LivingWithNullHostileCollections

Null-hostile collections

Many JDK collection types permit null elements:

• ArrayList
• LinkedList
• Hash{Set,Map}
• LinkedHash{Set,Map}
• Tree{Set,Map} (with suitable comparator)
• IdentityHashMap
• EnumMap (values)
• CopyOnWriteArray{List,Set}

But many don't:

• EnumSet
• EnumMap (keys)
• ConcurrentHashMap
• ConcurrentSkipList{Set,Map}
• All ten Queue implementations except LinkedList

Likewise in Guava we have many general-purpose collections that permit null:

• ArrayListMultimap
• HashBiMap
• HashMulti{set,map}
• LinkedHashMulti{set,map}
• TreeMulti{set,map} (with suitable comparator)
• LinkedListMultimap
• MutableClassToInstanceMap
• HashBasedTable
• Sets.union/intersection/difference

but many that do not:

• ConcurrentHashMultiset
• EnumBiMap
• EnumMultiset
• MinMaxPriorityQueue
• Interners
• MapMaker-made maps
• Sets.cartesianProduct/powerSet
• All implementations of ImmutableCollection and ImmutableMap

But what if?

What if you find yourself wanting to put a null element into one of these null-hostile beasts?

• If in a Set or as a key in a Map -- don't; it's clearer (less surprising) if you explicitly special-case null during lookup operations
• If as a value in a Map -- leave out that entry; keep a separate Set of non-null keys (or null keys)
• If in a List -- if the list is sparse, might you rather use a Map<Integer, E>?
• Consider if there is a natural "null object" that can be used. There isn't always. But sometimes.
  • example: if it's an enum, add a constant to mean whatever you're expecting null to mean here.
• Just use a different collection implementation, for example Collections.unmodifiableList(Lists.newArrayList()) instead of ImmutableList.
• Mask the nulls (this needs more detail)
• Use Optional<T>

See also: using and avoiding null.