Re: hashset contains wtf

[Martin Kuba]

Jiri Mares wrote:
>> Javadoc k contains() i remove() se dedi ze Set,
> 
> To neni pravda. Vynatek ze zdrojaku JDK6 k tride HashSet:
> Ja tu nic o dedeni javadocu nevidim.

Aha, omlouvam se za nepresnost, nedival jsem se do zdrojaku.
Nicmene javadoc ke contains() u HashSet jsou *stejne* jako u Set
a dokonce u Collection. A podle mne contains u HashSet porusuje
kontrakt od Collection. Zkuste si tohle:


import java.util.*;
import java.util.concurrent.CopyOnWriteArraySet;

public class MutovaniHash {
    public static void main(String[] args) {
        List<Integer> list;
        Set<List<Integer>> set;

        list = new ArrayList<Integer>();
        set = new HashSet<List<Integer>>();

        test(set, list);

        list = new ArrayList<Integer>();
        set = new CopyOnWriteArraySet<List<Integer>>();

        test(set, list);
    }

    private static void test(Collection<List<Integer>> collection, List<Integer>
list) {
        collection.add(list);
        list.add(1);
        System.out.println("collection = " + 
collection.getClass().getSimpleName());
        System.out.println("collection.contains(list) = " +
collection.contains(list));
        for(List<Integer> prvek : collection) {
            if(prvek.equals(list)) {
                System.out.println("list je obsazen ");
                System.out.println("prvek.hashCode() = " + prvek.hashCode());
                System.out.println("list.hashCode() = " + list.hashCode());
            }
        }
    }
}


Dostanete:

collection = HashSet
collection.contains(list) = false
list je obsazen
prvek.hashCode() = 32
list.hashCode() = 32

collection = CopyOnWriteArraySet
collection.contains(list) = true
list je obsazen
prvek.hashCode() = 32
list.hashCode() = 32


Cili HashSet ten zmutovany list *obsahuje* (contains),
protoze ho muzu najit iteratorem, pri porovnani equals()
dostanu true a hashCode() vraci to same, cili plni kontrakt.

Ale zaroven jeji metoda contains() vraci false, ackoliv jine
implementace Set se chovaji korektne.

IMHO tedy je javadoc ke contains() u HashSet spatne.

Makub
-- 
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Supercomputing Center Brno             Martin Kuba
Institute of Computer Science    email: [EMAIL PROTECTED]
Masaryk University             http://www.ics.muni.cz/~makub/
Botanicka 68a, 60200 Brno, CZ     mobil: +420-603-533775
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