主要介绍HashMap的四种循环遍历方式，各种方式的性能测试对比，根据HashMap的源码实现分析性能结果，总结结论。
下面只是简单介绍各种遍历示例(以HashMap为例)，各自优劣会在本文后面进行分析给出结论。
Map<String, String> map = new HashMap<String, String>();for (Entry<String, String> entry : map.entrySet()) { entry.getKey(); entry.getValue();}(2) 显示调用map.entrySet()的集合迭代器
Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();while (iterator.hasNext()) { Map.Entry<String, String> entry = iterator.next(); entry.getKey(); entry.getValue();}(3) for each map.keySet()，再调用get获取
Map<String, String> map = new HashMap<String, String>();for (String key : map.keySet()) { map.get(key);}(4) for each map.entrySet()，用临时变量保存map.entrySet()
Set<Entry<String, String>> entrySet = map.entrySet();for (Entry<String, String> entry : entrySet) { entry.getKey(); entry.getValue();}在测试前大家可以根据对HashMap的了解，想想上面四种遍历方式哪个性能更优。
2、HashMap四种遍历方式的性能测试及对比
以下是性能测试代码，会输出不同数量级大小的HashMap各种遍历方式所花费的时间。
package cn.trinea.java.test; import java.text.DecimalFormat;import java.util.Calendar;import java.util.HashMap;import java.util.Iterator;import java.util.Map;import java.util.Map.Entry;import java.util.Set; /** * JavaLoopTest * * @author www.trinea.cn 2013-10-28 */public class JavaLoopTest { public static void main(String[] args) { System.out.PRint("compare loop performance of HashMap"); loopMapCompare(getHashMaps(10000, 100000, 1000000, 2000000)); } public static Map<String, String>[] getHashMaps(int... sizeArray) { Map<String, String>[] mapArray = new HashMap[sizeArray.length]; for (int i = 0; i < sizeArray.length; i++) { int size = sizeArray[i]; Map<String, String> map = new HashMap<String, String>(); for (int j = 0; j < size; j++) { String s = Integer.toString(j); map.put(s, s); } mapArray[i] = map; } return mapArray; } public static void loopMapCompare(Map<String, String>[] mapArray) { printHeader(mapArray); long startTime, endTime; // Type 1 for (int i = 0; i < mapArray.length; i++) { Map<String, String> map = mapArray[i]; startTime = Calendar.getInstance().getTimeInMillis(); for (Entry<String, String> entry : map.entrySet()) { entry.getKey(); entry.getValue(); } endTime = Calendar.getInstance().getTimeInMillis(); printCostTime(i, mapArray.length, "for each entrySet", endTime - startTime); } // Type 2 for (int i = 0; i < mapArray.length; i++) { Map<String, String> map = mapArray[i]; startTime = Calendar.getInstance().getTimeInMillis(); Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator(); while (iterator.hasNext()) { Map.Entry<String, String> entry = iterator.next(); entry.getKey(); entry.getValue(); } endTime = Calendar.getInstance().getTimeInMillis(); printCostTime(i, mapArray.length, "for iterator entrySet", endTime - startTime); } // Type 3 for (int i = 0; i < mapArray.length; i++) { Map<String, String> map = mapArray[i]; startTime = Calendar.getInstance().getTimeInMillis(); for (String key : map.keySet()) { map.get(key); } endTime = Calendar.getInstance().getTimeInMillis(); printCostTime(i, mapArray.length, "for each keySet", endTime - startTime); } // Type 4 for (int i = 0; i < mapArray.length; i++) { Map<String, String> map = mapArray[i]; startTime = Calendar.getInstance().getTimeInMillis(); Set<Entry<String, String>> entrySet = map.entrySet(); for (Entry<String, String> entry : entrySet) { entry.getKey(); entry.getValue(); } endTime = Calendar.getInstance().getTimeInMillis(); printCostTime(i, mapArray.length, "for entrySet=entrySet()", endTime - startTime); } } static int FIRST_COLUMN_LENGTH = 23, OTHER_COLUMN_LENGTH = 12, TOTAL_COLUMN_LENGTH = 71; static final DecimalFormat COMMA_FORMAT = new DecimalFormat("#,###"); public static void printHeader(Map... mapArray) { printRowDivider(); for (int i = 0; i < mapArray.length; i++) { if (i == 0) { StringBuilder sb = new