Java Map.entry案例详解

   Map.entrySet() 这个方法返回的是一个Set<Map.Entry<K,V>>，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<Map.Entry<K,V>>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<String, String> map = new HashMap<String, String>();
  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<Map.Entry<String, String>> it = map.entrySet().iterator();
  while (it.hasNext()) {
   Map.Entry<String, String> 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<String, String> 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<K,V> implements Map.Entry<K,V> {
    final K key;
    V value;
    Entry<K,V> next;
    final int hash;  

    /**
     * Creates new entry.
     */
    Entry(int h, K k, V v, Entry<K,V> 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<K,V> m) {
    }  

    /**
     * This method is invoked whenever the entry is
     * removed from the table.
     */
    void recordRemoval(HashMap<K,V> 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<K,V> {
    /**
 * 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
 * <tt>remove</tt> 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 <tt>remove</tt> operation).
 *
        * @param value new value to be stored in this entry
        * @return old value corresponding to the entry
        * @throws UnsupportedOperationException if the <tt>put</tt> 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 <tt>true</tt> if the given object is also a map entry and
 * the two entries represent the same mapping.  More formally, two
 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
 * if<pre>
        *     (e1.getKey()==null ?
        *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &&
        *     (e1.getValue()==null ?
        *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
        * </pre>
 * This ensures that the <tt>equals</tt> method works properly across
 * different implementations of the <tt>Map.Entry</tt> interface.
 *
 * @param o object to be compared for equality with this map entry
 * @return <tt>true</tt> 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 <tt>e</tt> is defined to be: <pre>
 *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
 *     (e.getValue()==null ? 0 : e.getValue().hashCode())
        * </pre>
 * This ensures that <tt>e1.equals(e2)</tt> implies that
 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
 * <tt>e1</tt> and <tt>e2</tt>, as required by the general
 * contract of <tt>Object.hashCode</tt>.
 *
 * @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<K,V> header;  

初始化链表的方法：

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

TreeMap:

//定义根节点
 private transient Entry<K,V> 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<K,V> 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<K,V>(key, value, null);
        size = 1;
        modCount++;
        return null;
    }
    int cmp;
    Entry<K,V> parent;
    // split comparator and comparable paths
    Comparator<? super K> 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<? super K> k = (Comparable<? super K>) 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<K,V> e = new Entry<K,V>(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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