java.util.Map源码分析

/**
 * An object that maps keys to values.  A map cannot contain duplicate keys;
 * each key can map to at most one value.
 *
 * <p>This interface takes the place of the <tt>Dictionary</tt> class, which
 * was a totally abstract class rather than an interface.
 *
 * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
 * allow a map‘s contents to be viewed as a set of keys, collection of values,
 * or set of key-value mappings.  The <i>order</i> of a map is defined as
 * the order in which the iterators on the map‘s collection views return their
 * elements.  Some map implementations, like the <tt>TreeMap</tt> class, make
 * specific guarantees as to their order; others, like the <tt>HashMap</tt>
 * class, do not.
 *
 * <p>Note: great care must be exercised if mutable objects are used as map
 * keys.  The behavior of a map is not specified if the value of an object is
 * changed in a manner that affects <tt>equals</tt> comparisons while the
 * object is a key in the map.  A special case of this prohibition is that it
 * is not permissible for a map to contain itself as a key.  While it is
 * permissible for a map to contain itself as a value, extreme caution is
 * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
 * well defined on such a map.
 *
 * <p>All general-purpose map implementation classes should provide two
 * "standard" constructors: a void (no arguments) constructor which creates an
 * empty map, and a constructor with a single argument of type <tt>Map</tt>,
 * which creates a new map with the same key-value mappings as its argument.
 * In effect, the latter constructor allows the user to copy any map,
 * producing an equivalent map of the desired class.  There is no way to
 * enforce this recommendation (as interfaces cannot contain constructors) but
 * all of the general-purpose map implementations in the JDK comply.
 *
 * <p>The "destructive" methods contained in this interface, that is, the
 * methods that modify the map on which they operate, are specified to throw
 * <tt>UnsupportedOperationException</tt> if this map does not support the
 * operation.  If this is the case, these methods may, but are not required
 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
 * have no effect on the map.  For example, invoking the {@link #putAll(Map)}
 * method on an unmodifiable map may, but is not required to, throw the
 * exception if the map whose mappings are to be "superimposed" is empty.
 *
 * <p>Some map implementations have restrictions on the keys and values they
 * may contain.  For example, some implementations prohibit null keys and
 * values, and some have restrictions on the types of their keys.  Attempting
 * to insert an ineligible key or value throws an unchecked exception,
 * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
 * Attempting to query the presence of an ineligible key or value may throw an
 * exception, or it may simply return false; some implementations will exhibit
 * the former behavior and some will exhibit the latter.  More generally,
 * attempting an operation on an ineligible key or value whose completion
 * would not result in the insertion of an ineligible element into the map may
 * throw an exception or it may succeed, at the option of the implementation.
 * Such exceptions are marked as "optional" in the specification for this
 * interface.
 *
 * <p>Many methods in Collections Framework interfaces are defined
 * in terms of the {@link Object#equals(Object) equals} method.  For
 * example, the specification for the {@link #containsKey(Object)
 * containsKey(Object key)} method says: "returns <tt>true</tt> if and
 * only if this map contains a mapping for a key <tt>k</tt> such that
 * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
 * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
 * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
 * be invoked for any key <tt>k</tt>.  Implementations are free to
 * implement optimizations whereby the <tt>equals</tt> invocation is avoided,
 * for example, by first comparing the hash codes of the two keys.  (The
 * {@link Object#hashCode()} specification guarantees that two objects with
 * unequal hash codes cannot be equal.)  More generally, implementations of
 * the various Collections Framework interfaces are free to take advantage of
 * the specified behavior of underlying {@link Object} methods wherever the
 * implementor deems it appropriate.
 *
 * <p>Some map operations which perform recursive traversal of the map may fail
 * with an exception for self-referential instances where the map directly or
 * indirectly contains itself. This includes the {@code clone()},
 * {@code equals()}, {@code hashCode()} and {@code toString()} methods.
 * Implementations may optionally handle the self-referential scenario, however
 * most current implementations do not do so.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 *
 * @author  Josh Bloch
 * @see HashMap
 * @see TreeMap
 * @see Hashtable
 * @see SortedMap
 * @see Collection
 * @see Set
 * @since 1.2
 */
public interface Map<K,V>

