Can one do a for each loop in java in reverse order

Question

I need to run through a List in reverse order using Java    So where this does it forwards   for String string  stringList        do something     Is there some way to iterate the stringList in reverse order using the for each syntax   For clarity  I know how to iterate a list in reverse order but would like to know  for curiosity s sake   how to do it in the for each style

User · Answer

This may be an option  Hope there is a better way to start from last element than to while loop to the end   public static void main String   args                List lt String gt  a   new ArrayList lt String gt         a add  1   a add  2   a add  3   a add  4   a add  5         ListIterator lt String gt  aIter a listIterator                while aIter hasNext    aIter next         for   aIter hasPrevious                   String aVal   aIter previous            System out println aVal

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The Collections reverse method actually returns a new list with the elements of the original list copied into it in reverse order  so this has O n  performance with regards to the size of the original list   As a more efficient solution  you could write a decorator that presents a reversed view of a List as an Iterable  The iterator returned by your decorator would use the ListIterator of the decorated list to walk over the elements in reverse order   For example   public class Reversed lt T gt  implements Iterable lt T gt        private final List lt T gt  original       public Reversed List lt T gt  original            this original   original             public Iterator lt T gt  iterator             final ListIterator lt T gt  i   original listIterator original size              return new Iterator lt T gt                  public boolean hasNext     return i hasPrevious                  public T next     return i previous                  public void remove     i remove                            public static  lt T gt  Reversed lt T gt  reversed List lt T gt  original            return new Reversed lt T gt  original             And you would use it like   import static Reversed reversed        List lt String gt  someStrings   getSomeStrings    for  String s   reversed someStrings         doSomethingWith s

User · Answer

For a list  you could use the Google Guava Library   for  String item   Lists reverse stringList                    Note that Lists reverse doesn t reverse the whole collection  or do anything like it - it just allows iteration and random access  in the reverse order  This is more efficient than reversing the collection first   To reverse an arbitrary iterable  you d have to read it all and then  replay  it backwards    If you re not already using it  I d thoroughly recommend you have a look at the Guava  It s great stuff

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You can use the Collections class to reverse the list then loop

User · Answer

You d need to reverse your collection if you want to use the for each syntax out of the box and go in reverse order

User · Answer

A work Around     Collections reverse stringList  forEach str - gt          Or with guava     Lists reverse stringList  forEach str - gt

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AFAIK there isn t a standard  reverse iterator  sort of thing in the standard library that supports the for-each syntax which is already a syntactic sugar they brought late into the language    You could do something like for Item element  myList clone   reverse    and pay the associated price    This also seems fairly consistent with the apparent phenomenon of not giving you convenient ways to do expensive operations - since a list  by definition  could have O N  random access complexity  you could implement the interface with a single-link   reverse iteration could end up being O N 2   Of course  if you have an ArrayList  you don t pay that price

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As of the comment  You should be able to use Apache Commons ReverseListIterator  Iterable lt String gt  reverse        new IteratorIterable new ReverseListIterator stringList     for String string  reverse             do something     As  rogerdpack said  you need to wrap the ReverseListIterator as an Iterable

User · Answer

Definitely a late answer to this question  One possibility is to use the ListIterator in a for loop  It s not as clean as colon-syntax  but it works   List lt String gt  exampleList   new ArrayList lt  gt     exampleList add  One    exampleList add  Two    exampleList add  Three       Forward iteration for  String currentString   exampleList        System out println currentString         Reverse iteration for  ListIterator lt String gt  itr   exampleList listIterator exampleList size     itr hasPrevious      no-op           String currentString   itr previous        System out println currentString        Credit for the ListIterator syntax goes to  Ways to iterate over a list in Java

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All answers above only fulfill the requirement  either by wrapping another method or calling some foreign code outside   Here is the solution copied from the Thinking in Java 4th edition  chapter 11 13 1 AdapterMethodIdiom   Here is the code      The  Adapter Method  idiom allows you to use foreach    with additional kinds of Iterables  package holding  import java util      SuppressWarnings  serial   class ReversibleArrayList lt T gt  extends ArrayList lt T gt      public ReversibleArrayList Collection lt T gt  c    super c       public Iterable lt T gt  reversed         return new Iterable lt T gt            public Iterator lt T gt  iterator             return new Iterator lt T gt                int current   size   - 1    why this size   or super size   wrong            public boolean hasNext     return current  gt  -1              public T next     return get current--               public void remove        Not implemented             throw new UnsupportedOperationException                                                    public class AdapterMethodIdiom     public static void main String   args        ReversibleArrayList lt String gt  ral         new ReversibleArrayList lt String gt           Arrays asList  To be or not to be  split                Grabs the ordinary iterator via iterator        for String s   ral        System out print s             System out println           Hand it the Iterable of your choice     for String s   ral reversed          System out print s                  Output  To be or not to be be to not or be To

User · Answer

Not without writing some custom code which will give you an enumerator which will reverse the elements for you   You should be able to do it in Java by creating a custom implementation of Iterable which will return the elements in reverse order   Then  you would instantiate the wrapper  or call the method  what-have-you  which would return the Iterable implementation which reverses the element in the for each loop

User · Answer

This will mess with the original list and also needs to be called outside of the loop  Also you don t want to perform a reverse every time you loop - would that be true if one of the Iterables reverse ideas was applied   Collections reverse stringList    for String string  stringList        do something

User · Answer

The List  unlike the Set  is an ordered collection and iterating over it does preserve the order by contract  I would have expected a Stack to iterate in the reverse order but unfortunately it doesn t  So the simplest solution I can think of is this   for  int i   stack size   - 1  i  gt   0  i--        System out println stack get i        I realize that this is not a  for each  loop solution  I d rather use the for loop than introducing a new library like the Google Collections   Collections reverse   also does the job but it updates the list as opposed to returning a copy in reverse order

[java] Can one do a for each loop in java in reverse order?

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