Neat way to check whether a Set does not contain a null

Question

I have a method that is given a Set of objects. A method it delegates to requires that the Set does not contain any null elements. I would like to check the precondition that the Set contains no null elements early, in the method before the delegation. The obvious code do do so is this:

public void scan(Set<PlugIn> plugIns) {
   if (plugIns == null) {
      throw new NullPointerException("plugIns");
   } else if (plugIns.contains(null)) {
      throw new NullPointerException("plugIns null element");
   }
   // Body
 }

But this is incorrect, because Set.contains() may throw a NullPointerException if the Set implementation itself does not permit null elements. Catching then ignoring the NullPointerException in that case would work but would be inelegant. Is there a neat way to check this precondition?

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Is there a design flaw in the Set interface? If a Set implementation may never contain a null, why not instead require Set.contains(null) to always return false? Or have a isNullElementPermitted() predicate?

Answer 1

The simplest way would be to enumerate the Set and check for nulls.

public void scan(Set<PlugIn> plugIns) {
  if (plugIns == null) throw new NullPointerException("plugIns");
  for (PlugIn plugIn : plugIns) {
    if (plugIn == null) throw new NullPointerException("plugIns null element");
  }
}