CODEWITHSUNDEEP
javacollectionsconcurrency
Daniel Rikowski
Daniel Rikowski

Reputation: 72514

When I sort a List what happens to its iterators?

Let's say I have a List object and an iterator for that list.

Now I sort the list with java.util.Collections.sort()

I know, this problem could be circumvented by changing the program design, cloning the list for example, but I specificly want to know the "official" behavior of Java.

Upvotes: 12

Views: 6023

Answers (4)

sblundy
sblundy

Reputation: 61424

Most of the collections in java.util are "fail-fast" and may throw a ConcurrentModificationException if the underlying collection is changed. It should be pointed out that this is intended for debugging and so is not guaranteed. According to the javadocs, this is true of all decedents of AbstractList, but this is not true of CopyOnWriteArrayList, which is intended for multi-threaded use.

Upvotes: 16

MaitreKaio
MaitreKaio

Reputation: 31

I wrote some code to see what happens when a collection is sorted while you are iterating. It seems that the iterator does not throw any exceptions, but continues to iterate normally. Still it gives you wrong results if you are expecting to iterate over the collection unsorted. Look at that :

public static void main(String[] args) {
    List<String> list = new ArrayList<String>();
    list.add("D");
    list.add("B");
    list.add("A");
    list.add("C");
    list.add("E");

    Iterator<String> it = list.iterator();
    String s = it.next();
    System.out.println(s);
    s = it.next();
    System.out.println(s);

    Collections.sort(list);
    Iterator<String> it2 = list.iterator();

    s = it.next();
    System.out.println(s);
    s = it.next();
    System.out.println(s);
    s = it.next();
    System.out.println(s);

    while (it2.hasNext()) {
        System.out.println(it2.next());
    }
    }

Hope it helps.

Upvotes: 2

Tom Hawtin - tackline
Tom Hawtin - tackline

Reputation: 147164

Generally, any kind of mutation on a collection will invalidate its iterators. A mutation done through an iterator will not invalidate that iterator. There are some exceptional collection implementations, such as CopyOnWriteArrayList.

The general solution would be to sort a copy of the collection or recreate your iterators.

Upvotes: 4

Jon Skeet
Jon Skeet

Reputation: 1501163

Iterators are generally invalid after any modification to their underlying collections, except via the iterator itself. (For instance, ListIterator allows for insertion and removal.)

I'd certainly expect any iterators to become invalidated after a sort though - and if they weren't, I'd have no idea what order to expect.

Upvotes: 19

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