach works  but requires iterating over the list twice  The copy constructor  new ArrayList lt  gt  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  lt T gt  List lt T gt  reverse final List lt T gt  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 lt T gt  result   new ArrayList lt  gt  size           iterate through the list in reverse order and append to the result     for  int i   last  i  gt   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  lt T gt  List lt T gt  reverse final List lt T gt  list        final int last   list size   - 1      return IntStream rangeClosed 0  last     a stream of all valid indexes into the list          map i - gt   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

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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

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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 lt Object gt  reverse ArrayList lt Object gt  list        for  int i   0  i  lt  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

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ArrayList lt Integer gt  myArray   new ArrayList lt Integer gt      myArray add 1   myArray add 2   myArray add 3    int reverseArrayCounter   myArray size   - 1   for  int i   reverseArrayCounter  i  gt   0  i--        System out println myArray get i

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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 lt Object gt  reverse ArrayList lt Object gt  list        if list size    gt  1                               Object value   list remove 0           reverse list           list add value             return list      Or non-recursively   public ArrayList lt Object gt  reverse ArrayList lt Object gt  list        for int i   0  j   list size   - 1  i  lt  j  i              list add i  list remove j              return list

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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 lt String gt  arrayList   new ArrayList lt String gt             arrayList add  1            arrayList add  2            arrayList add  3            arrayList add  4            arrayList add  5            System out println  Reverse Order      reverse arrayList                public static  lt T gt  List lt T gt  reverse List lt T gt  arrayList            return reverse arrayList 0 arrayList size  -1             public static  lt T gt  List lt T gt  reverse List lt T gt  arrayList int startIndex int lastIndex             if startIndex lt lastIndex                T t arrayList get lastIndex               arrayList set lastIndex arrayList get startIndex                arrayList set startIndex t               startIndex                lastIndex--              reverse arrayList startIndex lastIndex                     return arrayList

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Simple way is that you have  quot Collections quot  in Java  You just need to call it and use  quot reverse   quot  method of it  Example usage  ArrayList lt Integer gt  yourArrayList   new ArrayList lt  gt         yourArrayList add 1       yourArrayList add 2       yourArrayList add 3         yourArrayList is  1 2 3  Collections reverse yourArrayList           Now  yourArrayList is  3 2 1  For more question   canerkaseler

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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 lt String gt  someDummyList   getDummyList            System out println someDummyList           int size   someDummyList size   - 1          ArrayList lt String gt  someDummyListRev   IntStream rangeClosed 0 size  mapToObj i- gt someDummyList get size-i   collect Collectors toCollection ArrayList  new            System out println someDummyListRev              private static ArrayList lt String gt  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

[java] What is the Simplest Way to Reverse an ArrayList?

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