Mapreduce(4)------遍历map的四种方法及Map.entry详解（转）

转自---https://blog.csdn.net/gm371200587/article/details/82108372

 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

1.  
   java] view plain copy
2.  
   <pre name="code" class="java"> /**
3.  
   * The table, resized as necessary. Length MUST Always be a power of two.
4.  
   */
5.  
   transient Entry[] table; //声明了一个数组
6.  
   ........
7.  
   public HashMap() {
8.  
   this.loadFactor = DEFAULT_LOAD_FACTOR;
9.  
   threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
10.  
    table = new Entry[DEFAULT_INITIAL_CAPACITY];//初始化数组的大小为DEFAULT_INITIAL_CAPACITY(这里是16)
11.  
    init();
12.  
    }</pre><br>

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

 

1.  
   static class Entry<K,V> implements Map.Entry<K,V> {
2.  
   final K key;
3.  
   V value;
4.  
   Entry<K,V> next;
5.  
   final int hash;
6.  
    
7.  
   /**
8.  
   * Creates new entry.
9.  
   */
10.  
    Entry(int h, K k, V v, Entry<K,V> n) {
11.  
    value = v;
12.  
    next = n;
13.  
    key = k;
14.  
    hash = h;
15.  
    }
16.  
     
17.  
    public final K getKey() {
18.  
    return key;
19.  
    }
20.  
     
21.  
    public final V getValue() {
22.  
    return value;
23.  
    }
24.  
     
25.  
    public final V setValue(V newValue) {
26.  
    V oldValue = value;
27.  
    value = newValue;
28.  
    return oldValue;
29.  
    }
30.  
     
31.  
    public final boolean equals(Object o) {
32.  
    if (!(o instanceof Map.Entry))
33.  
    return false;
34.  
    Map.Entry e = (Map.Entry)o;
35.  
    Object k1 = getKey();
36.  
    Object k2 = e.getKey();
37.  
    if (k1 == k2 || (k1 != null && k1.equals(k2))) {
38.  
    Object v1 = getValue();
39.  
    Object v2 = e.getValue();
40.  
    if (v1 == v2 || (v1 != null && v1.equals(v2)))
41.  
    return true;
42.  
    }
43.  
    return false;
44.  
    }
45.  
     
46.  
    public final int hashCode() {
47.  
    return (key==null ? 0 : key.hashCode()) ^
48.  
    (value==null ? 0 : value.hashCode());
49.  
    }
50.  
     
51.  
    public final String toString() {
52.  
    return getKey() + "=" + getValue();
53.  
    }
54.  
     
55.  
    /**
56.  
    * This method is invoked whenever the value in an entry is
57.  
    * overwritten by an invocation of put(k,v) for a key k that's already
58.  
    * in the HashMap.
59.  
    */
60.  
    void recordAccess(HashMap<K,V> m) {
61.  
    }
62.  
     
63.  
    /**
64.  
    * This method is invoked whenever the entry is
65.  
    * removed from the table.
66.  
    */
67.  
    void recordRemoval(HashMap<K,V> m) {
68.  
    }
69.  
    }

 

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​​​​​​​

1.  
   interface Entry<K,V> {
2.  
   /**
3.  
   * Returns the key corresponding to this entry.
4.  
   *
5.  
   * @return the key corresponding to this entry
6.  
   * @throws IllegalStateException implementations may, but are not
7.  
   * required to, throw this exception if the entry has been
8.  
   * removed from the backing map.
9.  
   */
10.  
    K getKey();
11.  
     
12.  
    /**
13.  
    * Returns the value corresponding to this entry. If the mapping
14.  
    * has been removed from the backing map (by the iterator's
15.  
    * <tt>remove</tt> operation), the results of this call are undefined.
16.  
    *
17.  
    * @return the value corresponding to this entry
18.  
    * @throws IllegalStateException implementations may, but are not
19.  
    * required to, throw this exception if the entry has been
20.  
    * removed from the backing map.
21.  
    */
22.  
    V getValue();
23.  
     
24.  
    /**
25.  
    * Replaces the value corresponding to this entry with the specified
26.  
    * value (optional operation). (Writes through to the map.) The
27.  
    * behavior of this call is undefined if the mapping has already been
28.  
    * removed from the map (by the iterator's <tt>remove</tt> operation).
29.  
    *
30.  
    * @param value new value to be stored in this entry
31.  
    * @return old value corresponding to the entry
32.  
    * @throws UnsupportedOperationException if the <tt>put</tt> operation
33.  
    * is not supported by the backing map
34.  
    * @throws ClassCastException if the class of the specified value
35.  
    * prevents it from being stored in the backing map
36.  
    * @throws NullPointerException if the backing map does not permit
37.  
    * null values, and the specified value is null
38.  
    * @throws IllegalArgumentException if some property of this value
39.  
    * prevents it from being stored in the backing map
40.  
    * @throws IllegalStateException implementations may, but are not
41.  
    * required to, throw this exception if the entry has been
42.  
    * removed from the backing map.
43.  
    */
44.  
    V setValue(V value);
45.  
     
