In which class should i locate static final instance?

[Garrett Rowe]

interface Filter<T>
{
   Collection<T> filter(Collection<T> coll);
}
 
interface Displayable
{
   String display;
}
 
class Entry implements Displayable
{
   //...
}
 
class UIDataModel implements Displayable
{
   private Set<Entry> entrySet;
 
   //The displayFilter is used to filter out any entries that I dont want to
   //display in a particular view.
   private Filter<Entry> displayFilter;
 
   private Comparator<Entry> entryComparator;
 
   //...
 
   public void setDisplayFilter(final Filter<Entry> filter)
   {
      //validateNonNull(filter);
      diaplayFilter = filter;
   }
   
   //This is how I want the body of this method to look
   public String display()
   {
      Set<Entry> displayableEntries = 
              new TreeSet<Entry>(entryComparator).addAll(displayFilter.filter(entrySet));
      
      //add each entry to a StringBuilder and return it
   }
}

For this to work, the displayFilter always has to be non - null. I thought about implementing the Null-Object pattern and creating a NULL_FILTER

final static Filter NULL_FILTER = new Filter()
{
   public Collection<T> filter(Collection<T> coll)
   {
      //do nothing
      return coll;
   }
};

But I couldn't decide where/how to implement it. I could implement it in my UIDataModel and set the displayFilter = NULL_FILTER whenever the client method called setFilter(null). But that would introduce ugly passing of null arguments and checking for nulls that always makes me wince. I could implement it as its own class but client classes would have to instantiate a new NullFilter() every time they wanted to stop filtering results. Seems kinda clumsy, but I'm not too experienced in such matters. Or should I not have a filter in this class at all and let the View handle the filtering of the results. Although I suspect this is a bad idea if the goal is to make the View as *dumb* as possible. How would you guys handle this situation?
[ July 06, 2006: Message edited by: Garrett Rowe ]

[Stan James]

I like the null object pattern idea, but this time you might just have a default implementation. Make it a full blown class and your UI can default its variable:

private Filter filter = new DefaultFilter();

Your setFilter() method would throw an exception on a null argument. Clients would create a DefaultFilter() or a new FancyFilter() or whatever they want to use.

Now that I've typed that I don't much like DefaultFilter as a name. Maybe PassAllFilter or something.

[Garrett Rowe]

Thanks for the advice Stan. I couldn't decide whether this was exposing too much of the implmentation to other classes.