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<a class="post-title-link" href="/2015/08/21/根据手机号码解析归属地/">
根据手机号码解析归属地
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发表于 2015-08-21
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<p>项目中有时候需要根据手机号得到用户的归属地信息，在网上查了下有免费的API接口可以根据手机号得到用户的归属地信息，感谢这些免费的API。<br>下面就用一个根据用户手机号码查询归属地的API接口解析的例子，API是淘宝提供的还是比较牛掰的。<br>比较简单：<br><figure class="highlight elixir"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="symbol">http:</span>/<span class="regexp">/tcc.taobao.com/cc</span><span class="regexp">/json/mobile</span>_tel_segment.htm?tel=<span class="number">15850781443</span></span><br><span class="line">参数：</span><br><span class="line">tel：手机号码</span><br><span class="line">返回：<span class="constant">JSON</span></span><br><span class="line"><span class="constant">__GetZoneResult_ </span>= {</span><br><span class="line"> <span class="symbol">mts:</span><span class="string">'1585078'</span>,</span><br><span class="line"> <span class="symbol">province:</span><span class="string">'江苏'</span>,</span><br><span class="line"> <span class="symbol">catName:</span><span class="string">'中国移动'</span>,</span><br><span class="line"> <span class="symbol">telString:</span><span class="string">'15850781443'</span>,</span><br><span class="line"> <span class="symbol">areaVid:</span><span class="string">'30511'</span>,</span><br><span class="line"> <span class="symbol">ispVid:</span><span class="string">'3236139'</span>,</span><br><span class="line"> <span class="symbol">carrier:</span><span class="string">'江苏移动'</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>根据给的API需要准备两样东西：<br>一、调用apache的HttpClient类实现httpGet方法，调用此API。<br>二、使用Jackson 解析返回来的json字符串成Map即可对应取其中的字段。</p>
<p>具体代码如下：<br><figure class="highlight processing"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> courage.me.utils;</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> java.util.Map;</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> org.apache.http.HttpEntity;</span><br><span class="line"><span class="keyword">import</span> org.apache.http.client.methods.CloseableHttpResponse;</span><br><span class="line"><span class="keyword">import</span> org.apache.http.client.methods.HttpGet;</span><br><span class="line"><span class="keyword">import</span> org.apache.http.impl.client.CloseableHttpClient;</span><br><span class="line"><span class="keyword">import</span> org.apache.http.impl.client.HttpClients;</span><br><span class="line"><span class="keyword">import</span> org.apache.http.util.EntityUtils;</span><br><span class="line"><span class="keyword">import</span> org.codehaus.jackson.JsonParser;</span><br><span class="line"><span class="keyword">import</span> org.codehaus.jackson.<span class="built_in">map</span>.ObjectMapper;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> class PhoneNumberUtils {</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">String</span> phoneLocationAPI = <span class="string">"http://tcc.taobao.com/cc/json/mobile_tel_segment.htm"</span>;</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">String</span> getProvinceByPhoneNumber(<span class="keyword">String</span> phoneNumber) {</span><br><span class="line"> <span class="keyword">String</span> param = <span class="string">"tel="</span>+phoneNumber;</span><br><span class="line"> <span class="keyword">String</span> resultJson = <span class="string">""</span>;</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> resultJson = httpGet(phoneLocationAPI, param);</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e1) {</span><br><span class="line"> e1.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//去掉=及其前面的字符串，格式化成规范的json字符串格式 :key：value</span></span><br><span class="line"> resultJson = resultJson.replaceAll(<span class="string">"^[__]\\w{14}+[_ = \']+"</span>, <span class="string">""</span>).<span class="built_in">trim</span>().replace(<span class="string">"\'"</span>, <span class="string">"\""</span>);</span><br><span class="line"> <span class="keyword">return</span> jsonToMap(resultJson).