android开发之-数据存储Map、HashMap、Hashtable、concurenthashmap区别

/**
 * A {@code Map} is a data structure consisting of a set of keys and values
 * in which each key is mapped to a single value. The class of the objects
 * used as keys is declared when the {@code Map} is declared, as is the
 * class of the corresponding values.
 * <p>
 * A {@code Map} provides helper methods to iterate through all of the
 * keys contained in it, as well as various methods to access and update
 * the key/value pairs.
 */

/**
 * HashMap is an implementation of {@link Map}. All optional operations are supported.
 *
 * <p>All elements are permitted as keys or values, including null.
 *
 * <p>Note that the iteration order for HashMap is non-deterministic. If you want
 * deterministic iteration, use {@link LinkedHashMap}.
 *
 * <p>Note: the implementation of {@code HashMap} is not synchronized.
 * If one thread of several threads accessing an instance modifies the map
 * structurally, access to the map needs to be synchronized. A structural
 * modification is an operation that adds or removes an entry. Changes in
 * the value of an entry are not structural changes.
 *
 * <p>The {@code Iterator} created by calling the {@code iterator} method
 * may throw a {@code ConcurrentModificationException} if the map is structurally
 * changed while an iterator is used to iterate over the elements. Only the
 * {@code remove} method that is provided by the iterator allows for removal of
 * elements during iteration. It is not possible to guarantee that this
 * mechanism works in all cases of unsynchronized concurrent modification. It
 * should only be used for debugging purposes.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */

/**
 * Hashtable is a synchronized implementation of {@link Map}. All optional operations are supported.
 *
 * <p>Neither keys nor values can be null. (Use {@code HashMap} or {@code LinkedHashMap} if you
 * need null keys or values.)
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 * @see HashMap
 */

/**
 * A {@link java.util.Map} providing additional atomic
 * <tt>putIfAbsent</tt>, <tt>remove</tt>, and <tt>replace</tt> methods.
 *
 * <p>Memory consistency effects: As with other concurrent
 * collections, actions in a thread prior to placing an object into a
 * {@code ConcurrentMap} as a key or value
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions subsequent to the access or removal of that object from
 * the {@code ConcurrentMap} in another thread.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */

/**
     * If the specified key is not already associated
     * with a value, associate it with the given value.
     * This is equivalent to
     * <pre>
     * if (!map.containsKey(key))
     * return map.put(key, value);
     * else
     * return map.get(key);</pre>
     * except that the action is performed atomically.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with the specified key, or
     * <tt>null</tt> if there was no mapping for the key.
     * (A <tt>null</tt> return can also indicate that the map
     * previously associated <tt>null</tt> with the key,
     * if the implementation supports null values.)
     * @throws UnsupportedOperationException if the <tt>put</tt> operation
     * is not supported by this map
     * @throws ClassCastException if the class of the specified key or value
     * prevents it from being stored in this map
     * @throws NullPointerException if the specified key or value is null,
     * and this map does not permit null keys or values
     * @throws IllegalArgumentException if some property of the specified key
     * or value prevents it from being stored in this map
     *
     */
    V putIfAbsent(K key, V value);

 if (!map.containsKey(key))
     return map.put(key, value);
      else
      return map.get(key);

/**
 * A hash table supporting full concurrency of retrievals and
 * adjustable expected concurrency for updates. This class obeys the
 * same functional specification as {@link java.util.Hashtable}, and
 * includes versions of methods corresponding to each method of
 * <tt>Hashtable</tt>. However, even though all operations are
 * thread-safe, retrieval operations do <em>not</em> entail locking,
 * and there is <em>not</em> any support for locking the entire table
 * in a way that prevents all access. This class is fully
 * interoperable with <tt>Hashtable</tt> in programs that rely on its
 * thread safety but not on its synchronization details.
 *
 * <p> Retrieval operations (including <tt>get</tt>) generally do not
 * block, so may overlap with update operations (including
 * <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results
 * of the most recently <em>completed</em> update operations holding
 * upon their onset. For aggregate operations such as <tt>putAll</tt>
 * and <tt>clear</tt>, concurrent retrievals may reflect insertion or
 * removal of only some entries. Similarly, Iterators and
 * Enumerations return elements reflecting the state of the hash table
 * at some point at or since the creation of the iterator/enumeration.
 * They do <em>not</em> throw {@link ConcurrentModificationException}.
 * However, iterators are designed to be used by only one thread at a time.
 *
 * <p> The allowed concurrency among update operations is guided by
 * the optional <tt>concurrencyLevel</tt> constructor argument
 * (default <tt>16</tt>), which is used as a hint for internal sizing. The
 * table is internally partitioned to try to permit the indicated
 * number of concurrent updates without contention. Because placement
 * in hash tables is essentially random, the actual concurrency will
 * vary. Ideally, you should choose a value to accommodate as many
 * threads as will ever concurrently modify the table. Using a
 * significantly higher value than you need can waste space and time,
 * and a significantly lower value can lead to thread contention. But
 * overestimates and underestimates within an order of magnitude do
 * not usually have much noticeable impact. A value of one is
 * appropriate when it is known that only one thread will modify and
 * all others will only read. Also, resizing this or any other kind of
 * hash table is a relatively slow operation, so, when possible, it is
 * a good idea to provide estimates of expected table sizes in
 * constructors.
 *
 * <p>This class and its views and iterators implement all of the
 * <em>optional</em> methods of the {@link Map} and {@link Iterator}
 * interfaces.
 *
 * <p> Like {@link Hashtable} but unlike {@link HashMap}, this class
 * does <em>not</em> allow <tt>null</tt> to be used as a key or value.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */

/**
 * LinkedHashMap is an implementation of {@link Map} that guarantees iteration order.
 * All optional operations are supported.
 *
 * <p>All elements are permitted as keys or values, including null.
 *
 * <p>Entries are kept in a doubly-linked list. The iteration order is, by default, the
 * order in which keys were inserted. Reinserting an already-present key doesn‘t change the
 * order. If the three argument constructor is used, and {@code accessOrder} is specified as
 * {@code true}, the iteration will be in the order that entries were accessed.
 * The access order is affected by {@code put}, {@code get}, and {@code putAll} operations,
 * but not by operations on the collection views.
 *
 * <p>Note: the implementation of {@code LinkedHashMap} is not synchronized.
 * If one thread of several threads accessing an instance modifies the map
 * structurally, access to the map needs to be synchronized. For
 * insertion-ordered instances a structural modification is an operation that
 * removes or adds an entry. Access-ordered instances also are structurally
 * modified by {@code put}, {@code get}, and {@code putAll} since these methods
 * change the order of the entries. Changes in the value of an entry are not structural changes.
 *
 * <p>The {@code Iterator} created by calling the {@code iterator} method
 * may throw a {@code ConcurrentModificationException} if the map is structurally
 * changed while an iterator is used to iterate over the elements. Only the
 * {@code remove} method that is provided by the iterator allows for removal of
 * elements during iteration. It is not possible to guarantee that this
 * mechanism works in all cases of unsynchronized concurrent modification. It
 * should only be used for debugging purposes.
 */