List Implementations - Java Tutorial 5.0 英文版

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List Implementations

List implementations are grouped into general-purpose and special-purpose implementations.

General-Purpose List Implementations

There are two general-purpose List implementations — ArrayList and LinkedList. Most of the time, you'll probably use ArrayList, which offers constant-time positional access and is just plain fast. It does not have to allocate a node object for each element in the List, and it can take advantage of System.arraycopy when it has to move multiple elements at the same time. Think of ArrayList as Vector without the synchronization overhead.

If you frequently add elements to the beginning of the List or iterate over the List to delete elements from its interior, you should consider using LinkedList. These operations require constant-time in a LinkedList and linear-time in an ArrayList. But you pay a big price in performance. Positional access requires linear-time in a LinkedList and constant-time in an ArrayList. Furthermore, the constant factor for LinkedList is much worse. If you think you want to use a LinkedList, measure the performance of your application with both LinkedList and ArrayList before making your choice: ArrayList is usually faster.

ArrayList has one tuning parameter — the initial capacity, which refers to the number of elements the ArrayList can hold before it has to grow. LinkedList has no tuning parameters and seven optional operations, one of which is clone. The other six are addFirst, getFirst, removeFirst, addLast, getLast, and removeLast. LinkedList also implements the Queue interface.

Special-Purpose List Implementations

CopyOnWriteArrayList is a List implementation backed up by a copy-on-write array. This implementation is similar in nature to CopyOnWriteArraySet. No synchronization is necessary, even during iteration, and iterators are guaranteed never to throw ConcurrentModificationException. This implementation is well suited to maintaining event-handler lists, in which change is infrequent, and traversal is frequent and potentially time-consuming.

If you need synchronization, a Vector will be slightly faster than an ArrayList synchronized with Collections.synchronizedList. But Vector has loads of legacy operations, so be careful to always manipulate the Vector with the List interface, or you won't be able to replace the implementation at a later time.

If your List is fixed in size — that is, you'll never use remove, add, or any of the bulk operations other than containsAll — you have a third option that's definitely worth considering. See Arrays.asList in the Convenience Implementations section for more information.

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