<span class="built_in">get</span>(<span class="string">"province"</span>);</span><br><span class="line"> } </span><br><span class="line"></span><br><span class="line"> <span class="comment">/**封装http get请求*/</span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">String</span> httpGet(<span class="keyword">String</span> url, <span class="keyword">String</span> params) <span class="keyword">throws</span> Exception {</span><br><span class="line"> CloseableHttpClient httpclient = HttpClients.createDefault();</span><br><span class="line"> HttpGet httpget = <span class="keyword">new</span> HttpGet(url+<span class="string">"?"</span>+params);</span><br><span class="line"> CloseableHttpResponse response = httpclient.execute(httpget);</span><br><span class="line"> <span class="keyword">String</span> result = <span class="string">""</span>;</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> HttpEntity entity = response.getEntity();</span><br><span class="line"> <span class="keyword">if</span> (entity != <span class="keyword">null</span>) {</span><br><span class="line"> result = EntityUtils.toString(entity);</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> response.close();</span><br><span class="line"> httpclient.close();</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> result;</span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="comment">/**jackson parse string to map*/</span></span><br><span class="line"> @SuppressWarnings(<span class="string">"unchecked"</span>)</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Map<<span class="keyword">String</span>, <span class="keyword">String</span>> jsonToMap(<span class="keyword">String</span> jsonString) {</span><br><span class="line"> </span><br><span class="line"> Map<<span class="keyword">String</span>, <span class="keyword">String</span>> resultMap = <span class="keyword">null</span>;</span><br><span class="line"> ObjectMapper mapper = <span class="keyword">new</span> ObjectMapper(); </span><br><span class="line"> <span class="comment">//保证可以解析不规则的json格式</span></span><br><span class="line"> mapper.configure(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES, <span class="keyword">true</span>); </span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> resultMap = mapper.readValue(jsonString, Map.class);</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } </span><br><span class="line"> <span class="keyword">return</span> resultMap;</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> main(<span class="keyword">String</span>[] args) {</span><br><span class="line"> <span class="keyword">String</span> province = getProvinceByPhoneNumber(<span class="string">"15850781443"</span>);</span><br><span class="line"> System.out.<span class="built_in">println</span>(province);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>开源中国社区原始链接：<a href="http://www.oschina.net/code/snippet_251355_18921" target="_blank" rel="external">http://www.oschina.net/code/snippet_251355_18921</a></p>
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<a class="post-title-link" href="/2015/08/19/Java-Generate-Xml/">
Java Generate Xml
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发表于 2015-08-19
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<p>在对http请求的接口进行解析处理时，经常会遇到传过来的是xml格式的数据，处理完成后需要返回一个xml格式的String回去。<br>下面就是一个简单的生成xml格式的数据返回功能。</p>
<figure class="highlight dart"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="literal">import</span> org.dom4j.Document;</span><br><span class="line"><span class="literal">import</span> org.dom4j.DocumentHelper;</span><br><span class="line"><span class="literal">import</span> org.dom4j.<span class="built_in">Element</span>;</span><br><span class="line">public <span class="class"><span class="keyword">class</span> <span class="title">GenerateXmlTest</span> </span>{</span><br><span class="line"> private <span class="literal">static</span> <span class="built_in">String</span> string2Xml(<span class="built_in">String</span> field, <span class="built_in">String</span> value) {</span><br><span class="line"> Document <span class="built_in">document</span> = DocumentHelper.createDocument();</span><br><span class="line"> <span class="built_in">Element</span> root = <span class="built_in">document</span>.addElement(field);</span><br><span class="line"> root.addText(value);</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">document</span>.asXML();</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> public <span class="literal">static</span> <span class="keyword">void</span> main(<span class="built_in">String</span>[] args) {</span><br><span class="line"> <span class="built_in">String</span> s = string2Xml(<span class="string">"root"</span>, <span class="string">"0"</span>);</span><br><span class="line"> System.out.println(s);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>out<br><figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="pi"><?xml version="1.0" encoding="UTF-8"?></span></span><br><span class="line"><span class="tag"><<span class="title">root</span>></span>0<span class="tag"></<span class="title">root</span>></span></span><br></pre></td></tr></table></figure></p>
