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From: Michal Kleczek <kleku75@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Concurrent bidirectional one-to-many map?
Date: Mon, 09 May 2011 16:42:53 +0200
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Sebastian wrote:

>
> Thanks for the idea. I do not yet see how to deal with the
> one-to-many aspect of my problem.
> 
> To give an example, I'm trying to solve a problem like this:
> Associate tasks with workspaces, where a workspace may hold many
> tasks,but a task may be associate with at most one workspace.
> 
> Idea:
> 
>     private ConcurrentMap<Item, Workspace> wsMap =
>               new ConcurrentHashMap<Item, Workspace>();
> 
>      public boolean isAssigned( Item item ) {
>          return wsMap.containsKey( item );
>      }
> 
>      public void assign( Item item, Workspace ws ) {
>          wsMap.putIfAbsent( item, ws );
>          if( !ws.equals( wsMap.get( item) ) ) {
>             throw new NotAssignableException();
>          }
>      }
> 
> 
> Now I want to be able to close a workspace, releasing all tasks
> to be assignable again to other workspaces.
> 
>      public void closeWorkspace( Workspace ws ) {
>        // how do this efficiently? iterate over all
>        // entries (holding a write lock) and remove it
>        // when the value equals ws?
>      }
> 

Maybe it is a dumb question but: is there any reason not to implement it 
simply as

class Item {
  private Workspace workspace;
  //getters setters
}

class Workspace {
  private Collection<Item> items;
  //getters setters

  public close() {
    for (final Item item : items) {
      //release item
    }
  }
}

-- 
Michal