Map.entrySet() 這個方法返回的是一個Set<Map.Entry<K,V>>，Map.Entry 是Map中的一個接口，他的用途是表示一個映射項（里面有Key和Value），而Set<Map.Entry<K,V>>表示一個映射項的Set。Map.Entry里有相應的getKey和getValue方法，即JavaBean，讓我們能夠從一個項中取出Key和Value。

下面是遍歷Map的四種方法:

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'); //第一種：普遍使用，二次取值 System.out.println('通過Map.keySet遍歷key和value：'); for (String key : map.keySet()) { System.out.println('key= '+ key + ' and value= ' + map.get(key)); } //第二種 System.out.println('通過Map.entrySet使用iterator遍歷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()); } //第三種：推薦，尤其是容量大時 System.out.println('通過Map.entrySet遍歷key和value'); for (Map.Entry<String, String> entry : map.entrySet()) { System.out.println('key= ' + entry.getKey() + ' and value= ' + entry.getValue()); } //第四種 System.out.println('通過Map.values()遍歷所有的value，但不能遍歷key'); for (String v : map.values()) { System.out.println('value= ' + v); } }

下面是HashMap的源代碼：

首先HashMap的底層實現用的時候一個Entry數組

/** * The table, resized as necessary. Length MUST Always be a power of two. */ transient Entry[] table; //聲明了一個數組 ........ public HashMap() { this.loadFactor = DEFAULT_LOAD_FACTOR; threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR); table = new Entry[DEFAULT_INITIAL_CAPACITY];//初始化數組的大小為DEFAULT_INITIAL_CAPACITY(這里是16) init(); }

再來看一下Entry是在什么地方定義的，繼續上源碼,我們在HashMap的源碼的674行發現了它的定義，原來他是HashMap的一個內部類，并且實現了Map.Entry接口，以下有些地方是轉載

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) { } }

既然這樣那我們再看一下Map.Entry這個接口是怎么定義的，原來他是Map的一個內部接口并且定義了一些方法

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(); }

看到這里的時候大伙兒估計都明白得差不多了為什么HashMap為什么要選擇Entry數組來存放key-value對了吧，因為Entry實現的Map.Entry接口里面定義了getKey(),getValue()，setKey(),setValue()等方法相當于一個javaBean，對鍵值對進行了一個封裝便于后面的操作，從這里我們其實也可以聯想到不光是HashMap，譬如LinkedHashMap,TreeMap 等繼承自map的容器存儲key-value對都應該使用的是Entry只不過組織Entry的形式不一樣，HashMap用的是數組加鏈表的形式，LinkedHashMap用的是鏈表的形式，TreeMap應該使用的二叉樹的形式，不信的話上源碼

LinkedHashMap:

/** * The head of the doubly linked list. */ /定義了鏈頭 private transient Entry<K,V> header;

初始化鏈表的方法：

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

TreeMap:

//定義根節點 private transient Entry<K,V> root = null;

再看他的put方法，是不是很面熟（二叉排序樹的插入操作）

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，明白了各種Map的底層存儲key-value對的方式后，再來看看如何遍歷map吧，這里用HashMap來演示吧

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個Set集合，此集合的類型為Map.Entry。

so,很容易寫出如下的遍歷代碼

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，還有一種遍歷方法是，單純的遍歷value值，Map有一個values方法，返回的是value的Collection集合。通過遍歷collection也可以遍歷value,如[java] view plain copyMap map = new HashMap(); Collection c = map.values(); Iterator iterator = c.iterator(); while(iterator.hasNext()) { Object value = iterator.next();

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