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<a class="post-title-link" href="/2015/08/18/java-BigInteger-to-string/">
java BigInteger to string
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<span class="post-time">
发表于 2015-08-18
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<p>从数据库取出一个字段的值,赋值给自己的创建的变量，出现如下异常告警：<br><figure class="highlight stylus"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">java<span class="class">.lang</span><span class="class">.ClassCastException</span>: java<span class="class">.math</span><span class="class">.BigInteger</span> cannot be cast to java<span class="class">.lang</span><span class="class">.String</span></span><br></pre></td></tr></table></figure></p>
<p>后来发现原来在mysql中建表的时候，设置字段的类型为bigint型<br>我在代码中要转成Integer类型的时候用到：<br>Integer.parseInt((String)map.get(vaule))；这句报错</p>
<p>具体的解决方案：通过Object封装作为中间层进行转换</p>
<figure class="highlight nix"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Object <span class="variable">ob =</span> <span class="built_in">map</span>.get(keyField);</span><br><span class="line">Integer.parseInt(ob.<span class="built_in">toString</span>());</span><br></pre></td></tr></table></figure>
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<a class="post-title-link" href="/2015/08/18/mysql保留关键字和表字段冲突/">
mysql保留关键字和表字段冲突
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发表于 2015-08-18
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<p>mysql数据库表的字段名称不能和数据库本身的保留关键字相同<br>问题：插入数据库的sql报异常，检查了好多遍没发现到底是啥原因，问题如下：<br><figure class="highlight livecodeserver"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">Thread-<span class="number">6</span> ERROR [com.sdk.biz.snowfox.common.db.SaveCacheToDB] doBatchDB <span class="number">1</span> exception: </span><br><span class="line">java.sql.SQLException: You have <span class="operator">an</span> error <span class="operator">in</span> your SQL syntax; </span><br><span class="line">check <span class="operator">the</span> manual that corresponds <span class="built_in">to</span> your MySQL server <span class="built_in">version</span> <span class="keyword">for</span> <span class="operator">the</span> <span class="constant">right</span> syntax <span class="built_in">to</span> use </span><br><span class="line">near <span class="string">'desc,create_time) values('</span>t<span class="string">','</span><span class="operator">a</span><span class="string">','</span>d<span class="string">','</span>o<span class="string">','</span><span class="operator">a</span><span class="string">','</span>test<span class="string">','</span><span class="number">2015</span>-<span class="number">08</span>-<span class="number">14</span> <span class="number">19</span>:<span class="number">25</span>:<span class="number">35</span><span class="string">')'</span> <span class="keyword">at</span> <span class="built_in">line</span> <span class="number">1</span> </span><br><span class="line">Query: insert ignore <span class="keyword">into</span> testTable(ptxId,userId,txId,priceId,statusCode,desc,create_time) values(?,?,?,?,?,?,?); </span><br><span class="line">Parameters: [[t, <span class="operator">a</span>, d, o, <span class="operator">a</span>, test, <span class="number">2015</span>-<span class="number">08</span>-<span class="number">14</span> <span class="number">19</span>:<span class="number">21</span>:<span class="number">04</span>], [t, <span class="operator">a</span>, d, o, <span class="operator">a</span>, test, <span class="number">2015</span>-<span class="number">08</span>-<span class="number">14</span> <span class="number">19</span>:<span class="number">25</span>:<span class="number">35</span>]]</span><br><span class="line"> <span