Petr proposed an interesting change of the FileSet
API in order to enable to generalize current
filtering to operations like union or intersection.

So I would integrate following modifications to
realize this enhancement:

1. introduce interface ContentListener + class
ContentEvent(extending EventObject)
  ContentListener
    contentChanged(ContentEvent)
  ContentEvent(Content source)
    
2. Replace Filter class with abstract class
Content containing
  public void addContentListener(ContentListener);
  public void removeContentListener(ContentListener);
  public abstract boolean contains(URL); // moved
from FileSet
  protected final void
fireContentChanged(ContentEvent) // moved from FileSet

3. FileSet will extend Content
  remove addPropertyChangeListener,
removePropertyChangeListener, fireContentChanged,
contains, PROP_CONTENT
  
4. MutableFileSet
  replace removeAll(FileSet) with removeAll(Content)
  
5. FileSets
  remove filterSet
  add methods:
  Content union(Content, Content)
  Content intersection(Content, Content)
   

The original proposal is accessible at
http://projects.netbeans.org/servlets/ReadMsg?msgId=381936&listName=dev