HashMap两种遍历方式的异同（大数据的情况下）

1.两种遍历方式：

第一种：

Iterator<String> keySetIterator = keySetMap.keySet().iterator();            while (keySetIterator.hasNext()) {                 String key = keySetIterator.next();                 String value = keySetMap.get(key);             //  System.out.println("key:="+key+" ,value:="+value);           // System.out.println(value);             } 

第二种：

        Iterator<Entry<String, String>> entryKeyIterator = entrySetMap.entrySet().iterator();             while (entryKeyIterator.hasNext()) {                 Entry<String, String> e = entryKeyIterator.next();             //  System.out.println("key:="+e.getKey()+" ,value:="+e.getValue());          //  System.out.println(e.getValue());             } 

性能比较：

import java.util.HashMap;import java.util.Iterator;import java.util.Map.Entry;/** * @author Administrator * */public class testHashMap1 {    public static void Test(){        HashMap<String, String> keySetMap = new HashMap<String, String>();            HashMap<String, String> entrySetMap = new HashMap<String, String>();          /**         * i=9000000keyset spent times:313entrySet spent times:169i=4000000keyset spent times:191entrySet spent times:111i=9999999keyset spent times:400entrySet spent times:342         * */        for (int i = 0; i < 9999999; i++) {                 keySetMap.put("" + i, "keySet");             }             for (int i = 0; i < 9999999; i++) {                 entrySetMap.put("" + i, "entrySet");             }             long startTimeOne = System.currentTimeMillis();             Iterator<String> keySetIterator = keySetMap.keySet().iterator();            while (keySetIterator.hasNext()) {                 String key = keySetIterator.next();                 String value = keySetMap.get(key);             //  System.out.println("key:="+key+" ,value:="+value);           // System.out.println(value);             }             System.out.println("keyset spent times:"                     + (System.currentTimeMillis() - startTimeOne));            long startTimeTwo = System.currentTimeMillis();             Iterator<Entry<String, String>> entryKeyIterator = entrySetMap                    .entrySet().iterator();             while (entryKeyIterator.hasNext()) {                 Entry<String, String> e = entryKeyIterator.next();             //  System.out.println("key:="+e.getKey()+" ,value:="+e.getValue());          //  System.out.println(e.getValue());             }             System.out.println("entrySet spent times:"                    + (System.currentTimeMillis() - startTimeTwo));         }    /**     * @param args     */    public static void main(String[] args) {        Test();    }}

通过测试发现,第二种方式的性能通常要比第一种方式高一倍.

2.原因分析（jdk1.8）

调用keySetMap.keySet()时执行的源代码是返回一个KeyIterator迭代器,其next方法只返回其key值.

    final class KeyIterator extends HashIterator        implements Iterator<K> {        public final K next() { return nextNode().key; }    }

而调用entrySetMap.entrySet()方法会生成EntryIterator 迭代器,其next方法返回一个Entry对象的一个实例,其中包含key和value.

    final class EntryIterator extends HashIterator        implements Iterator<Map.Entry<K,V>> {        public final Map.Entry<K,V> next() { return nextNode(); }    }

二者在此时的性能应该是相同的,但方式一再取得key所对应的value时,此时还要访问Map的这个方法,这时,方式一多遍历了一次
hashmap。

    /**     * Returns the value to which the specified key is mapped,     * or {@code null} if this map contains no mapping for the key.     *     * <p>More formally, if this map contains a mapping from a key     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :     * key.equals(k))}, then this method returns {@code v}; otherwise     * it returns {@code null}.  (There can be at most one such mapping.)     *     * <p>A return value of {@code null} does not <i>necessarily</i>     * indicate that the map contains no mapping for the key; it's also     * possible that the map explicitly maps the key to {@code null}.     * The {@link #containsKey containsKey} operation may be used to     * distinguish these two cases.     *     * @see #put(Object, Object)     */    public V get(Object key) {        Node<K,V> e;        return (e = getNode(hash(key), key)) == null ? null : e.value;    }

    /**     * Implements Map.get and related methods     *     * @param hash hash for key     * @param key the key     * @return the node, or null if none     */    final Node<K,V> getNode(int hash, Object key) {        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;        if ((tab = table) != null && (n = tab.length) > 0 &&            (first = tab[(n - 1) & hash]) != null) {            if (first.hash == hash && // always check first node                ((k = first.key) == key || (key != null && key.equals(k))))                return first;            if ((e = first.next) != null) {                if (first instanceof TreeNode)                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);                do {                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k))))                        return e;                } while ((e = e.next) != null);            }        }        return null;    }

这个 String value = keySetMap.get(key); 的get方法就是二者性能差别的主要原因,重新调用了do ..while循环遍历了一遍hashmap.