Re: Collections and Decorators

ankur wrote:


It can't.

All synchronized collection does is wrap calls to Collection methods
like this:

   @Override
   public synchronized boolean add( E e ) {
     internal.add( e );
   }

where "internal" is the collection you wrapped. That's all the code is
capable of doing. It doesn't know that you might want to do something
else with the collection after add() completes, and anyway
synchronization in Java is in blocks and automatically releases once the
block is exited.

What it can't do is this:

    if( sync_c.contains( object ) ) {
       sync_c.remove( object )
    }

because in between the call to "contains" and "remove" another thread
might modify the sync_c and you can't control that. Thus, the external
lock is required:

   synchronized( sync_c ) {
      if( sync_c.contains( object ) ) {
         sync_c.remove( object )
      }
    }

Same with iterators -- you have to lock manually because the code inside
sync_c doesn't know, can't know, when you are done using the iterator.