java sets

Sets

• HashSet – a set implementation based on a HashMap with dummy values (same Object is used for every value). Has the same properties as a HashMap. Due to such implementation, consumes more memory than actually required for this data structure.
• EnumSet – a set of enum values. Each enum in Java is mapped into an int: one distinct int for each enum value. This allows to use a BitSet-like structure for this collection, where each bit is mapped to a distinct enum value. There are 2 actual implementations – RegularEnumSet backed up by a single long (and capable to store up to 64 enum values, which covers 99.9% use cases) and JumboEnumSet backed by a long[].
• BitSet – a bit set. You should always keep in mind that you may use a BitSet for representing a dense set of integers (like ids starting from a number known in advance). This class uses a long[] for bit storage.
• LinkedHashSet – like HashSet, this class is implemented on top of a LinkedHashMap. This is the only set which keeps its elements in the insertion order.
• TreeSet – like HashSet, this class is based on a TreeMap instance. This is the only sorted set in the single threaded part of the standard JDK.