HashMap循环遍历方式及其性能对比
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。
1. Map的四种遍历方式
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。
(1) for each map.entrySet()
Map<String, String> map = new HashMap<String, String>(); for (Entry<String, String> entry : map.entrySet()) { entry.getKey(); entry.getValue(); }
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Map<String, String> map = new HashMap<String, String>();
for (Entry<String, String> entry : map.entrySet()) {
entry.getKey();
entry.getValue();
}
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(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(); }
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Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry<String, String> entry = iterator.next();
entry.getKey();
entry.getValue();
}
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(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); }
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Map<String, String> map = new HashMap<String, String>();
for (String key : map.keySet()) {
map.get(key);
}
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(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(); }
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Set<Entry<String, String>> entrySet = map.entrySet();
for (Entry<String, String> entry : entrySet) {
entry.getKey();
entry.getValue();
}
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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(); } } }
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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();
}
}
}
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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 -----------------------------------------------------------------------
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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
-----------------------------------------------------------------------
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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(); } }
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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();
}
}
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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; }
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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;
}
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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(); }
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Map<String, String> map = new HashMap<String, String>();
for (Entry<String, String> entry : map.entrySet()) {
entry.getKey();
entry.getValue();
}
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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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Map<String, String> map = new HashMap<String, String>();
for (String key : map.keySet()) {
// key process
}
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