Collections/HashMap and threads

[Anamitra Bhattacharyya]

Hi All
I know that a Hashmap object should not be modified in multiple threads - but just on curousity basis -
If multiple threads add different values to the hashmap object - but for the same key - is there any chance that the hashmap might contain multiple entries for the same key?
I mean is it possible somehow for a hashmap to have mutiple entries for the same key?
thanks
Anamitra

[Dirk Schreckmann]

Anamitra,
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Thanks Pardners!

[Dirk Schreckmann]

is it possible somehow for a hashmap to have mutiple entries for the same key?
I sure hope not: "A map cannot contain duplicate keys; each key can map to at most one value."

[Jim Yingst]

The Map interface does not allow this directly. But you can create this effect yourself by putting a Collection of values into the Map. When you get() a value, test if (value instanceof Collection) - if so, cast it to Collection and iterate to get the values.

[Rajasekaran Anand]

Usually if u try to give multiple entries for the
same key the last entry will be the value for that
key.That is key will get overriden.So it is adopting one --> one policy.one -->many policy
is not achievable upto my knowledge that too
directly.
Anand

[Zakaria Haque]

Very interesting question. From HashMap.put(Object,Object) source in 1.3, it looks like multiple threads can actually put multiple entries in the hashtable with same key !!! Here is the code that adds new entry to the hashtable if there is no entry with the same key already esisting
----
Entry e = new Entry(hash, key, value, tab[index]);
tab[index] = e;
count++;
return null;
----
If there os no entry in the hashtable with the same key and one thread executes upto before this block, and then the other thread starts executing, you will end up with to Entry instances with the same key. !!
Please prove me wrong.

[Dirk Schreckmann]

import java.util.HashMap ;
 
public class Whatever extends Thread 
{
    static HashMap hm = new HashMap() ;
    
    String string = "" ;
    
    public Whatever( String string ) 
    {
        this.string = string ;
    }
    
    public void run() 
    {
        for ( int i = 0 ; i < 3 ; i++ ) 
        {
            hm.put( String.valueOf( i ) , string );
            yield() ;
        }
    }

public static void main( String[] args ) throws Exception 
    {
        Thread t1 = new Thread( new Whatever( "t1" ) );
        Thread t2 = new Thread( new Whatever( "t2" ) );
        
        t1.start() ;
        t2.start() ;
        
        Thread.sleep( 1000 );
        System.out.println( hm ); /* {2=t2, 1=t2, 0=t2} */
    }
}

[Ilja Preuss]

Originally posted by Zakaria Haque:
Very interesting question. From HashMap.put(Object,Object) source in 1.3, it looks like multiple threads can actually put multiple entries in the hashtable with same key !!!

Yes, they can. There can even happen far nastier things (like count being incremented just once when two entries were inserted). The new Collection classes just don't work properly in a multithreaded environment without proper synchronization.