Set implementations are grouped into general-purpose and special-purpose implementations.
General-Purpose Set Implementations
There are three general-purpose
Set implementations —
TreeSet , and
LinkedHashSet . Which of these three to use is generally straightforward.
HashSet is much faster than
TreeSet (constant-time versus log-time for most operations) but offers no ordering guarantees. If you need to use the operations in the
SortedSet interface, or if value-ordered iteration is required, use
TreeSet; otherwise, use
HashSet. It's a fair bet that you'll end up using
HashSet most of the time.
LinkedHashSet is in some sense intermediate between
TreeSet. Implemented as a hash table with a linked list running through it, it provides insertion-ordered iteration (least recently inserted to most recently) and runs nearly as fast as
LinkedHashSet implementation spares its clients from the unspecified, generally chaotic ordering provided by
HashSet without incurring the increased cost associated with
One thing worth keeping in mind about
HashSet is that iteration is linear in the sum of the number of entries and the number of buckets (the capacity). Thus, choosing an initial capacity that's too high can waste both space and time. On the other hand, choosing an initial capacity that's too low wastes time by copying the data structure each time it's forced to increase its capacity. If you don't specify an initial capacity, the default is 16. In the past, there was some advantage to choosing a prime number as the initial capacity. This is no longer true. Internally, the capacity is always rounded up to a power of two. The initial capacity is specified by using the
int constructor. The following line of code allocates a
HashSet whose initial capacity is 64.
Set<String> s = new HashSet<String>(64);
HashSet class has one other tuning parameter called the load factor. If you care a lot about the space consumption of your
HashSet, read the
HashSet documentation for more information. Otherwise, just accept the default; it's almost always the right thing to do.
If you accept the default load factor but want to specify an initial capacity, pick a number that's about twice the size to which you expect the set to grow. If your guess is way off, you may waste a bit of space, time, or both, but it's unlikely to be a big problem.
LinkedHashSet has the same tuning parameters as
HashSet, but iteration time is not affected by capacity.
TreeSet has no tuning parameters.
Special-Purpose Set Implementations
EnumSet is a high-performance
Set implementation for enum types. All of the members of an enum set must be of the same enum type. Internally, it is represented by a bit-vector, typically a single
long. Enum sets support iteration over ranges of enum types. For example, given the enum declaration for the days of the week, you can iterate over the weekdays. The
EnumSet class provides a static factory that makes it easy.
for (Day d : EnumSet.range(Day.MONDAY, Day.FRIDAY)) System.out.println(d);
Enum sets also provide a rich, typesafe replacement for traditional bit flags.
CopyOnWriteArraySet is a
Set implementation backed up by a copy-on-write array. All mutative operations, such as
remove, are implemented by making a new copy of the array; no locking is ever required. Even iteration may safely proceed concurrently with element insertion and deletion. Unlike most
Set implementations, the
contains methods require time proportional to the size of the set. This implementation is only appropriate for sets that are rarely modified but frequently iterated. It is well suited to maintaining event-handler lists that must prevent duplicates.