Java LinkedHashSet backwards iteration

Question

How can I iterate through items of a LinkedHashSet from the last item to the first one?

Answer 1

If you want to continue to use collections, you could use the following:

LinkedHashSet<T> set = ...

LinkedList<T> list = new LinkedList<>(set);
Iterator<T> itr = list.descendingIterator();
while(itr.hasNext()) {
    T item = itr.next();
    // do something
}

If you're fine with using an array instead, you could take a look at hvgotcodes' answer.

Answer 2

er, assuming you mean LinkedHashSet...

I would use toArray and just use a reverse for loop.

There might be a better way to do it, but that should work. toArray guarantees any order is preserved

If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

Something like

Set<MyType> mySet = new LinkedHashSet();
...
MyType[] asArray = mySet.toArray(new MyType[0]);

for (int i = asArray.length - 1; i>=0; i--){
..
}

Answer 3

From the javadoc: "This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order)."

So you can simply:

LinkedHashSet<Integer> numbers = new LinkedHashSet<Integer>();
numbers.add(1);
numbers.add(2);
numbers.add(33);
numbers.add(44);
numbers.add(108);

for (Integer i : numbers) {
    System.out.println(i);
}

Answer 4

This is another way:

LinkedHashSet<T> set = ...

List<T> list = new ArrayList<>(set);
Collections.reverse(list);

for( T item : list ){
   ...
}

Answer 5

If you really meant LinkedHashSet, you could put the elements into an ArrayList and then use the ArrayList's ListIterator.

ListIterator<T> l = new ArrayList<T>(yourLinkedHashList).listIterator();
// ListIterator can iterate in reverse
while(l.hasPrevious()) {
    T obj = l.previous();
}

Answer 6

As of Java 21, the reversed() method on LinkedHashSet can be used. This reversed view of the set can be used to iterate in reverse order.

for (Object o : myLinkedHashSet.reversed()) {
  System.out.println(o);
}

Answer 7

Now from java 21 onwards you can use reversed() method of SequancedSet to reverse a collection element.

LinkedHashSet lhs=....
....
.....
SequencedSet sequencedSet=lhs.reversed();