class="keyword">at</span> org.apache.commons.dbutils.AbstractQueryRunner.rethrow(AbstractQueryRunner.java:<span class="number">392</span>) ~[commons-dbutils-<span class="number">1.6</span>.jar:<span class="number">1.6</span>]</span><br><span class="line"> <span class="keyword">at</span> org.apache.commons.dbutils.QueryRunner.batch(QueryRunner.java:<span class="number">155</span>) ~[commons-dbutils-<span class="number">1.6</span>.jar:<span class="number">1.6</span>]</span><br><span class="line"> <span class="keyword">at</span> org.apache.commons.dbutils.QueryRunner.batch(QueryRunner.java:<span class="number">92</span>) ~[commons-dbutils-<span class="number">1.6</span>.jar:<span class="number">1.6</span>]</span><br><span class="line"> <span class="keyword">at</span> com.sdk.core.jdbc.DBUtils.batch(DBUtils.java:<span class="number">405</span>) ~[classes/:na]</span><br><span class="line"> <span class="keyword">at</span> com.sdk.core.jdbc.DBModel.batch(DBModel.java:<span class="number">64</span>) ~[classes/:na]</span><br></pre></td></tr></table></figure></p>
<p>仔细看上面的报错原因：You have an error in your SQL syntax，use near ‘desc,create_time)<br>sql语法错误，在desc这个字段附近。不注意还真不知道到底为啥会出现这种错误，后来在本地测试出现一样的问题。<br>找同事过来帮忙看原因也是一头雾水，后来终于发现，原因这个desc和mysql数据库中的排序关键字冲突了。</p>
<p>解决方案：1、修改字段名称desc，如desc_name，以后建表的时候要注意不要使用关键字了。<br>2、insert ignore into sdk_callback_pulzesdk(ptxId,userId,txId,priceId,statusCode,<code>desc</code>,create_time)<br> values(‘a’,’b’,’c’,’d’,’e’,’test’, ‘2015-08-14 19:21:04’)<br>注意desc和其他字段的区别，多了两个符号括起来。就是键盘上数字键的前面那个键盘上的符号（不晓得名字了）</p>
<p>总结：遇到问题，解决问题的过程是相当苦逼的，并且遇到这种坑爹的问题，更是要细心，成长的过程就是这么酸爽。下次建表的时候要长点心了！</p>
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<a class="post-title-link" href="/2015/08/03/Top-10-Java中Strings的常见问题/">
Top 10 Java中Strings的常见问题
</a>
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<span class="post-time">
发表于 2015-08-03
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| 分类于
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<p>下面是关于Java String最常见的10个问题</p>
<h2 id="1-_如何比较String类型的变量？用“==”还是equals()?">1. 如何比较String类型的变量？用“==”还是equals()?</h2><p>简言之，“==”用于测试引用是否相等，equals()测试值是否相等。如果你想检查两个string是不是同一个对象，你需要使用equals()方法。<br>如果你知道String的intering(link)概念就更好了。</p>
<h2 id="2-_为什么说char[]比string好对于安全敏感的信息来说？">2. 为什么说char[]比string好对于安全敏感的信息来说？</h2><p>String是不可变长的，这就意味着一旦被创建，就不会改变直到垃圾回收掉。用一个数组你可以显式的更改它的元素值。用这种方式，安全敏感信息（像密码）将不会被显式在系统中。</p>
<h2 id="3-_在switch语句中可以使用string吗？">3. 在switch语句中可以使用string吗？</h2><p>从JDK1.7开始，我们可以使用string作为switch语句的条件。<br><figure class="highlight ceylon"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//java 7 </span></span><br><span class="line"><span class="keyword">switch</span>(str.toLowerCase()) {</span><br><span class="line"> <span class="keyword">case</span> <span class="string">"a"</span>:</span><br><span class="line"> <span class="keyword">value</span> = <span class="number">1</span>;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> <span class="keyword">case</span> <span class="string">"b"</span>:</span><br><span class="line"> <span class="keyword">value</span> = <span class="number">2</span>;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<h2 id="4-_如何将string转换成int？">4. 如何将string转换成int？</h2><figure class="highlight aspectj"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> n = Integer.parseInt(<span class="string">"10"</span>);</span><br></pre></td></tr></table></figure>
<h2 id="5-_如何用空格分割一个字符串？">5. 如何用空格分割一个字符串？</h2><figure class="highlight armasm"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">String[] </span><span class="keyword">strArray </span>= aString.split(<span class="string">"\\s+"</span>)<span class="comment">;</span></span><br></pre></td></tr></table></figure>
<h2 id="6-_substring()方法到底是干什么用的？">6. substring()方法到底是干什么用的？</h2><p>在JDK1.6中，substring()方法引用一个已经存在的string的字符数组，而不是重新创建一个。创建一个新的string代表一个新的字符数组。你可以+一个空字符串来创建一个新的string<br><figure class="highlight openscad"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">str</span>.substring<span class="params">(m, n)</span> + <span class="string">""</span>;</span><br></pre></td></tr></table></figure></p>
<p>这句代码将会创建一个新的字符数组来表示一个新的string，有时候这种方式会使你的代码更快，因为垃圾回收器可以收集不用的大的string并且只保留其sub string.</p>
<h2 id="7-_String_vs_StringBuffer_vs_StringBuilder">7. String vs StringBuffer vs StringBuilder</h2><p>String vs StringBuilder:StringBuilder是可变长的，这就意味着你可以在创建一个字符串后修改它。StringBuilder vs StringBuilder: StringBuffer是同步的，也就是说是线程安全的，但是比StringBuilder速度慢。</p>
