注意点1：
Map的容量如何定义
Map xMap = new HashMap();在Map的子类HashMap中，可以在创建的时候指定容量,没有指定的情况是默认16，源代码一直都是很好的证明工具之一，下面是HashMap()的源代码块
从这两个块中可以看到默认的容量是16。
而Map xMap = new HashMap(6);这里所指定的6，是用户期望的数量，为了提高效率，Map的容量会是2的幂，就是一些移位操作的原因了，内部其实是这样的:
关键看这个：
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;
所以，传入6，实际上市会生成容量为8的HashMap，传入9的话，就是16了。
注意点2：
更容错的Map取值方法
参考文章：http://hellosure.iteye.com/blog/1255080
当需要用String.valueOf(Object obj)来取值作为map.get(key)中的key值的时候，建议使用map.get(String.valueOf((Object)obj));这样可以预防在取到null值时候的比较难找的错误。
/**
* 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NullPointerExceptionboolean 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mapvoid 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 codeint 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());
}
}