文章目录
  1. 1. HashMap的四种遍历方式
  2. 2. HashMap四种遍历方式的性能测试及对比
  3. 3. HashMap遍历方式结果分析
  4. 4. 结论总结

HashMap的四种遍历方式

  1. for each map.entrySet()

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    Map<String, String> map = new HashMap<String, String>();
    for (Entry<String, String> entry : map.entrySet()) {
    entry.getKey();
    entry.getValue();
    }
  2. 调用map.entrySet()的集合迭代器

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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();
    }
  3. for each map.keySet(),再调用get获取

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    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()

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    Set<Entry<String, String>> entrySet = map.entrySet();
    for (Entry<String, String> entry : entrySet) {
    entry.getKey();
    entry.getValue();
    }

HashMap四种遍历方式的性能测试及对比

性能测试代码:

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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();
}
}
}

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:

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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
-----------------------------------------------------------------------

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。

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

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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();
}
}

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:

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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;
}

get的时间复杂度取决于for循环循环次数,即hash算法。

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

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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

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Map<String, String> map = new HashMap<String, String>();
for (String key : map.keySet()) {
// key process
}

参考:HashMap循环遍历方式及其性能对比

文章目录
  1. 1. HashMap的四种遍历方式
  2. 2. HashMap四种遍历方式的性能测试及对比
  3. 3. HashMap遍历方式结果分析
  4. 4. 结论总结