Java Map.entry案例详解

   Map.entrySet() 这个方法返回的是一个Set>，Map.Entry 是Map中的一个接口，他的用途是表示一个映射项（里面有Key和Value），而Set>表示一个映射项的Set。Map.Entry里有相应的getKey和getValue方法，即JavaBean，让我们能够从一个项中取出Key和Value。

下面是遍历Map的四种方法:

public static void main(String[] args) {
 
 
  Map map = new HashMap();
  map.put("1", "value1");
  map.put("2", "value2");
  map.put("3", "value3");
  
  //第一种：普遍使用，二次取值
  System.out.println("通过Map.keySet遍历key和value：");
  for (String key : map.keySet()) {
   System.out.println("key= "+ key + " and value= " + map.get(key));
  }
  
  //第二种
  System.out.println("通过Map.entrySet使用iterator遍历key和value：");
  Iterator> it = map.entrySet().iterator();
  while (it.hasNext()) {
   Map.Entry entry = it.next();
   System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
  }
  
  //第三种：推荐，尤其是容量大时
  System.out.println("通过Map.entrySet遍历key和value");
  for (Map.Entry entry : map.entrySet()) {
   System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
  }
 
  //第四种
  System.out.println("通过Map.values()遍历所有的value，但不能遍历key");
  for (String v : map.values()) {
   System.out.println("value= " + v);
  }
 }

下面是HashMap的源代码：

首先HashMap的底层实现用的时候一个Entry数组

/** 
     * The table, resized as necessary. Length MUST Always be a power of two. 
     */  
   transient Entry[] table; //声明了一个数组  
   ........  
   public HashMap() {  
        this.loadFactor = DEFAULT_LOAD_FACTOR;  
        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);  
        table = new Entry[DEFAULT_INITIAL_CAPACITY];//初始化数组的大小为DEFAULT_INITIAL_CAPACITY(这里是16)  
        init();  
    }

再来看一下Entry是在什么地方定义的，继续上源码,我们在HashMap的源码的674行发现了它的定义，原来他是HashMap的一个内部类，并且实现了Map.Entry接口，以下有些地方是转载

static class Entry implements Map.Entry {  
    final K key;  
    V value;  
    Entry next;  
    final int hash;  
  
    /** 
     * Creates new entry. 
     */  
    Entry(int h, K k, V v, Entry n) {  
        value = v;  
        next = n;  
        key = k;  
        hash = h;  
    }  
  
    public final K getKey() {  
        return key;  
    }  
  
    public final V getValue() {  
        return value;  
    }  
  
    public final V setValue(V newValue) {  
 V oldValue = value;  
        value = newValue;  
        return oldValue;  
    }  
  
    public final boolean equals(Object o) {  
        if (!(o instanceof Map.Entry))  
            return false;  
        Map.Entry e = (Map.Entry)o;  
        Object k1 = getKey();  
        Object k2 = e.getKey();  
        if (k1 == k2 || (k1 != null && k1.equals(k2))) {  
            Object v1 = getValue();  
            Object v2 = e.getValue();  
            if (v1 == v2 || (v1 != null && v1.equals(v2)))  
                return true;  
        }  
        return false;  
    }  
  
    public final int hashCode() {  
        return (key==null   ? 0 : key.hashCode()) ^  
               (value==null ? 0 : value.hashCode());  
    }  
  
    public final String toString() {  
        return getKey() + "=" + getValue();  
    }  
  
    /** 
     * This method is invoked whenever the value in an entry is 
     * overwritten by an invocation of put(k,v) for a key k that's already 
     * in the HashMap. 
     */  
    void recordAccess(HashMap m) {  
    }  
  
    /** 
     * This method is invoked whenever the entry is 
     * removed from the table. 
     */  
    void recordRemoval(HashMap m) {  
    }  
}  

既然这样那我们再看一下Map.Entry这个接口是怎么定义的，原来他是Map的一个内部接口并且定义了一些方法

  interface Entry {  
    /** 
 * Returns the key corresponding to this entry. 
 * 
 * @return the key corresponding to this entry 
        * @throws IllegalStateException implementations may, but are not 
        *         required to, throw this exception if the entry has been 
        *         removed from the backing map. 
 */  
K getKey();  
  
    /** 
 * Returns the value corresponding to this entry.  If the mapping 
 * has been removed from the backing map (by the iterator's 
 * remove operation), the results of this call are undefined. 
 * 
 * @return the value corresponding to this entry 
        * @throws IllegalStateException implementations may, but are not 
        *         required to, throw this exception if the entry has been 
        *         removed from the backing map. 
 */  
V getValue();  
  
    /** 
 * Replaces the value corresponding to this entry with the specified 
 * value (optional operation).  (Writes through to the map.)  The 
 * behavior of this call is undefined if the mapping has already been 
 * removed from the map (by the iterator's remove operation). 
 * 
        * @param value new value to be stored in this entry 
        * @return old value corresponding to the entry 
        * @throws UnsupportedOperationException if the put operation 
        *         is not supported by the backing map 
        * @throws ClassCastException if the class of the specified value 
        *         prevents it from being stored in the backing map 
        * @throws NullPointerException if the backing map does not permit 
        *         null values, and the specified value is null 
        * @throws IllegalArgumentException if some property of this value 
        *         prevents it from being stored in the backing map 
        * @throws IllegalStateException implementations may, but are not 
        *         required to, throw this exception if the entry has been 
        *         removed from the backing map. 
        */  
V setValue(V value);  
  
