java collections

ArrayList

• permit null.

• not synchronized.

  List list = Collections.synchronizedList(new ArrayList(...))

• an application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation.

  ensureCapacity(int minCapacity)

Vector

• synchronized.

Stack

• extends class Vector.
• a more complete and consistent set of LIFO stack operations is provided by the Deque interface and its implementations.

LinkedList

see also Queue.

Hashtable

• the keys must implement the hashCode method and the equals method.
• only non-null object can be used as a key or as a value.

• synchronized.

  Hashtable(int initialCapacity, float loadFactor=.75)

HashMap

• permits null values and the null key.

• not synchronized.

  Map m = Collections.synchronizedMap(new HashMap(...))

LinkedHashMap

• permits null values and the null key.

• maintains a doubly-linked list running through all of its entries which defines the iteration ordering (insertion-order or access order which its entries were last accessed, from least-recently accessed to most-recently. This is well-suited to building LRU caches).

  LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)

• the removeEldestEntry(Map.Entry) method may be overridden to impose a policy for removing stale mappings automatically when new mappings are added to the map.

  1 2 3 4

  private static final int MAX_ENTRIES = 100; protected boolean (Map.Entry eldest) { return size() > MAX_ENTRIES; }

• not synchronized.

TreeMap

• the implementation based on Red-Black tree.
• permits null values.
• sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time.
• the ordering maintained by a sorted map must be consistent with equals if this sorted map is to correctly implement the Map interface.

• not synchronized.

  SortedMap m = Collections.synchronizedSortedMap(new TreeMap(...))

ConcurrentHashMap

Queue

each of these methods exists in two forms: one throws an exception if the operation fails, the other returns a special value (null or false).

• Queue implementations generally do not allow insertion of null elements, although some implementations, such as LinkedList, but it should not be inserted into a Queue, as null is also used as a special return value by the poll method to indicate that the queue contains no elements.

PriorityQueue

• based on a priority heap. Ordered according to their natural ordering, or by a Comparator provided at queue construction time.

  public PriorityQueue(Comparator<? super E> comparator)

• not synchronized.

Deque

each of these methods exists in two forms: one throws an exception if the operation fails, the other returns a special value (null or false).

• this interface provides two methods to remove interior elements, removeFirstOccurrence and removeLastOccurrence.

ArrayDeque

• resizable-array implementation of the Deque interface.
• null elements are prohibited.
• not thread-safe.
• faster than Stack when used as a stack, and faster than LinkedList when used as a queue.

LinkedList

• doubly-linked list implementation of the List and Deque interfaces.
• permits all elements (including null).

• not synchronized.

  List list = Collections.synchronizedList(new LinkedList(...))

BlockingQueue

the third method blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up.

• does not accept null elements.
• thread-safe.
• designed to be used primarily for producer-consumer queues.