[Java] Java源码解析HashMap简介

/**
 * hash table based implementation of the <tt>map</tt> interface. this
 * implementation provides all of the optional map operations, and permits
 * <tt>null</tt> values and the <tt>null</tt> key. (the <tt>hashmap</tt>
 * class is roughly equivalent to <tt>hashtable</tt>, except that it is
 * unsynchronized and permits nulls.) this class makes no guarantees as to
 * the order of the map; in particular, it does not guarantee that the order
 * will remain constant over time.
 * <p>this implementation provides constant-time performance for the basic
 * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
 * disperses the elements properly among the buckets. iteration over
 * collection views requires time proportional to the "capacity" of the
 * <tt>hashmap</tt> instance (the number of buckets) plus its size (the number
 * of key-value mappings). thus, it's very important not to set the initial
 * capacity too high (or the load factor too low) if iteration performance is
 * important.
 * <p>an instance of <tt>hashmap</tt> has two parameters that affect its
 * performance: <i>initial capacity</i> and <i>load factor</i>. the
 * <i>capacity</i> is the number of buckets in the hash table, and the initial
 * capacity is simply the capacity at the time the hash table is created. the
 * <i>load factor</i> is a measure of how full the hash table is allowed to
 * get before its capacity is automatically increased. when the number of
 * entries in the hash table exceeds the product of the load factor and the
 * current capacity, the hash table is <i>rehashed</i> (that is, internal data
 * structures are rebuilt) so that the hash table has approximately twice the
 * number of buckets.
 * <p>as a general rule, the default load factor (.75) offers a good
 * tradeoff between time and space costs. higher values decrease the
 * space overhead but increase the lookup cost (reflected in most of
 * the operations of the <tt>hashmap</tt> class, including
 * <tt>get</tt> and <tt>put</tt>). the expected number of entries in
 * the map and its load factor should be taken into account when
 * setting its initial capacity, so as to minimize the number of
 * rehash operations. if the initial capacity is greater than the
 * maximum number of entries divided by the load factor, no rehash
 * operations will ever occur.
 * <p>if many mappings are to be stored in a <tt>hashmap</tt>
 * instance, creating it with a sufficiently large capacity will allow
 * the mappings to be stored more efficiently than letting it perform
 * automatic rehashing as needed to grow the table. note that using
 * many keys with the same {@code hashcode()} is a sure way to slow
 * down performance of any hash table. to ameliorate impact, when keys
 * are {@link comparable}, this class may use comparison order among
 * keys to help break ties.
 * <p><strong>note that this implementation is not synchronized.</strong>
 * if multiple threads access a hash map concurrently, and at least one of
 * the threads modifies the map structurally, it <i>must</i> be
 * synchronized externally. (a structural modification is any operation
 * that adds or deletes one or more mappings; merely changing the value
 * associated with a key that an instance already contains is not a
 * structural modification.) this is typically accomplished by
 * synchronizing on some object that naturally encapsulates the map.
 * if no such object exists, the map should be "wrapped" using the
 * {@link collections#synchronizedmap collections.synchronizedmap}
 * method. this is best done at creation time, to prevent accidental
 * unsynchronized access to the map:<pre>
 *  map m = collections.synchronizedmap(new hashmap(...));</pre>
 * <p>the iterators returned by all of this class's "collection view methods"
 * are <i>fail-fast</i>: if the map is structurally modified at any time after
 * the iterator is created, in any way except through the iterator's own
 * <tt>remove</tt> method, the iterator will throw a
 * {@link concurrentmodificationexception}. thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the
 * future.
 * <p>note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification. fail-fast iterators
 * throw <tt>concurrentmodificationexception</tt> on a best-effort basis.
 * therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 * <p>this class is a member of the
 * <a href="{@docroot}/../technotes/guides/collections/index.html" rel="external nofollow" >
 * java collections framework</a>.
 * @param <k> the type of keys maintained by this map
 * @param <v> the type of mapped values
 * @author doug lea
 * @author josh bloch
 * @author arthur van hoff
 * @author neal gafter
 * @see   object#hashcode()
 * @see   collection
 * @see   map
 * @see   treemap
 * @see   hashtable
 * @since  1.2
 **/