Java Map遍历方式的选择

忘是亡心i 2022-08-06 15:15 175阅读 0赞

# 1. 阐述 #

对于Java中Map的遍历方式，很多文章都推荐使用entrySet，认为其比keySet的效率高很多。理由是：entrySet方法一次拿到所有key和value的集合；而keySet拿到的只是key的集合，针对每个key，都要去Map中额外查找一次value，从而降低了总体效率。那么实际情况如何呢？

为了解遍历性能的真实差距，包括在遍历key+value、遍历key、遍历value等不同场景下的差异，我试着进行了一些对比测试。

# 2. 对比测试 #

一开始只进行了简单的测试，但结果却表明keySet的性能更好，这一点让我很是费解，不都说entrySet明显好于keySet吗？为了进一步地进行验证，于是采用了不同的测试数据进行更详细的对比测试。

## 2.1 测试数据 ##

### 2.1.1 HashMap测试数据 ###

*  HashMap-1，大小为100万，key和value均为String，key的值为1、2、3……1000000：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Map&lt;String, String&gt; map = new HashMap&lt;String, String&gt;();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">String key, value;</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (i = 1; i &lt;= num; i++) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = "" + i;</span></span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = "value";</span></span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; map.put(key, value);</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  HashMap-2，大小为100万，key和value均为String，key的值为50、100、150、200、……、50000000：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; height:219px; background-color:rgb(240,248,255); width:717px"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Map&lt;String, String&gt; map = new HashMap&lt;String, String&gt;();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">String key, value;</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (i = 1; i &lt;= num; i++) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = "" + (i * 50);</span></span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = "value";</span></span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; map.put(key, value);</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

2.1.2 TreeMap测试数据

*  TreeMap-1，大小为100万，key和value均为String，key的值为1、2、3……1000000：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Map&lt;String, String&gt; map = new TreeMap&lt;String, String&gt;();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">String key, value;</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (i = 1; i &lt;= num; i++) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = "" + i;</span></span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = "value";</span></span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; map.put(key, value);</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  TreeMap-2，大小为100万，key和value均为String，key的值为50、100、150、200、……、50000000，更离散：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Map&lt;String, String&gt; map = new TreeMap&lt;String, String&gt;();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">String key, value;</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (i = 1; i &lt;= num; i++) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = "" + (i * 50);</span></span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = "value";</span></span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; map.put(key, value);</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

2.2 测试场景

分别使用keySet、entrySet和values的多种写法测试三种场景：遍历key+value、遍历key、遍历value的场景。

### 2.2.1 遍历key+value ###

*  keySet遍历key+value（写法1）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Iterator&lt;String&gt; iter = map.keySet().iterator();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">while (iter.hasNext()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = iter.next();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = map.get(key);</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  keySet遍历key+value（写法2）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (String key : map.keySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = map.get(key);</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  entrySet遍历key+value（写法1）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Iterator&lt;Entry&lt;String, String&gt;&gt; iter = map.entrySet().iterator();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Entry&lt;String, String&gt; entry;</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">while (iter.hasNext()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; entry = iter.next();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = entry.getKey();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = entry.getValue();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*   entrySet遍历key+value（写法2）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (Entry&lt;String, String&gt; entry: map.entrySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = entry.getKey();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = entry.getValue();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

### 2.2.2 遍历key ###

*  keySet遍历key（写法1）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Iterator&lt;String&gt; iter = map.keySet().iterator();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">while (iter.hasNext()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = iter.next();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  keySet遍历key（写法2）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (String key : map.keySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*   entrySet遍历key（写法1）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Iterator&lt;Entry&lt;String, String&gt;&gt; iter = map.entrySet().iterator();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">while (iter.hasNext()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = iter.next().getKey();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  entrySet遍历key（写法2）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (Entry&lt;String, String&gt; entry: map.entrySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = entry.getKey();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

2.2.3 遍历value

*  keySet遍历value（写法1）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Iterator&lt;String&gt; iter = map.keySet().iterator();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">while (iter.hasNext()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = map.get(iter.next());</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  keySet遍历value（写法2）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (String key : map.keySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = map.get(key);</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  entrySet遍历value（写法1）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Iterator&lt;Entry&lt;String, String&gt;&gt; iter = map.entrySet().iterator();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">while (iter.hasNext()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">value = iter.next().getValue();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  entrySet遍历value（写法2）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (Entry&lt;String, String&gt; entry: map.entrySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = entry.getValue();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  values遍历value（写法1）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">Iterator&lt;String&gt; iter = map.values().iterator();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">while (iter.hasNext()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">value = iter.next();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