/** 
 * Compares the specified object with this entry for equality. 
 * Returns true if the given object is also a map entry and 
 * the two entries represent the same mapping.  More formally, two 
 * entries e1 and e2 represent the same mapping 
 * if
 
        *     (e1.getKey()==null ? 
        *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  && 
        *     (e1.getValue()==null ? 
        *      e2.getValue()==null : e1.getValue().equals(e2.getValue())) 
        * 
 
 * This ensures that the equals method works properly across 
 * different implementations of the Map.Entry interface. 
 * 
 * @param o object to be compared for equality with this map entry 
 * @return true if the specified object is equal to this map 
 *         entry 
        */  
boolean equals(Object o);  
  
/** 
 * Returns the hash code value for this map entry.  The hash code 
 * of a map entry e is defined to be: 
 
 *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^ 
 *     (e.getValue()==null ? 0 : e.getValue().hashCode()) 
        * 
 
 * This ensures that e1.equals(e2) implies that 
 * e1.hashCode()==e2.hashCode() for any two Entries 
 * e1 and e2, as required by the general 
 * contract of Object.hashCode. 
 * 
 * @return the hash code value for this map entry 
 * @see Object#hashCode() 
 * @see Object#equals(Object) 
 * @see #equals(Object) 
 */  
int hashCode();  
   }  

看到这里的时候大伙儿估计都明白得差不多了为什么HashMap为什么要选择Entry数组来存放key-value对了吧，因为Entry实现的Map.Entry接口里面定义了getKey(),getValue()，setKey(),setValue()等方法相当于一个javaBean，对键值对进行了一个封装便于后面的操作，从这里我们其实也可以联想到不光是HashMap，譬如LinkedHashMap,TreeMap 等继承自map的容器存储key-value对都应该使用的是Entry只不过组织Entry的形式不一样，HashMap用的是数组加链表的形式，LinkedHashMap用的是链表的形式，TreeMap应该使用的二叉树的形式，不信的话上源码

LinkedHashMap:

/** 
    * The head of the doubly linked list. 
    */  
/定义了链头  
   private transient Entry header;  

初始化链表的方法：

void init() {  
    header = new Entry(-1, null, null, null);  
    header.before = header.after = header;  
}  

TreeMap:

//定义根节点  
 private transient Entry root = null;  

再看他的put方法，是不是很面熟（二叉排序树的插入操作）

public V put(K key, V value) {  
    Entry t = root;  
    if (t == null) {  
 // TBD:  
 // 5045147: (coll) Adding null to an empty TreeSet should  
 // throw NullPointerException  
 //  
 // compare(key, key); // type check  
        root = new Entry(key, value, null);  
        size = 1;  
        modCount++;  
        return null;  
    }  
    int cmp;  
    Entry parent;  
    // split comparator and comparable paths  
    Comparator cpr = comparator;  
    if (cpr != null) {  
        do {  
            parent = t;  
            cmp = cpr.compare(key, t.key);  
            if (cmp < 0)  
                t = t.left;  
            else if (cmp > 0)  
                t = t.right;  
            else  
                return t.setValue(value);  
        } while (t != null);  
    }  
    else {  
        if (key == null)  
            throw new NullPointerException();  
        Comparable k = (Comparable) key;  
        do {  
            parent = t;  
            cmp = k.compareTo(t.key);  
            if (cmp < 0)  
                t = t.left;  
            else if (cmp > 0)  
                t = t.right;  
            else  
                return t.setValue(value);  
        } while (t != null);  
    }  
    Entry e = new Entry(key, value, parent);  
    if (cmp < 0)  
        parent.left = e;  
    else  
        parent.right = e;  
    fixAfterInsertion(e);  
    size++;  
    modCount++;  
    return null;  
}  

ok，明白了各种Map的底层存储key-value对的方式后，再来看看如何遍历map吧，这里用HashMap来演示吧

Map提供了一些常用方法，如keySet()、entrySet()等方法，keySet()方法返回值是Map中key值的集合；entrySet()的返回值也是返回一个Set集合，此集合的类型为Map.Entry。

so,很容易写出如下的遍历代码

1.  Map map = new HashMap();  
  
         Irerator iterator = map.entrySet().iterator();  
  
         while(iterator.hasNext()) {  
  
                 Map.Entry entry = iterator.next();  
  
                 Object key = entry.getKey();  
  
                 //  
  
         }  
  
     2.Map map = new HashMap();   
  
         Set  keySet= map.keySet();  
  
         Irerator iterator = keySet.iterator;  
  
         while(iterator.hasNext()) {  
  
                 Object key = iterator.next();  
  
                 Object value = map.get(key);  
  
                 //  
  
         }  
另外，还有一种遍历方法是，单纯的遍历value值，Map有一个values方法，返回的是value的Collection集合。通过遍历collection也可以遍历value,如
[java] view plain copy
Map map = new HashMap();  
  
Collection c = map.values();  
  
Iterator iterator = c.iterator();  
  
while(iterator.hasNext()) {  
  
       Object value = iterator.next();   

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