Re: Cannot seem to lock HashMap

Lew <>
Thu, 16 Aug 2007 18:37:43 -0400
<> wrote:

I found the following from Sun:

"Note that constructors cannot be synchronized - using the
synchronized keyword with a constructor is a syntax error.
Synchronizing constructors doesn't make sense, because only the thread
that creates an object should have access to it while it is being


So it would seem that I should use clone() method, but this is not
available from the Map that is returned from the
Collections.synchronizedMap method. So it seems that I cannot do what
I would like because of this. I think I either have to use Hashtable,
or use a wrapper class (above example) to do the locking.

Constructors have nothing to do with your issue.

Your problem stemmed from not synchronizing your puts or gets on the Map.

You might also consider just declaring the Map instance variable as a
synchronized Map in the first place, thus avoiding the Hashtable gotchas.
(Speed isn't one of them.)

public static class TestContainer

A "static" class? Was this a nested class? If so, you should post the outer
class, too, to make an SSCCE. If not, then you should post code that will

And stop embedding TAB characters in Usenet posts.
   private final Map<Date, Date> values;
// use the interface, not the concrete class

  public TestContainer()

       this( new HashMap<Date, Date>() );



  public TestContainer( Map<Date, Date> v)

  //again, use the interface
       if ( v == null )
          throw new IllegalArgumentException( "Map must not be null." );

    values = Collections.synchronizedMap( v );

  private void put(Date key, Date value)


      values.put( key, value );

// See? /Now/ this is synchronized. It wasn't before.


  public Date get(Date key)


    return values.get(key);

// See? /Now/ this is synchronized. It wasn't before.


  public Object clone()

       // prevent concurrent mod during clone()
       synchronized ( values )
         Map<Date, Date> cValues = new HashMap<Date, Date>( values );
         return new TestContainer( cValues );


Another approach is to copy the Map in the constructor, thus avoiding most of
the issue.


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