iterator to go backwards

Discussion in 'Java' started by Ike, Jul 8, 2003.

  1. Ike

    Ike Guest

    How do I make an Iterator go through the elements,
    (SortedMap.keySet().iterator() ) in reverse order? Thanks Ike
    Ike, Jul 8, 2003
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  2. Ike wrote:
    > How do I make an Iterator go through the elements,
    > (SortedMap.keySet().iterator() ) in reverse order? Thanks Ike

    You can't. General Iterators don't work that way. You have some

    (1) Use a Comparator on your SortedMap that reverses the sense of the
    comparison. That's only useful if it is reasonable to use the reverse
    order globally within the map.

    (2) Dump the key set into a different collection (or an array) where you
    can sort it how you want. You will still need an appropriate Comparator
    with which to define the order. You could use a SortedSet, a List, or
    an array (via toArray()) as the intermediate object; in the latter two
    cases you would first populate it and than sort it with Collections.sort
    or Arrays.sort, respectively. If performance matters then the array
    approach will probably be fastest, the List should be reasonably fast
    (depending on the type of list), and the SortedSet will probably be
    considerably slower (because it is re-sorted with every insertion).
    None will be as fast as keeping the map in the opposite order in the
    first place, of course.

    John Bollinger
    John C. Bollinger, Jul 8, 2003
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  3. Søren Bak

    Søren Bak Guest

    There is no support for backward iteration in Iterators.

    You can use recursion or a stack to solve the problem, but you should only
    do this for relatively small sets (or if performance is not an issue).

    Here is a sketch of a new iterator-class that uses a stack internally to do
    the reversal:

    class ReverseIterator implements Iterator {

    Stack stack;

    public ReverseIterator(Iterator iterator) {
    stack = new Stack();
    while (iterator.hasNext())

    public boolean hasNext() {
    return !stack.isEmpty();

    public Object next() {
    return stack.pop();


    Søren Bak

    "Ike" <> skrev i en meddelelse
    > How do I make an Iterator go through the elements,
    > (SortedMap.keySet().iterator() ) in reverse order? Thanks Ike
    Søren Bak, Jul 8, 2003
  4. Roedy Green

    Roedy Green Guest

    On Tue, 08 Jul 2003 20:56:32 GMT, "Ike" <> wrote or
    quoted :

    >How do I make an Iterator go through the elements,
    >(SortedMap.keySet().iterator() ) in reverse order? Thanks Ike

    you define the Comparable in reverse for a different set. You can
    extract the elements as an array and sort them in reverse order.
    Canadian Mind Products, Roedy Green.
    Coaching, problem solving, economical contract programming.
    See for The Java Glossary.
    Roedy Green, Jul 9, 2003
  5. I'll have to do some reading about Map, but couldn't aMap's elements be copied
    to a collection that could be reverse()d and iterated over?

    No need to answer. I'll read.

    remove dashes in email for replies
    =?ISO-8859-15?Q?Thomas_Gagn=E9?=, Jul 14, 2003
  6. Jason

    Jason Guest

    With an iterator you can't, but where there's a need, a way presents.
    What you can do is extend the iterator class and add methods for
    navigating backwards by keeping track of the current index of the
    ArrayList. You would also have to overwrite hasNext() and next()
    while providing your own hasPrevious() and previous() methods. The
    critical element is that every time you get an element, you will have
    to increment or decrement the index pointer to make sure that you are
    where you're supposed to be.


    "Ike" <> wrote in message news:<4eGOa.90219$>...
    > How do I make an Iterator go through the elements,
    > (SortedMap.keySet().iterator() ) in reverse order? Thanks Ike
    Jason, Jul 15, 2003
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