遍历map的四种方法

 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

   
   
   

    
    
 
     
 
      
 
     
 
     
 
      

        java] view plain copy 
      
 
     
    
    
 
     
 
      
 
     
 
     
 
      

        <pre name= 
       "code" 
       class= 
       "java"> 
       /**  
      
 
     
    
    
 
     
 
      
 
     
 
     
 
      
 
        * 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(); 
      
 
     
    
    
 
     
 
      
 
     
 
     
 
      

        }</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

 
 
 
 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:

 
 
 
 [ java] view plain copy
 
 
 
 //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​​​​​​​

 
 
 
 [java] view plain copy
 
 
 
 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();

原文转至：https://blog.csdn.net/kyi_zhu123/article/details/52769469