Make iterators cloneable?

[Dave Brosius]

Hi,


The failfast argument is is irrelevant. It only matters if you use 
it.remove() which is no different than any other failfast situation.

Obviously the 3rd argument is bogus too, as you say.

The second argument, ok, is not arguable, as it's just a persons 
opinion. The bug report is in the genre of subList implementation which 
is different than this use, but ok. The problem is that there are no 
reasonable concurrent indexable collections, and the thought of copying 
large collections into a List everytime you want to do this is crazy.


Unfortunately you can't even have one-off fixes for specific collections 
by having an iterator with a 'copy constructor' for instance, as in the 
case of collection iterators you _really_ have no business knowing what 
the real class is.


On 09/11/2016 05:40 AM, Remi Forax wrote:

> Hi,
> digging a little bit in the bug database, there is a bug from 1999
>    https://bugs.openjdk.java.net/browse/JDK-4244952
>
> answers seems to be
> - having different iterators on the same collection is error prone because iterators are failfast,
>    the proposed workaround to use an array or a List with several indexes
> - it's too C++ish, C++ defines clone and equals, Java tries to define a simpler iteration protocol
>    (from now, given that this question is raised only once in a while, it's doesn't seem to be a bad idea)
> - iterators are interfaces, so having several iterators means several virtual dispatch calls
>    (this one is not true anymore because any JITs routinely devirtualize this kind of call).
>
> cheers,
> Rémi
>
> ----- Mail original -----
>> De: "Dave Brosius" <dbrosius at mebigfatguy.com>
>> À: "Louis Wasserman" <lowasser at google.com>, "Jonathan Bluett-Duncan" <jbluettduncan at gmail.com>
>> Cc: core-libs-dev at openjdk.java.net
>> Envoyé: Dimanche 11 Septembre 2016 02:23:41
>> Objet: Re: Make iterators cloneable?
>>>> Iterators reading from a BufferedReader
>> yes that's true. altho given the contract of Cloneable, adding the clone
>> method on Iterator would be safe, as it would only be available if the
>> real implementing class 'extends Cloneable'... it's actually kind of
>> funny that Object methods aren't available on interface references, anyway.
>>
>>
>> But i get the "age of Iterator" answer.. Shame there isn't an answer, tho.
>>
>> thanks, dave
>>
>> On 09/10/2016 07:44 PM, Louis Wasserman wrote:
>>> Some iterators might be.  Many may not be. Certainly Iterator as an
>>> interface has been out there for long enough there are Iterator
>>> implementations out there that aren't cloneable -- say, Iterators
>>> reading from a BufferedReader, where there really won't be any way to
>>> do what you're hoping for; BufferedReaders certainly aren't cloneable.
>>>
>>> On Sat, Sep 10, 2016 at 4:33 PM Dave Brosius <dbrosius at mebigfatguy.com
>>> <mailto:dbrosius at mebigfatguy.com>> wrote:
>>>
>>>      Yes Louis is correct.
>>>
>>>      I want the pair wise associations or all elements of a set.
>>>
>>>      Fee-Fi
>>>
>>>      Fee-Fo
>>>
>>>      Fee-Fum
>>>
>>>      Fi-Fo
>>>
>>>      Fi-Fum
>>>
>>>      Fo-Fum
>>>
>>>
>>>      the independent iterators produce Fee-Fee (etc) as well as the
>>>      duplicate Fee-Fi and Fi-Fee (etc), both of which i don't want.
>>>
>>>
>>>      This is obviously simplistic with index based collections, but not
>>>      with sets/maps
>>>
>>>      I don't see why an Iterator isn't by nature easily cloneable.
>>>
>>>
>>>
>>>      On 09/10/2016 06:45 PM, Jonathan Bluett-Duncan wrote:
>>>>      Ah okay Louis, if that's the case then that certainly makes
>>>>      sense, and I'd agree that there's no good way of doing so, as one
>>>>      would need to copy the set into a list.
>>>>
>>>>      Dave, did Louis hit the mark? If not, would you kindly go into
>>>>      further detail as to exactly what it is you're trying to do?
>>>>
>>>>      Best,
>>>>      Jonathan
>>>>
>>>>      On 10 September 2016 at 23:36, Jonathan Bluett-Duncan
>>>>      <jbluettduncan at gmail.com <mailto:jbluettduncan at gmail.com>> wrote:
>>>>
>>>>          Hi Dave,
>>>>
>>>>          Rather than using Iterator.clone(), how about you just call
>>>>          collection.iterator() 2 times to return 2 unique, non-same
>>>>          iterators; something like the following:
>>>>
>>>>          import java.util.Collections;
>>>>          import java.util.Iterator;
>>>>          import java.util.Set;
>>>>          import java.util.concurrent.ConcurrentHashMap;
>>>>
>>>>          public class Example {public static void main(String[] args) { Set<String> s =
>>>>          Collections.newSetFromMap(new ConcurrentHashMap<String,
>>>>          Boolean>()); s.add("Fee"); s.add("Fi"); s.add("Fo");
>>>>          s.add("Fum"); Iterator<String> it1 = s.iterator();      for (String v1 =null;
>>>>          it1.hasNext(); v1 =it1.next()) {
>>>>                 Iterator<String> it2 = s.iterator();// a completely separate iterator to it1 for
>>>>                 (String v2 =null; it2.hasNext(); v2 = it2.next()) {System.out.println(v1 + "
>>>>                 <-->" + v2); } } } }
>>>>
>>>>          Or, even better, if you're using Java 5+, you can skip using
>>>>          Iterators altogether and use for-loops directly:
>>>>
>>>>          import java.util.Collections;
>>>>          import java.util.Set;
>>>>          import java.util.concurrent.ConcurrentHashMap;
>>>>
>>>>          public class Example {public static void main(String[] args) { Set<String> s =
>>>>          Collections.newSetFromMap(new ConcurrentHashMap<String,
>>>>          Boolean>()); s.add("Fee"); s.add("Fi"); s.add("Fo");
>>>>          s.add("Fum");      for (String v1 : s) {
>>>>                 for (String v2 : s) {System.out.println(v1 + "<-->" + v2); } } } }
>>>>
>>>>          Kind regards,
>>>>          Jonathan
>>>>          On 10 September 2016 at 23:13, Dave Brosius
>>>>          <dbrosius at mebigfatguy.com <mailto:dbrosius at mebigfatguy.com>>
>>>>          wrote:
>>>>
>>>>              It would be nice to be able to associate each element in
>>>>              a collection with another element in the collection,
>>>>              which is something very easily done with index based
>>>>              collections, but with sets, etc this isn't so easy...
>>>>              unless i'm having a brainfart. So i'd like to do this,
>>>>              but Iterator doesn't implement Cloneable... Any reason
>>>>              not to? or is there another way that's missing me? public
>>>>              class ItClone {     public static void main(String[]
>>>>              args) {         Set<String> s =
>>>>              Collections.newSetFromMap(new ConcurrentHashMap<String,
>>>>              Boolean>());         s.add("Fee");         s.add("Fi");
>>>>                    s.add("Fo");         s.add("Fum");
>>>>              Iterator<String> it1 = s.iterator();         while
>>>>              (it1.hasNext()) {             String v1 = it1.next();
>>>>                      Iterator<String> it2 = (Iterator<String>)
>>>>              it1.*clone*();             while (it2.hasNext()) {
>>>>                        String v2 = it2.next();
>>>>              System.out.println(v1 + " <-->" + v2);             }
>>>>                  }     } }