java.util.map源码＋学习心得

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

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1. /** 
2.  * The default initial capacity - MUST be a power of two. 
3.  */  
4. static final int DEFAULT_INITIAL_CAPACITY = 16;  
5.   
6. /** 
7.  * Constructs an empty <tt>HashMap</tt> with the default initial capacity 
8.  * (16) and the default load factor (0.75). 
9.  */  
10. public HashMap() {  
11.     this.loadFactor = DEFAULT_LOAD_FACTOR;  
12.     threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);  
13.     table = new Entry[DEFAULT_INITIAL_CAPACITY];  
14.     init();  
15. }  

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:

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1. /** 
2.      * Constructs an empty <tt>HashMap</tt> with the specified initial 
3.      * capacity and load factor. 
4.      * 
5.      * @param  initialCapacity the initial capacity 
6.      * @param  loadFactor      the load factor 
7.      * @throws IllegalArgumentException if the initial capacity is negative 
8.      *         or the load factor is nonpositive 
9.      */  
10.     public HashMap(int initialCapacity, float loadFactor) {  
11.         if (initialCapacity < 0)  
12.             throw new IllegalArgumentException("Illegal initial capacity: " +  
13.                                                initialCapacity);  
14.         if (initialCapacity > MAXIMUM_CAPACITY)  
15.             initialCapacity = MAXIMUM_CAPACITY;  
16.         if (loadFactor <= 0 || Float.isNaN(loadFactor))  
17.             throw new IllegalArgumentException("Illegal load factor: " +  
18.                                                loadFactor);  
19.   
20.         // Find a power of 2 >= initialCapacity  
21.         int capacity = 1;  
22.         while (capacity < initialCapacity)  
23.             capacity <<= 1;  
24.   
25.         this.loadFactor = loadFactor;  
26.         threshold = (int)(capacity * loadFactor);  
27.         table = new Entry[capacity];  
28.         init();  
29.     }  

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());        }}