Map.entry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

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<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();

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