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

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<T> implements Iterable<T> {
    private final List<T> original;

    public Reversed(List<T> original) {
        this.original = original;
    }

    public Iterator<T> iterator() {
        final ListIterator<T> i = original.listIterator(original.size());

        return new Iterator<T>() {
            public boolean hasNext() { return i.hasPrevious(); }
            public T next() { return i.previous(); }
            public void remove() { i.remove(); }
        };
    }

    public static <T> Reversed<T> reversed(List<T> original) {
        return new Reversed<T>(original);
    }
}

And you would use it like:

import static Reversed.reversed;

...

List<String> someStrings = getSomeStrings();
for (String s : reversed(someStrings)) {
    doSomethingWith(s);
}

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.)

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
}

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.

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<String> a = new ArrayList<String>();
    a.add("1");a.add("2");a.add("3");a.add("4");a.add("5");

    ListIterator<String> aIter=a.listIterator();        
    while(aIter.hasNext()) aIter.next();

    for (;aIter.hasPrevious();)
    {
        String aVal = aIter.previous();
        System.out.println(aVal);           
    }
}

A work Around :

Collections.reverse(stringList).forEach(str -> ...);

Or with guava :

Lists.reverse(stringList).forEach(str -> ...);

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.

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

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.

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<String> exampleList = new ArrayList<>();
exampleList.add("One");
exampleList.add("Two");
exampleList.add("Three");

//Forward iteration
for (String currentString : exampleList) {
    System.out.println(currentString); 
}

//Reverse iteration
for (ListIterator<String> 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"

E.g.

Integer[][] a = {
                { 1, 2, 3 }, 
                { 4, 5, 6 }, 
                { 9, 8, 9 }, 
                };

List<List<Integer>> arr = Arrays.stream(a)
                .map(Arrays::asList)
                .collect(Collectors.toList());

Reverse it now.

Collections.reverse(arr);
System.out.println("Reverse Array" + arr);
for (List<Integer> obj : arr) {
    // TODO
}