<h2 id="8-_如何重复一个string？">8. 如何重复一个string？</h2><p>在Python中我们可以用一个数字就可以重复一个string。Java中，我们可以使用Apache的StringUtils.repeat()方法。<br><figure class="highlight processing"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">String</span> <span class="built_in">str</span> = <span class="string">"adcs"</span>;</span><br><span class="line"><span class="keyword">String</span> repeated = StringUtils.repeat(<span class="built_in">str</span>,<span class="number">3</span>);</span><br></pre></td></tr></table></figure></p>
<h2 id="9-_如何将字符串转换成日期类型？">9. 如何将字符串转换成日期类型？</h2><figure class="highlight lasso"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">String</span> str <span class="subst">=</span> <span class="string">"Sep 17, 2013"</span>;</span><br><span class="line"><span class="built_in">Date</span> <span class="built_in">date</span> <span class="subst">=</span> <span class="literal">new</span> SimpleDateFormat(<span class="string">"MMMM d, yy"</span>, <span class="built_in">locale</span><span class="built_in">.</span>ENGLISH)<span class="built_in">.</span>parse(str);</span><br></pre></td></tr></table></figure>
<h2 id="10-_如何计数一个char在一个string中出现的次数？">10. 如何计数一个char在一个string中出现的次数？</h2><p>使用Apache的StringUtils类的方法<br><figure class="highlight aspectj"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> n = StringUtils.countMatches(<span class="string">"111222"</span>, <span class="string">"1"</span>);</span><br></pre></td></tr></table></figure></p>
<p><a href="www.programcreek.com/2013/09/top-10-faqs-of-java-strings/">原文链接</a></p>
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<a class="post-title-link" href="/2015/08/03/LittleSummry/">
LittleSummry
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<span class="post-time">
发表于 2015-08-03
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<p>2015年转眼间已经过了一大半了，生活继而有了很大的变化，而不是改善，当初为了自己所谓的梦想和追求，走了出来，虽说不后悔，却也有些伤感。一方面在收支上直接跟财政挂钩，搞了个财政赤字出来。另一方面在学习上，虽然学习了很多东西，实际上并没有想象中学到的多而且深入。理想和现实之间总有无法逾越的鸿沟，即使计划的总体安排是这样，但是并没有起到太多决定性的作用。</p>
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<a class="post-title-link" href="/2015/07/20/Top-10-Java常见的Exceptions/">
Top 10 Java常见的Exceptions
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发表于 2015-07-20
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<p>这篇文章总结了关于Java Exceptions 10个最常被问的问题</p>
<h2 id="1-_Checked_vs_UnChecked">1. Checked vs UnChecked</h2><p>简言之，检查异常必须在一个方法中被显式捕获，或者在方法的throws从句中被声明。非检查时异常是由于除数为0、空指针异常等问题导致的。非检查时异常特别重要，因为你希望其他使用你的API的开发者知道如何处理这些异常。<br>例如，IOException是一种常见的检查时异常，运行时异常是一种非检查时异常。在读余下的文章之前你可以看下<a href="http://www.programcreek.com/2009/02/diagram-for-hierarchy-of-exception-classes/" target="_blank" rel="external">Java Exception Hierarchy Diagram</a></p>
<h2 id="2-_管理异常的最佳实践">2. 管理异常的最佳实践</h2><p>如果一个Exception可以被合理的处理，那么它就应该被caught，否则就应该被thrown抛出。</p>
<h2 id="3-_为什么在try中声明的变量不能在catch和finally语句块的中被调用？">3. 为什么在try中声明的变量不能在catch和finally语句块的中被调用？</h2><p>下面的代码中，字符串s在try块中被声明，不能被用在catch从句中。如下代码不能通过编译。</p>
<figure class="highlight gradle"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">File</span> <span class="keyword">file</span> = <span class="keyword">new</span> <span class="keyword">File</span>(<span class="string">"path"</span>);</span><br><span class="line"> FileInputStream fis = <span class="keyword">new</span> FileInputStream(<span class="keyword">file</span>);</span><br><span class="line"> String s = <span class="string">"insideTry"</span>;</span><br><span class="line">}<span class="keyword">catch</span>(FileNotFoundException e){</span><br><span class="line"> e.printStactTrace();</span><br><span class="line"> System.out.<span class="keyword">print</span>(s);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>原因是你不知道在try语句块中Exception什么时候被抛出。对于一些特殊的例子来说，异常很有可能会在对象声明之前被抛出。</p>
<h2 id="4-_为什么Double-parseDouble(null)和Integer-parseInt(null)抛出不同的异常?">4. 为什么Double.parseDouble(null)和Integer.parseInt(null)抛出不同的异常?</h2><p>首先，它们确实抛出不同的异常。这是JDK的问题，主要是它们是不同的开发人员开发的，不用多想。<br><figure class="highlight gradle"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">Integer.parseInt(<span class="keyword">null</span>);</span><br><span class="line"><span class="comment">//throws java.lang.NumberFormatException: null</span></span><br><span class="line"><span class="keyword">Double</span>.parseDouble(<span class="keyword">null</span>);</span><br><span class="line"><span class="comment">//throws java.lang.NullPointerException</span></span><br></pre></td></tr></table></figure></p>
