Iterating through a list in reverse order in java

Question

I m migrating a piece of code to make use of generics  One argument for doing so is that the for loop is much cleaner than keeping track of indexes  or using an explicit iterator   In about half the cases  the list  an ArrayList  is being iterated in reverse order by using an index today   Can someone suggest a cleaner way of doing this  since I dislike the indexed for loop when working with collections   though it does work    for  int i   nodes size   - 1  i  gt   0  i--        final Node each    Node  nodes get i                Note  I can t add any new dependencies outside the JDK

User · Answer

To have code which looks like this   List lt Item gt  items      for  Item item   In reverse items                 Put this code into a file called  In java    import java util     public enum In        public static final  lt T gt  Iterable lt T gt  reverse final List lt T gt  list            return new ListReverseIterable lt T gt  list              class ListReverseIterable lt T gt  implements Iterable lt T gt            private final List lt T gt  mList           public ListReverseIterable final List lt T gt  list                mList   list                     public Iterator lt T gt  iterator                 return new Iterator lt T gt                      final ListIterator lt T gt  it   mList listIterator mList size                      public boolean hasNext                         return it hasPrevious                                      public T next                         return it previous                                      public void remove                         it remove

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This is an old question  but it s lacking a java8-friendly answer  Here are some ways of reverse-iterating the list  with the help of the Streaming API   List lt Integer gt  list   new ArrayList lt Integer gt  Arrays asList 1  3  3  7  5    list stream   forEach System out  println      1 3 3 7 5  int size   list size     ListIterator lt Integer gt  it   list listIterator size   Stream generate it  previous  limit size       forEach System out  println      5 7 3 3 1  ListIterator lt Integer gt  it2   list listIterator size   Stream iterate it2 previous    i - gt  it2 previous    limit size       forEach System out  println      5 7 3 3 1     If list is RandomAccess  i e  an ArrayList  IntStream range 0  size  map i - gt  size - i - 1  map list  get       forEach System out  println      5 7 3 3 1     If list is RandomAccess  i e  an ArrayList   less efficient due to sorting IntStream range 0  size  boxed   sorted Comparator reverseOrder         map list  get  forEach System out  println      5 7 3 3 1

User · Answer

You could use ReverseListIterator from Apache Commons-Collections   https   commons apache org proper commons-collections apidocs org apache commons collections4 iterators ReverseListIterator html

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You could use the concrete class LinkedList instead of the general interface List  Then you have a descendingIterator for iterating with the reverse direction   LinkedList lt String  gt  linkedList  for  Iterator lt String  gt  it   linkedList descendingIterator    it hasNext            String text   it next        Don t know why there is no descendingIterator with ArrayList

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Also found google collections reverse method

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Option 1  Have you thought about reversing the List with Collections reverse   and then using foreach   Of course  you may also want to refactor your code such that the list is ordered correctly so you don t have to reverse it  which uses extra space time     EDIT   Option 2  Alternatively  could you use a Deque instead of an ArrayList   It will allow you to iterate forwards and backwards    EDIT   Option 3  As others have suggested  you could write an Iterator that will go through the list in reverse  here is an example   import java util Iterator  import java util List   public class ReverseIterator lt T gt  implements Iterator lt T gt   Iterable lt T gt         private final List lt T gt  list      private int position       public ReverseIterator List lt T gt  list            this list   list          this position   list size   - 1              Override     public Iterator lt T gt  iterator             return this              Override     public boolean hasNext             return position  gt   0              Override     public T next             return list get position--               Override     public void remove             throw new UnsupportedOperationException               List lt String gt  list   new ArrayList lt String gt     list add  A    list add  B    list add  C    list add  D    list add  E     for  String s   new ReverseIterator lt String gt  list         System out println s

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Guava offers Lists reverse List  and ImmutableList reverse    As in most cases for Guava  the former delegates to the latter if the argument is an ImmutableList  so you can use the former in all cases   These do not create new copies of the list but just  reversed views  of it   Example  List reversed   ImmutableList copyOf myList  reverse

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Reason    Don t know why there is no descendingIterator with ArrayList      Since array list doesnot keep the list in the same order as data has been added to list  So  never use Arraylist    Linked list will keep the data in same order of ADD to list   So   above in my example  i used ArrayList   in order to make user to twist their mind and make them to workout something from their side   Instead of this  List lt String gt  list   new ArrayList lt String gt       USE   List lt String gt  list   new LinkedList lt String gt      list add  ravi     list add  kant     list add  soni        Iterate to disply   result will be as ---     ravi kant soni  for  String name   list               Now call this method  Collections reverse list       iterate and print index wise   result will be as ---     soni kant ravi  for  String name   list

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Try this      Substitute appropriate type  ArrayList lt     gt  a   new ArrayList lt     gt         Add elements to list      Generate an iterator  Start just after the last element  ListIterator li   a listIterator a size         Iterate in reverse  while li hasPrevious        System out println li previous

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As has been suggested at least twice  you can use descendingIterator with a Deque  in particular with a LinkedList  If you want to use the for-each loop  i e   have an Iterable   you can construct and use a wraper like this   import java util     public class Main        public static class ReverseIterating lt T gt  implements Iterable lt T gt            private final LinkedList lt T gt  list           public ReverseIterating LinkedList lt T gt  list                this list   list                      Override         public Iterator lt T gt  iterator                 return list descendingIterator                         public static void main String    args            LinkedList lt String gt  list   new LinkedList lt String gt             list add  A            list add  B            list add  C            list add  D            list add  E             for  String s   new ReverseIterating lt String gt  list                 System out println s

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Create a custom reverseIterable

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I don t think it s possible using the for loop syntax   The only thing I can suggest is to do something like   Collections reverse list   for  Object o   list                  but I wouldn t say this is  cleaner  given that it s going to be less efficient

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If the lists are fairly small so that performance is not a real issue  one can use the reverse-metod of the Lists-class in Google Guava  Yields pretty for-each-code  and the original list stays the same  Also  the reversed list is backed by the original list  so any change to the original list will be reflected in the reversed one   import com google common collect Lists          final List lt String gt  myList   Lists newArrayList  one    two    three    final List lt String gt  myReverseList   Lists reverse myList    System out println myList   System out println myReverseList    myList add  four     System out println myList   System out println myReverseList     Yields the following result    one  two  three   three  two  one   one  two  three  four   four  three  two  one    Which means that reverse iteration of myList can be written as   for  final String someString   Lists reverse myList           do something

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Very simple Example   List lt String gt  list   new ArrayList lt String gt      list add  ravi     list add  kant     list add  soni        Iterate to disply   result will be as ---     ravi kant soni  for  String name   list               Now call this method  Collections reverse list       iterate and print index wise   result will be as ---     soni kant ravi  for  String name   list

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Here is an  untested  implementation of a ReverseIterable   When iterator   is called it creates and returns a private ReverseIterator implementation  which simply maps calls to hasNext   to hasPrevious   and calls to next   are mapped to previous     It means you could iterate over an ArrayList in reverse as follows   ArrayList lt String gt  l       for  String s   new ReverseIterable l       System err println s       Class Definition  public class ReverseIterable lt T gt  implements Iterable lt T gt      private static class ReverseIterator lt T gt  implements Iterator       private final ListIterator lt T gt  it       public boolean hasNext           return it hasPrevious               public T next           return it previous               public void remove           it remove                 private final ArrayList lt T gt  l     public ReverseIterable ArrayList lt T gt  l        this l   l         public Iterator lt T gt  iterator         return new ReverseIterator l listIterator l size

[java] Iterating through a list in reverse order in java

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