主要介紹HashMap的四種循環(huán)遍歷方式，各種方式的性能測(cè)試對(duì)比，根據(jù)HashMap的源碼實(shí)現(xiàn)分析性能結(jié)果，總結(jié)結(jié)論。
下面只是簡(jiǎn)單介紹各種遍歷示例(以HashMap為例)，各自優(yōu)劣會(huì)在本文后面進(jìn)行分析給出結(jié)論。
Map<String, String> map = new HashMap<String, String>();for (Entry<String, String> entry : map.entrySet()) { entry.getKey(); entry.getValue();}(2) 顯示調(diào)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()，再調(diào)用get獲取
Map<String, String> map = new HashMap<String, String>();for (String key : map.keySet()) { map.get(key);}(4) for each map.entrySet()，用臨時(shí)變量保存map.entrySet()
Set<Entry<String, String>> entrySet = map.entrySet();for (Entry<String, String> entry : entrySet) { entry.getKey(); entry.getValue();}在測(cè)試前大家可以根據(jù)對(duì)HashMap的了解，想想上面四種遍歷方式哪個(gè)性能更優(yōu)。
2、HashMap四種遍歷方式的性能測(cè)試及對(duì)比
以下是性能測(cè)試代碼，會(huì)輸出不同數(shù)量級(jí)大小的HashMap各種遍歷方式所花費(fèi)的時(shí)間。
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：如果運(yùn)行報(bào)異常in thread “main” java.lang.OutOfMemoryError: Java heap space，請(qǐng)將main函數(shù)里面map size的大小減小。
其中g(shù)etHashMaps函數(shù)會(huì)返回不同size的HashMap。loopMapCompare函數(shù)會(huì)分別用上面的遍歷方式1-4去遍歷每一個(gè)map數(shù)組(包含不同大小HashMap)中的HashMap。print開頭函數(shù)為輸出輔助函數(shù)，可忽略。
測(cè)試環(huán)境為Windows7 32位系統(tǒng) 3.2G雙核CPU 4G內(nèi)存，Java 7，Eclipse -Xms512m -Xmx512m最終測(cè)試結(jié)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 -----------------------------------------------------------------------表橫向?yàn)橥槐闅v方式不同大小HashMap遍歷的時(shí)間消耗，縱向?yàn)橥籋ashMap不同遍歷方式遍歷的時(shí)間消耗。PS：由于首次遍歷HashMap會(huì)稍微多耗時(shí)一點(diǎn)，for each的結(jié)果稍微有點(diǎn)偏差，將測(cè)試代碼中的幾個(gè)Type順序調(diào)換會(huì)發(fā)現(xiàn)，for each entrySet耗時(shí)和for iterator entrySet接近。
3、遍歷方式性能測(cè)試結(jié)果分析(1) foreach介紹見：ArrayList和LinkedList的幾種循環(huán)遍歷方式及性能對(duì)比分析中介紹。
(2) HashMap遍歷方式結(jié)果分析從上面知道for each與顯示調(diào)用Iterator等價(jià)，上表的結(jié)果中可以看出除了第三種方式(for each map.keySet())，再調(diào)用get獲取方式外，其他三種方式性能相當(dāng)。本例還是hash值散列較好的情況，若散列算法較差，第三種方式會(huì)更加耗時(shí)。我們看看HashMap entrySet和keySet的源碼
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(); }}分別是keySet()和entrySet()返回的set的迭代器，從中我們可以看到只是返回值不同而已，父類相同，所以性能相差不多。只是第三種方式多了一步根據(jù)key get得到value的操作而已。get的時(shí)間復(fù)雜度根據(jù)hash算法而異，源碼如下：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的時(shí)間復(fù)雜度取決于for循環(huán)循環(huán)次數(shù)，即hash算法。
4、結(jié)論總結(jié)
從上面的分析來看：a. HashMap的循環(huán)，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簡(jiǎn)潔易懂。
b. 如果只是遍歷key而無需value的話，可以直接用
Map<String, String> map = new HashMap<String, String>();for (String key : map.keySet()) { // key process}
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