Concrete Collections

Rather than getting into more details about all the interfaces, we thought it would be helpful to first discuss the concrete data structures that the Java library supplies. Once we have thoroughly described the classes you might want to use, we will return to abstract considerations and see how the collections framework organizes these classes. Table 13–1 shows the collections in the Java library and briefly describes the purpose of each collection class. (For simplicity, we omit the thread-safe collections that will be discussed in Chapter 14.) All classes in Table 13–1 implement the Collection interface, with the exception of the classes with names ending in Map. Those classes implement the Map interface instead. We will discuss the Map interface in the section “Maps” on page 680.

Table 13–1. Concrete Collections in the Java Library

Collection Type	Description	See Page
ArrayList	An indexed sequence that grows and shrinks dynamically	668
LinkedList	An ordered sequence that allows efficient insertions and removal at any location	659
ArrayDeque	A double-ended queue that is implemented as a circular array	678
HashSet	An unordered collection that rejects duplicates	668
TreeSet	A sorted set	672
EnumSet	A set of enumerated type values	687
LinkedHashSet	A set that remembers the order in which elements were inserted	686
PriorityQueue	A collection that allows efficient removal of the smallest element	679
HashMap	A data structure that stores key/value associations	680
TreeMap	A map in which the keys are sorted	680
EnumMap	A map in which the keys belong to an enumerated type	687
LinkedHashMap	A map that remembers the order in which entries were added	686
WeakHashMap	A map with values that can be reclaimed by the garbage collector if they are not used elsewhere	685
IdentityHashMap	A map with keys that are compared by ==, not equals	688