遍历map的四种方法及Map.entry详解 |
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遍历map的四种方法及Map.entry详解
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Map.entrySet() 这个方法返回的是一个Set，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 map = new HashMap(); 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 it = map.entrySet().iterator(); while (it.hasNext()) { Map.Entry 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 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 /** * 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 implements Map.Entry { final K key; V value; Entry next; final int hash; /** * Creates new entry. */ Entry(int h, K k, V v, Entry 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 m) { } /** * This method is invoked whenever the entry is * removed from the table. */ void recordRemoval(HashMap 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 { /** * 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 * remove 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 remove operation). * * @param value new value to be stored in this entry * @return old value corresponding to the entry * @throws UnsupportedOperationException if the put 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 true if the given object is also a map entry and * the two entries represent the same mapping. More formally, two * entries e1 and e2 represent the same mapping * if * (e1.getKey()==null ? * e2.getKey()==null : e1.getKey().equals(e2.getKey())) && * (e1.getValue()==null ? * e2.getValue()==null : e1.getValue().equals(e2.getValue())) * * This ensures that the equals method works properly across * different implementations of the Map.Entry interface. * * @param o object to be compared for equality with this map entry * @return true 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 e is defined to be: * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^ * (e.getValue()==null ? 0 : e.getValue().hashCode()) * * This ensures that e1.equals(e2) implies that * e1.hashCode()==e2.hashCode() for any two Entries * e1 and e2, as required by the general * contract of Object.hashCode. * * @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 header;
初始化链表的方法： void init() { header = new Entry(-1, null, null, null); header.before = header.after = header; } TreeMap: [java] view plain copy //定义根节点 private transient Entry 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 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(key, value, null); size = 1; modCount++; return null; } int cmp; Entry parent; // split comparator and comparable paths Comparator |
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