Is there a basic Java Set implementation that does

Question

The API for the Java Set interface states:

For example, some implementations prohibit null elements and some have restrictions on the types of their elements

I am looking for a basic Set implementation that does not require ordering (as ArrayList provides for the List interface) and that does not permit null. TreeSet, HashSet, and LinkedHashSet all allow null elements. Additionally, TreeSet has the requirement that elements implement Comparable.

It seems that no such basic Set exists currently. Does anyone know why? Or if one does exist where I can find it?

[Edit]: I do not want to allow nulls, because later in the code my class will iterate over all elements in the collection and call a specific method. (I'm actually using HashSet<MyRandomObject>). I would rather fail fast than fail later or accidentally incur some bizarre behavior due to a null being in the set.

Answer 1

I would say use composition instead of inheritance... it might be more work but it'll be more stable in the face of any changes that Sun might make to the Collections Framework.

public class NoNullSet<E> implements Set<E>
{
   /** The set that is wrapped. */
   final private Set<E> wrappedSet = new HashSet<E>();

   public boolean add(E e)
   {
     if (e == null) 
       throw new IllegalArgumentException("You cannot add null to a NoNullSet");
     return wrappedSet.add(e);
   }

   public boolean addAll(Collection<? extends E> c)
   {
     for (E e : c) add(e);
   }

   public void clear()
   { wrappedSet.clear(); }

   public boolean contains(Object o)
   { return wrappedSet.contains(o); }

   ... wrap the rest of them ...
}

Note that this implementation does not depend on addAll calling add (which is an implementation detail and should not be used because it cannot be guaranteed to remain true in all Java releases).

Answer 2

You could use apache collections and its PredicatedCollection class, and set the predicate to not allow nulls. You will get exceptions if someone sends nulls in.

Answer 3

Better than extending a particular implementation, you can easily write a proxy implementation of Set that checks for nulls. This analogous to Collections.checkedSet. Other than being applicable to any implementation, you can also be sure that you have overridden all applicable methods. Many flaws have been found by extending concrete collections which then have additional methods added in later versions.