Map是一个接口，一个map不能包含重复的key，每个key只能映射唯一一个value。

Map接口是用来取代Dictionary抽象类的。

Map接口提供三个集合视图，1.key的集合 2.value的集合 3.key-value的集合。map内元素的顺序取决于Iterator的具体实现，获取集合视图其实是获取一个迭代器，实现对遍历元素细节的隐藏。TreeMap类能保证遍历元素的顺序，而HashMap就无法保证遍历元素的顺序。

注意：当使用一个可变对象作为key的时候要小心，map是根据hashCode和equals方法决定存放的位置的。一个特殊的案例是不允许一个map将自己作为一个key，但允许将自己作为一value。

所有多种用途的map实现类应该提供两个“标准”构造器，一个无参构造器用来创建一个空map，一个只有一个参数，参数类型是map的构造器，用来创建一个新的和传入参数有一样key-value映射的map。实际上，后者允许复制任何一个map，这仅仅是一个建议，并没有强制要求，因为接口是无法包含构造器的，不过这个建议在JDK被遵守。

如果一个方法的操作是不被支持的，这个方法指定抛出UnsupportedOperationException异常。如果这个操作对mao是没有影响的，那么也可以不抛出UnsupportedOperationException异常。例如，在一个不能被修改的map调用putAll（Map）方法，如果该map的映射是空的，就不要求抛出UnsupportedOperationException异常。

Map接口是Java Collections Framework的一员。

Map里面的方法：

int size();//返回map中key-value映射的数量

boolean isEmpty();//如果map中没有key-value映射返回true
//如果map不含key映射，返回false，当key的类型不符合，抛出ClassCastException，当key是//null且该map不支持key的值是null时，抛出NullPointerException
boolean containsKey(Object key);
//如果map含有一个以上的key映射的参数value，返回true，异常抛出的情况和containKey一样
boolean containsValue(Object value);
//根据key得到对应的value，如果没有对应的映射，返回null，如果map允许value为null，返回//null可能是有一对key-null的映射或没有对应的映射V get(Object key);
//往map放入一对key-value映射
V put(K key, V value);
//根据key删除对应映射
V remove(Object key);
//复制一份与参数一样的map
void putAll(Map<? extends K, ? extends V> m);
//清空map中所有的映射
void clear();
//返回map中所有key的集合
Set<K> keySet();
//返回map中所有value的集合
Collection<V> values();
//返回key-value的集合
Set<Map.Entry<K, V>> entrySet();
//比较调用者与参数是否相等
boolean equals(Object o);
//计算map的hash code
int hashCode();

//还有其他default方法...，都是jdk1.8发布的

Map接口里有一个内部接口Entry<K,V>，其实它就是Map存放key-value映射的数据结构

interface Entry<K,V> {
    　　//返回对应的key
    K getKey();
　　//返回对应的value
    V getValue();
    　　//设置用新value替换旧value，返回值是旧value
    V setValue(V value);
　　//如果两个entry的映射一样，返回true
    boolean equals(Object o);
　　//计算entry的hash code
    int hashCode();
　　//下面的静态方法是JDK1.8才发布的　　//返回一个比较器，比较的规则是key的自然大小
    public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() {
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> c1.getKey().compareTo(c2.getKey());//这里用的是lambda表达式
        }
　　　　　　//返回一个比较器，比较规则是value的自然大小
    public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue() {
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> c1.getValue().compareTo(c2.getValue());
        }
　　//返回一个比较器，比较规则用参数传入，比较的是key
    public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
            Objects.requireNonNull(cmp);
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
        }
　　//返回一个比较器，比较规则用参数传入，比较的是value
    public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
            Objects.requireNonNull(cmp);
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
        }
}