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Java Standard Edition (SE)

Outline of the Collections Framework


Outline of the Collections Framework

The collections framework consists of:
  • Collection interfaces - The primary means by which collections are manipulated.
    • Collection - A group of objects. No assumptions are made about the order of the collection (if any) or whether it can contain duplicate elements.
    • Set - The familiar set abstraction. No duplicate elements permitted. May or may not be ordered. Extends the Collection interface.
    • List - Ordered collection, also known as a sequence. Duplicates are generally permitted. Allows positional access. Extends the Collection interface.
    • Queue - A collection designed for holding elements before processing. Besides basic Collection operations, queues provide additional insertion, extraction, and inspection operations.
    • Deque - A double ended queue, supporting element insertion and removal at both ends. Extends the Queue interface.
    • Map - A mapping from keys to values. Each key can map to one value.
    • SortedSet - A set whose elements are automatically sorted, either in their natural ordering (see the Comparable interface) or by a Comparator object provided when a SortedSet instance is created. Extends the Set interface.
    • SortedMap - A map whose mappings are automatically sorted by key, either using the natural ordering of the keys or by a comparator provided when a SortedMap instance is created. Extends the Map interface.
    • NavigableSet - A SortedSet extended with navigation methods reporting closest matches for given search targets. A NavigableSet may be accessed and traversed in either ascending or descending order.
    • NavigableMap - A SortedMap extended with navigation methods returning the closest matches for given search targets. A NavigableMap can be accessed and traversed in either ascending or descending key order.
    • BlockingQueue - A Queue with operations that wait for the queue to become nonempty when retrieving an element and that wait for space to become available in the queue when storing an element. (This interface is part of the java.util.concurrent package.)
    • TransferQueue - A BlockingQueue in which producers can wait for consumers to receive elements. (This interface is part of the java.util.concurrent package.)
    • BlockingDeque - A Deque with operations that wait for the deque to become nonempty when retrieving an element and wait for space to become available in the deque when storing an element. Extends both the Deque and BlockingQueue interfaces. (This interface is part of the java.util.concurrent package.)
    • ConcurrentMap - A Map with atomic putIfAbsent, remove, and replace methods. (This interface is part of the java.util.concurrent package.)
    • ConcurrentNavigableMap - A ConcurrentMap that is also a NavigableMap.
  • General-purpose implementations - The primary implementations of the collection interfaces.
    • HashSet - Hash table implementation of the Set interface. The best all-around implementation of the Set interface.
    • TreeSet - Red-black tree implementation of the NavigableSet interface.
    • LinkedHashSet - Hash table and linked list implementation of the Set interface. An insertion-ordered Set implementation that runs nearly as fast as HashSet.
    • ArrayList - Resizable array implementation of the List interface (an unsynchronized Vector). The best all-around implementation of the List interface.
    • ArrayDeque - Efficient, resizable array implementation of the Deque interface.
    • LinkedList - Doubly-linked list implementation of the List interface. Provides better performance than the ArrayList implementation if elements are frequently inserted or deleted within the list. Also implements the Deque interface. When accessed through the Queue interface, LinkedList acts as a FIFO queue.
    • PriorityQueue - Heap implementation of an unbounded priority queue.
    • HashMap - Hash table implementation of the Map interface (an unsynchronized Hashtable that supports null keys and values). The best all-around implementation of the Map interface.
    • TreeMap Red-black tree implementation of the NavigableMap interface.
    • LinkedHashMap - Hash table and linked list implementation of the Map interface. An insertion-ordered Map implementation that runs nearly as fast as HashMap. Also useful for building caches (see removeEldestEntry(Map.Entry) ).
  • Wrapper implementations - Functionality-enhancing implementations for use with other implementations. Accessed solely through static factory methods.
    • Collections.unmodifiableInterface - Returns an unmodifiable view of a specified collection that throws an UnsupportedOperationException if the user attempts to modify it.
    • Collections.synchronizedInterface - Returns a synchronized collection that is backed by the specified (typically unsynchronized) collection. As long as all accesses to the backing collection are through the returned collection, thread safety is guaranteed.
    • Collections.checkedInterface - Returns a dynamically type-safe view of the specified collection, which throws a ClassCastException if a client attempts to add an element of the wrong type. The generics mechanism in the language provides compile-time (static) type checking, but it is possible to bypass this mechanism. Dynamically type-safe views eliminate this possibility.
  • Adapter implementations - Implementations that adapt one collections interface to another:
    • newSetFromMap(Map) - Creates a general-purpose Set implementation from a general-purpose Map implementation.
    • asLifoQueue(Deque) - Returns a view of a Deque as a Last In First Out (LIFO) Queue.
  • Convenience implementations - High-performance "mini-implementations" of the collection interfaces.
  • Legacy implementations - Older collection classes were retrofitted to implement the collection interfaces.
    • Vector - Synchronized resizable array implementation of the List interface with additional legacy methods.
    • Hashtable - Synchronized hash table implementation of the Map interface that does not allow null keys or values, plus additional legacy methods.