<h2 id="5-_Java中常见的运行时异常">5. Java中常见的运行时异常</h2><p>这里给出一部分，IllegalArgumentException，ArrayIndexOutBoundsException<br>它们可以用于如下语句条件不满足的时候。<br><figure class="highlight actionscript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span>(obj == <span class="literal">null</span>){</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> IllegalArgumentException(<span class="string">"obj can not be null"</span>);</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<h2 id="6-_我们可以在一个catch从句中捕捉多个Exception吗？">6. 我们可以在一个catch从句中捕捉多个Exception吗？</h2><p>答案是肯定的。只要这些异常类在类的继承中都继承同一个父类，你可以只用父类就可以了。</p>
<h2 id="7-_Java中构造函数可以抛出异常吗？">7. Java中构造函数可以抛出异常吗？</h2><p>答案是肯定的，构造函数是一种特殊的函数。</p>
<h2 id="8-_在finally从句中抛出异常。">8. 在finally从句中抛出异常。</h2><p>做下面的处理是合法的<br><figure class="highlight gradle"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> mani(String[] args) {</span><br><span class="line"> <span class="keyword">File</span> file1 = <span class="keyword">new</span> <span class="keyword">File</span>(<span class="string">"path1"</span>);</span><br><span class="line"> <span class="keyword">File</span> file2 = <span class="keyword">new</span> <span class="keyword">File</span>(<span class="string">"path2"</span>);</span><br><span class="line"> <span class="keyword">try</span>{</span><br><span class="line"> FileInputStream fis = <span class="keyword">new</span> FileInputStream(file1);</span><br><span class="line"> }<span class="keyword">catch</span>(FileNotFoundException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }<span class="keyword">finally</span> {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> FileInputStream fis = <span class="keyword">new</span> FileInputStream(file2);</span><br><span class="line"> }<span class="keyword">catch</span>(FileNotFoundException e) {</span><br><span class="line"> e.printStackTrace(); </span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>但是要有更好的代码可读性，你应该把嵌入式的try-catch块包起来作为一种新方法，然后将方法调用放在finally一个子句中。<br><figure class="highlight gradle"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> mani(String[] args) {</span><br><span class="line"> <span class="keyword">File</span> file1 = <span class="keyword">new</span> <span class="keyword">File</span>(<span class="string">"path1"</span>);</span><br><span class="line"> <span class="keyword">File</span> file2 = <span class="keyword">new</span> <span class="keyword">File</span>(<span class="string">"path2"</span>);</span><br><span class="line"> <span class="keyword">try</span>{</span><br><span class="line"> FileInputStream fis = <span class="keyword">new</span> FileInputStream(file1);</span><br><span class="line"> }<span class="keyword">catch</span>(FileNotFoundException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }<span class="keyword">finally</span> {</span><br><span class="line"> methodThrowException();</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<h2 id="9-_在finally语句块中可以用return关键字吗？">9. 在finally语句块中可以用return关键字吗？</h2><p>可以。</p>
<h2 id="10-_为什么开发者默认处理异常？">10. 为什么开发者默认处理异常？</h2><pre><code>有这么多的时间段类似的代码段发生。如果正确处理异常是非常重要的，为什么开发人员一直这样做？
</code></pre><figure class="highlight julia"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">try</span> {</span><br><span class="line"> ....</span><br><span class="line">}<span class="keyword">catch</span>(<span class="built_in">Exception</span> <span class="literal">e</span>) {</span><br><span class="line"> <span class="literal">e</span>.printStackTrace();</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>忽略虽然很容易。频繁发生并不意味着正确性。</p>
<p><a href="http://www.programcreek.com/2013/10/top-10-questions-about-java-exceptions/" target="_blank" rel="external">原文链接</a></p>
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<a class="post-title-link" href="/2015/07/16/Top-10-Java中常用于Array的方法/">
Top 10 Java中常用于Array的方法
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发表于 2015-07-16
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<p>下面是经常在Java Array中用到的10个方法。在StackOverFlow上被提出最多的问题。</p>