46.  
    /**
47.  
    * Compares the specified object with this entry for equality.
48.  
    * Returns <tt>true</tt> if the given object is also a map entry and
49.  
    * the two entries represent the same mapping. More formally, two
50.  
    * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
51.  
    * if<pre>
52.  
    * (e1.getKey()==null ?
53.  
    * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
54.  
    * (e1.getValue()==null ?
55.  
    * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
56.  
    * </pre>
57.  
    * This ensures that the <tt>equals</tt> method works properly across
58.  
    * different implementations of the <tt>Map.Entry</tt> interface.
59.  
    *
60.  
    * @param o object to be compared for equality with this map entry
61.  
    * @return <tt>true</tt> if the specified object is equal to this map
62.  
    * entry
63.  
    */
64.  
    boolean equals(Object o);
65.  
     
66.  
    /**
67.  
    * Returns the hash code value for this map entry. The hash code
68.  
    * of a map entry <tt>e</tt> is defined to be: <pre>
69.  
    * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
70.  
    * (e.getValue()==null ? 0 : e.getValue().hashCode())
71.  
    * </pre>
72.  
    * This ensures that <tt>e1.equals(e2)</tt> implies that
73.  
    * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
74.  
    * <tt>e1</tt> and <tt>e2</tt>, as required by the general
75.  
    * contract of <tt>Object.hashCode</tt>.
76.  
    *
77.  
    * @return the hash code value for this map entry
78.  
    * @see Object#hashCode()
79.  
    * @see Object#equals(Object)
80.  
    * @see #equals(Object)
81.  
    */
82.  
    int hashCode();
83.  
    }

 

     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:​​​​​​​

1.  
   /**
2.  
   * The head of the doubly linked list.
3.  
   */
4.  
   /定义了链头
5.  
   private transient Entry<K,V> header;

 

初始化链表的方法：

1.  
   void init() {
2.  
   header = new Entry<K,V>(-1, null, null, null);
3.  
   header.before = header.after = header;
4.  
   }

​​​​​​

TreeMap:

1.  
   [java] view plain copy
2.  
   //定义根节点
3.  
   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）​​​​​​​

1.  
   public V put(K key, V value) {
2.  
   Entry<K,V> t = root;
3.  
   if (t == null) {
4.  
   // TBD:
5.  
   // 5045147: (coll) Adding null to an empty TreeSet should
6.  
   // throw NullPointerException
7.  
   //
8.  
   // compare(key, key); // type check
9.  
   root = new Entry<K,V>(key, value, null);
10.  
    size = 1;
11.  
    modCount++;
12.  
    return null;
13.  
    }
14.  
    int cmp;
15.  
    Entry<K,V> parent;
16.  
    // split comparator and comparable paths
17.  
    Comparator<? super K> cpr = comparator;
18.  
    if (cpr != null) {
19.  
    do {
20.  
    parent = t;
21.  
    cmp = cpr.compare(key, t.key);
22.  
    if (cmp < 0)
23.  
    t = t.left;
24.  
    else if (cmp > 0)
25.  
    t = t.right;
26.  
    else
27.  
    return t.setValue(value);
28.  
    } while (t != null);
29.  
    }
30.  
    else {
31.  
    if (key == null)
32.  
    throw new NullPointerException();
33.  
    Comparable<? super K> k = (Comparable<? super K>) key;
34.  
    do {
35.  
    parent = t;
36.  
    cmp = k.compareTo(t.key);
37.  
    if (cmp < 0)
38.  
    t = t.left;
39.  
    else if (cmp > 0)
40.  
    t = t.right;
41.  
    else
42.  
    return t.setValue(value);
43.  
    } while (t != null);
44.  
    }
45.  
    Entry<K,V> e = new Entry<K,V>(key, value, parent);
46.  
    if (cmp < 0)
47.  
    parent.left = e;
48.  
    else
49.  
    parent.right = e;
50.  
    fixAfterInsertion(e);
51.  
    size++;
52.  
    modCount++;
53.  
    return null;
54.  
    }

 

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.  
   [java] view plain copy
2.  
   1. Map map = new HashMap();
3.  
    
4.  
   Irerator iterator = map.entrySet().iterator();
5.  
    
6.  
   while(iterator.hasNext()) {
7.  
    
8.  
   Map.Entry entry = iterator.next();
9.  
    
10.  
    Object key = entry.getKey();
11.  
     
12.  
    //
13.  
     
14.  
    }
15.  
     
16.  
    2.Map map = new HashMap();
17.  
     
18.  
    Set keySet= map.keySet();
19.  
     
20.  
    Irerator iterator = keySet.iterator;
21.  
     
22.  
    while(iterator.hasNext()) {
23.  
     
24.  
    Object key = iterator.next();
25.  
     
26.  
    Object value = map.get(key);
27.  
     
28.  
    //
29.  
     
30.  
    }
31.  
    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
32.  
    [java] view plain copy
33.  
    Map map = new HashMap();
34.  
     
35.  
    Collection c = map.values();
36.  
     
37.  
    Iterator iterator = c.iterator();
38.  
     
39.  
    while(iterator.hasNext()) {
40.  
     
41.  
    Object value = iterator.next();