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copied!<p>In Scala 2.7.7, you get a NullPointer straight away when adding null:</p> <pre><code> scala> val s = new scala.collection.mutable.HashSet[String] s: scala.collection.mutable.HashSet[String] = Set() scala> s += null java.lang.NullPointerException at scala.collection.mutable.FlatHashTable$class.elemHashCode(FlatHashTable.scala:144) at scala.collection.mutable.HashSet.elemHashCode(HashSet.scala:31) at scala.collection.mutable.FlatHashTable$class.addEntry(FlatHashTable.scala:66) at scala.collection.mutable.HashSet.addEntry(HashSet.scala:31) at scala.collection.mutable.HashSet.$plus$eq(HashSet.s... </code></pre> <p>Using Scala 2.8.0.RC7, that is no longer the case. However, as <a href="https://stackoverflow.com/questions/3152167/scala-2-8-mutable-set-null-handling/3152988#3152988">Randall has observed</a>, the behaviour is not entirely consistent either:</p> <pre><code> scala> import scala.collection.mutable.HashSet import scala.collection.mutable.HashSet scala> val s = new HashSet[String] s: scala.collection.mutable.HashSet[String] = Set() scala> s += null res0: s.type = Set() scala> s += null res1: s.type = Set() scala> s.size res2: Int = 2 scala> s.head java.util.NoSuchElementException: next on empty iterator at scala.collection.Iterator$$anon$3.next(Iterator.scala:29) at scala.collection.Iterator$$anon$3.next(Iterator.scala:27) at scala.collection.mutable.FlatHashTable$$anon$1.next(FlatHashTable.scala:176) at scala.collection.IterableLike$class.head(IterableLike.scala:102) at scala.collection.mutable.HashSet.head(HashSet.scala:38) at .(:8) </pre> <p>Most of the work when adding an entry is done in <a href="https://lampsvn.epfl.ch/trac/scala/browser/scala/trunk/src/library/scala/collection/mutable/FlatHashTable.scala" rel="nofollow noreferrer">FlatHashTable</a> When you add an entery, <code>addEntry</code> is invoked, which looks like this:</p> <pre><code> /** The actual hash table. */ @transient protected var table: Array[AnyRef] = new Array(initialCapacity) def addEntry(elem: A) : Boolean = { var h = index(elemHashCode(elem)) var entry = table(h) while (null != entry) { if (entry == elem) return false h = (h + 1) % table.length entry = table(h) } table(h) = elem.asInstanceOf[AnyRef] tableSize = tableSize + 1 if (tableSize >= threshold) growTable() true } </code></pre> <p><code>null.asInstanceOf[AnyRef]</code> just gives you back <code>null</code>, and the <code>elemHashCode(elem)<code> returns zero if the elem is <code>null</code>. So in this case, the element with index zero is 'initialized' with <code>null</code> (which is the value it already had), but the tableSize is increased. This explains the results that are seen in the REPL.</p> <p>If you call <code>head</code> on the Set, ultimately the iterator in <code>FlatHashTable</code> is called, as can be seen from the stacktrace. This is implemented as follows:</p> <pre><code> iterator = new Iterator[A] { private var i = 0 def hasNext: Boolean = { while (i < table.length && (null == table(i))) i += 1 i < table.length } def next(): A = if (hasNext) { i += 1; table(i - 1).asInstanceOf[A] } else Iterator.empty.next } </code></pre> <p><code>hasNext</code> will return false, since the element that we stored in the Array is null, therefore, the call to <code>next</code> will call Iterator.empty.next</code>, which throws an exception. Since adding null</code> to a HashSet is not handled consistently in Scala 2.8, this indeed looks like a bug. Also, as already has been remarked, it provides another good reason to stay away from using null</code> where possible.</p> <p><strong>Edit</strong> Since I couldn't find a ticket for this, I <a href="https://lampsvn.epfl.ch/trac/scala/ticket/3630" rel="nofollow noreferrer">added it</a>.</p>

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