Iterating through a list in reverse order in java

Allain_Lalonde_2443 0
Viewed 257056

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 >= 0; i--) {
    final Node each = (Node) nodes.get(i);
    ...
 }

Note: I can't add any new dependencies outside the JDK.

collections java
17 Answers

Try this:

// Substitute appropriate type.
ArrayList<...> a = new ArrayList<...>();

// 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());
}
user_751700

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.

user_1274790

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<T> implements Iterator<T>, Iterable<T> {

    private final List<T> list;
    private int position;

    public ReverseIterator(List<T> list) {
        this.list = list;
        this.position = list.size() - 1;
    }

    @Override
    public Iterator<T> iterator() {
        return this;
    }

    @Override
    public boolean hasNext() {
        return position >= 0;
    }

    @Override
    public T next() {
        return list.get(position--);
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException();
    }

}


List<String> list = new ArrayList<String>();
list.add("A");
list.add("B");
list.add("C");
list.add("D");
list.add("E");

for (String s : new ReverseIterator<String>(list)) {
    System.out.println(s);
}
Kevin_266200

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<String> l = ...
for (String s : new ReverseIterable(l)) {
  System.err.println(s);
}

Class Definition

public class ReverseIterable<T> implements Iterable<T> {
  private static class ReverseIterator<T> implements Iterator {
    private final ListIterator<T> it;

    public boolean hasNext() {
      return it.hasPrevious();
    }

    public T next() {
      return it.previous();
    }

    public void remove() {
      it.remove();
    }
  }

  private final ArrayList<T> l;

  public ReverseIterable(ArrayList<T> l) {
    this.l = l;
  }

  public Iterator<T> iterator() {
    return new ReverseIterator(l.listIterator(l.size()));
  }
}
user_1274790

Create a custom reverseIterable.

user_1536210

Also found google collections reverse method.

Allain_Lalonde_24430

How about using DeQue:

  var queue = new ArrayDeque<>(list);
  while (!queue.isEmpty()) {
    var first = reversed ? queue.removeLast() : queue.removeFirst();
    var second = reversed ? queue.peekLast() : queue.peekFirst();
    if (second != null) {
      //your code goes here
    }
  }
user_183202500 
Valid for Java 9+

List<String> strList = List.of("a", "b", "c", "d", "e");

IntStream.iterate(strList.size() - 1, i -> i >= 0, i -> --i)
         .mapToObj(strList::get)
         .forEach(System.out::println);
user_160780830
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