*  values遍历value（写法2）：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (String value : map.values()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

2.3 测试结果

### 2.3.1 HashMap测试结果 ###

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">单位：毫秒</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">HashMap-1</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">HashMap-2</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历key+value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">39</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">93</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历key+value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">38</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">87</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历key+value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">43</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">86</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历key+value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">43</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">85</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">单位：毫秒</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">HashMap-1</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">HashMap-2</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历key（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">27</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">65</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历key（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">26</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">64</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历key（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">35</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">75</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历key（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">34</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">74</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">单位：毫秒</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">HashMap-1</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">HashMap-2</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">38</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">87</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">37</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">87</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">34</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">61</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">32</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">62</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">values遍历value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">26</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">48</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">values遍历value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">26</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">48</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

2.3.2 TreeMap测试结果

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">单位：毫秒</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">TreeMap-1</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">TreeMap-2</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历key+value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">430</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">451</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历key+value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">429</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">450</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历key+value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">77</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">84</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历key+value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">70</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">68</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">单位：毫秒</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">TreeMap-1</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">TreeMap-2</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历key（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">50</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">49</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历key（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">49</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">48</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历key（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">66</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">64</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历key（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">65</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">63</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">单位：毫秒</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">TreeMap-1</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">TreeMap-2</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">432</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">448</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">keySet遍历value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">430</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">448</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">62</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">61</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">entrySet遍历value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">62</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">61</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">values遍历value（写法1）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">46</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">46</span></span></p> </td> 
  </tr> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">values遍历value（写法2）</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">45</span></span></p> </td> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:14px"><span style="font-family:KaiTi_GB2312">46</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

3. 结论

## 3.1 如果你使用HashMap ##

1.  同时遍历key和value时，keySet与entrySet方法的性能差异取决于key的具体情况，如复杂度（复杂对象）、离散度、冲突率等。换言之，取决于HashMap查找value的开销。entrySet一次性取出所有key和value的操作是有性能开销的，当这个损失小于HashMap查找value的开销时，entrySet的性能优势就会体现出来。例如上述对比测试中，当key是最简单的数值字符串时，keySet可能反而会更高效，耗时比entrySet少10%。总体来说还是推荐使用entrySet。因为当key很简单时，其性能或许会略低于keySet，但却是可控的；而随着key的复杂化，entrySet的优势将会明显体现出来。当然，我们可以根据实际情况进行选择
2.  只遍历key时，keySet方法更为合适，因为entrySet将无用的value也给取出来了，浪费了性能和空间。在上述测试结果中，keySet比entrySet方法耗时少23%。
3.  只遍历value时，使用vlaues方法是最佳选择，entrySet会略好于keySet方法。
4.  在不同的遍历写法中，推荐使用如下写法，其效率略高一些：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (String key : map.keySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = map.get(key);</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (Entry&lt;String, String&gt; entry: map.entrySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = entry.getKey();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = entry.getValue();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (String value : map.values()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

3.2 如果你使用TreeMap

1.  同时遍历key和value时，与HashMap不同，entrySet的性能远远高于keySet。这是由TreeMap的查询效率决定的，也就是说，TreeMap查找value的开销较大，明显高于entrySet一次性取出所有key和value的开销。因此，遍历TreeMap时强烈推荐使用entrySet方法。
2.  只遍历key时，keySet方法更为合适，因为entrySet将无用的value也给取出来了，浪费了性能和空间。在上述测试结果中，keySet比entrySet方法耗时少24%。
3.  只遍历value时，使用vlaues方法是最佳选择，entrySet也明显优于keySet方法。
4.  在不同的遍历写法中，推荐使用如下写法，其效率略高一些：

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (String key : map.keySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = map.get(key);</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (Entry&lt;String, String&gt; entry: map.entrySet()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; key = entry.getKey();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">&nbsp;&nbsp;&nbsp; value = entry.getValue();</span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px; padding:0px; border-collapse:collapse; border-spacing:0px; border:1px solid silver; word-break:break-word; background-color:rgb(240,248,255)"> 
 <tbody style="margin:0px; padding:0px"> 
  <tr style="margin:0px; padding:0px"> 
   <td style="margin:0px; padding:3px; border:1px solid silver; border-collapse:collapse"> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">for (String value : map.values()) { </span></span></p> <p style="margin:10px auto; padding-top:0px; padding-bottom:0px"><span style="margin:0px; padding:0px; font-size:13px"><span style="font-family:KaiTi_GB2312">}</span></span></p> </td> 
  </tr> 
 </tbody> 
</table>

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