Colin Decker edited this page Oct 17, 2017 · 4 revisions

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.

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