StringBuilder().append("map size"); while (sb.length() < FIRST_COLUMN_LENGTH) { sb.append(" "); } System.out.print(sb); } StringBuilder sb = new StringBuilder().append("| ").append(COMMA_FORMAT.format(mapArray[i].size())); while (sb.length() < OTHER_COLUMN_LENGTH) { sb.append(" "); } System.out.print(sb); } TOTAL_COLUMN_LENGTH = FIRST_COLUMN_LENGTH + OTHER_COLUMN_LENGTH * mapArray.length; printRowDivider(); } public static void printRowDivider() { System.out.println(); StringBuilder sb = new StringBuilder(); while (sb.length() < TOTAL_COLUMN_LENGTH) { sb.append("-"); } System.out.println(sb); } public static void printCostTime(int i, int size, String caseName, long costTime) { if (i == 0) { StringBuilder sb = new StringBuilder().append(caseName); while (sb.length() < FIRST_COLUMN_LENGTH) { sb.append(" "); } System.out.print(sb); } StringBuilder sb = new StringBuilder().append("| ").append(costTime).append(" ms"); while (sb.length() < OTHER_COLUMN_LENGTH) { sb.append(" "); } System.out.print(sb); if (i == size - 1) { printRowDivider(); } }}PS：如果运行报异常in thread “main” java.lang.OutOfMemoryError: Java heap space，请将main函数里面map size的大小减小。
其中getHashMaps函数会返回不同size的HashMap。loopMapCompare函数会分别用上面的遍历方式1-4去遍历每一个map数组(包含不同大小HashMap)中的HashMap。print开头函数为输出辅助函数，可忽略。
测试环境为Windows7 32位系统 3.2G双核CPU 4G内存，Java 7，Eclipse -Xms512m -Xmx512m最终测试结果如下：
compare loop performance of HashMap-----------------------------------------------------------------------map size | 10,000 | 100,000 | 1,000,000 | 2,000,000 -----------------------------------------------------------------------for each entrySet | 2 ms | 6 ms | 36 ms | 91 ms -----------------------------------------------------------------------for iterator entrySet | 0 ms | 4 ms | 35 ms | 89 ms -----------------------------------------------------------------------for each keySet | 1 ms | 6 ms | 48 ms | 126 ms -----------------------------------------------------------------------for entrySet=entrySet()| 1 ms | 4 ms | 35 ms | 92 ms -----------------------------------------------------------------------表横向为同一遍历方式不同大小HashMap遍历的时间消耗，纵向为同一HashMap不同遍历方式遍历的时间消耗。PS：由于首次遍历HashMap会稍微多耗时一点，for each的结果稍微有点偏差，将测试代码中的几个Type顺序调换会发现，for each entrySet耗时和for iterator entrySet接近。
3、遍历方式性能测试结果分析(1) foreach介绍见：ArrayList和LinkedList的几种循环遍历方式及性能对比分析中介绍。
(2) HashMap遍历方式结果分析从上面知道for each与显示调用Iterator等价，上表的结果中可以看出除了第三种方式(for each map.keySet())，再调用get获取方式外，其他三种方式性能相当。本例还是hash值散列较好的情况，若散列算法较差，第三种方式会更加耗时。我们看看HashMap entrySet和keySet的源码
private final class KeyIterator extends HashIterator<K> { public K next() { return nextEntry().getKey(); }} private final class EntryIterator extends HashIterator<Map.Entry<K,V>> { public Map.Entry<K,V> next() { return nextEntry(); }}分别是keySet()和entrySet()返回的set的迭代器，从中我们可以看到只是返回值不同而已，父类相同，所以性能相差不多。只是第三种方式多了一步根据key get得到value的操作而已。get的时间复杂度根据hash算法而异，源码如下：public V get(Object key) { if (key == null) return getForNullKey(); Entry<K,V> entry = getEntry(key); return null == entry ? null : entry.getValue();} /** * Returns the entry associated with the specified key in the * HashMap. Returns null if the HashMap contains no mapping * for the key. */final Entry<K,V> getEntry(Object key) { int hash = (key == null) ? 0 : hash(key); for (Entry<K,V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } return null;}get的时间复杂度取决于for循环循环次数，即hash算法。
4、结论总结
从上面的分析来看：a. HashMap的循环，如果既需要key也需要value，直接用
Map<String, String> map = new HashMap<String, String>();for (Entry<String, String> entry : map.entrySet()) { entry.getKey(); entry.getValue();}即可，foreach简洁易懂。
b. 如果只是遍历key而无需value的话，可以直接用
Map<String, String> map = new HashMap<String, String>();for (String key : map.keySet()) { // key process}
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