  • Special-purpose implementations
    • WeakHashMap - An implementation of the Map interface that stores only weak references to its keys. Storing only weak references enables key-value pairs to be garbage collected when the key is no longer referenced outside of the WeakHashMap. This class is the easiest way to use the power of weak references. It is useful for implementing registry-like data structures, where the utility of an entry vanishes when its key is no longer reachable by any thread.
    • IdentityHashMap - Identity-based Map implementation based on a hash table. This class is useful for topology-preserving object graph transformations (such as serialization or deep copying). To perform these transformations, you must maintain an identity-based "node table" that keeps track of which objects have already been seen. Identity-based maps are also used to maintain object-to-meta-information mappings in dynamic debuggers and similar systems. Finally, identity-based maps are useful in preventing "spoof attacks" resulting from intentionally perverse equals methods. (IdentityHashMap never invokes the equals method on its keys.) An added benefit of this implementation is that it is fast.
    • CopyOnWriteArrayList - A List implementation backed by an copy-on-write array. All mutative operations (such as add, set, and remove) are implemented by making a new copy of the array. 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 (where change is infrequent, and traversal is frequent and potentially time-consuming).
    • CopyOnWriteArraySet - A Set implementation backed by a copy-on-write array. This implementation is similar to CopyOnWriteArrayList. Unlike most Set implementations, the add, remove, and contains methods require time proportional to the size of the set. This implementation is well suited to maintaining event-handler lists that must prevent duplicates.
    • EnumSet - A high-performance Set implementation backed by a bit vector. All elements of each EnumSet instance must be elements of a single enum type.
    • EnumMap - A high-performance Map implementation backed by an array. All keys in each EnumMap instance must be elements of a single enum type.
  • Concurrent implementations - These implementations are part of java.util.concurrent.
    • ConcurrentLinkedQueue - An unbounded first in, first out (FIFO) queue based on linked nodes.
    • LinkedBlockingQueue - An optionally bounded FIFO blocking queue backed by linked nodes.
    • ArrayBlockingQueue - A bounded FIFO blocking queue backed by an array.
    • PriorityBlockingQueue - An unbounded blocking priority queue backed by a priority heap.
    • DelayQueue - A time-based scheduling queue backed by a priority heap.
    • SynchronousQueue - A simple rendezvous mechanism that uses the BlockingQueue interface.
    • LinkedBlockingDeque - An optionally bounded FIFO blocking deque backed by linked nodes.
    • LinkedTransferQueue - An unbounded TransferQueue backed by linked nodes.
    • ConcurrentHashMap - A highly concurrent, high-performance ConcurrentMap implementation based on a hash table. This implementation never blocks when performing retrievals and enables the client to select the concurrency level for updates. It is intended as a drop-in replacement for Hashtable. In addition to implementing ConcurrentMap, it supports all of the legacy methods of Hashtable.
    • ConcurrentSkipListSet - Skips list implementation of the NavigableSet interface.
    • ConcurrentSkipListMap - Skips list implementation of the ConcurrentNavigableMap interface.
  • Abstract implementations - Skeletal implementations of the collection interfaces to facilitate custom implementations.
    • AbstractCollection - Skeletal Collection implementation that is neither a set nor a list (such as a "bag" or multiset).
    • AbstractSet - Skeletal Set implementation.
    • AbstractList - Skeletal List implementation backed by a random access data store (such as an array).
    • AbstractSequentialList - Skeletal List implementation backed by a sequential access data store (such as a linked list).
    • AbstractQueue - Skeletal Queue implementation.
    • AbstractMap - Skeletal Map implementation.
  • Algorithms - The Collections class contains these useful static methods.
  • Infrastructure
    • Iterators - Similar to the familiar Enumeration interface, but more powerful, and with improved method names.
      • Iterator - In addition to the functionality of the Enumeration interface, enables the user to remove elements from the backing collection with well-defined, useful semantics.
      • ListIterator - Iterator for use with lists. In addition to the functionality of the Iterator interface, supports bidirectional iteration, element replacement, element insertion, and index retrieval.
    • Ordering
      • Comparable - Imparts a natural ordering to classes that implement it. The natural ordering can be used to sort a list or maintain order in a sorted set or map. Many classes were retrofitted to implement this interface.
      • Comparator - Represents an order relation, which can be used to sort a list or maintain order in a sorted set or map. Can override a type's natural ordering or order objects of a type that does not implement the Comparable interface.
    • Runtime exceptions
      • UnsupportedOperationException - Thrown by collections if an unsupported optional operation is called.
      • ConcurrentModificationException - Thrown by iterators and list iterators if the backing collection is changed unexpectedly while the iteration is in progress. Also thrown by sublist views of lists if the backing list is changed unexpectedly.
    • Performance
      • RandomAccess - Marker interface that lets List implementations indicate that they support fast (generally constant time) random access. This lets generic algorithms change their behavior to provide good performance when applied to either random or sequential access lists.
  • Array Utilities
    • Arrays - Contains static methods to sort, search, compare, hash, copy, resize, convert to String, and fill arrays of primitives and objects.


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