<h2 id="1-_声明一个数组">1. 声明一个数组</h2><figure class="highlight dart"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">String</span>[] aArray = <span class="keyword">new</span> <span class="built_in">String</span>[<span class="number">5</span>];</span><br><span class="line"><span class="built_in">String</span>[] bArray = {<span class="string">"a"</span>,<span class="string">"b"</span>,<span class="string">"c"</span>,<span class="string">"d"</span>,<span class="string">"e"</span>};</span><br><span class="line"><span class="built_in">String</span>[] cArray = <span class="keyword">new</span> <span class="built_in">String</span>[]{<span class="string">"a"</span>,<span class="string">"b"</span>,<span class="string">"c"</span>,<span class="string">"d"</span>,<span class="string">"e"</span>};</span><br></pre></td></tr></table></figure>
<h2 id="2-_打印一个数组">2. 打印一个数组</h2><figure class="highlight stata"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">int[] intArray = { 1, 2, 3, 4, 5 };</span><br><span class="line">String intArrayString = Arrays.<span class="keyword">toString</span>(intArray);</span><br><span class="line">System.<span class="keyword">out</span>.println(intArray);</span><br><span class="line"><span class="comment">// [I@7150bd4d</span></span><br><span class="line">System.<span class="keyword">out</span>.println(intArrayString);</span><br><span class="line"><span class="comment">// [1, 2, 3, 4, 5]</span></span><br><span class="line"></span><br><span class="line"><span class="comment">//foreach</span></span><br><span class="line"><span class="keyword">for</span>(int <span class="keyword">n</span> : intArray) {</span><br><span class="line"> System.<span class="keyword">out</span>.<span class="keyword">print</span>(<span class="keyword">n</span>);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="3-_用array创建一个list">3. 用array创建一个list</h2><p><strong>Arrays.asList(stringArray)</strong>传入一个数组，返回一个list<br><figure class="highlight processing"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">String</span>[] stringArray = { <span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>, <span class="string">"d"</span>, <span class="string">"e"</span> };</span><br><span class="line">ArrayList<<span class="keyword">String</span>> arrayList = <span class="keyword">new</span> ArrayList<<span class="keyword">String</span>>(Arrays.asList(stringArray));</span><br><span class="line">System.out.<span class="built_in">println</span>(arrayList);</span><br><span class="line"><span class="comment">// [a, b, c, d, e]</span></span><br></pre></td></tr></table></figure></p>
<h2 id="4-_连接两个数组(首尾连接)">4. 连接两个数组(首尾连接)</h2><p>用到apache一个jar包 commons-lang3-3.4.jar<br><figure class="highlight aspectj"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span>[] intArray = { <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span> };</span><br><span class="line"><span class="keyword">int</span>[] intArray2 = { <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>, <span class="number">9</span>, <span class="number">10</span> };</span><br><span class="line"><span class="keyword">int</span>[] combinedIntArray = ArrayUtils.addAll(intArray, intArray2);</span><br><span class="line"><span class="comment">// combinedIntArray {1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }</span></span><br></pre></td></tr></table></figure></p>
<h2 id="5-_数组中插入分割字符">5. 数组中插入分割字符</h2><figure class="highlight processing"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// containing the provided list of elements</span></span><br><span class="line"><span class="comment">// Apache common lang</span></span><br><span class="line"><span class="keyword">String</span> j = StringUtils.<span class="built_in">join</span>(<span class="keyword">new</span> <span class="keyword">String</span>[] { <span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span> }, <span class="string">"|"</span>);</span><br><span class="line">System.out.<span class="built_in">println</span>(j);</span><br><span class="line"><span class="comment">// a| b| c</span></span><br></pre></td></tr></table></figure>
<h2 id="6-_ArrayList_转换_Array">6. ArrayList 转换 Array</h2><p>思路：用到List.toArray(array)方法<br><figure class="highlight processing"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">String</span>[] stringArray = { <span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>, <span class="string">"d"</span>, <span class="string">"e"</span> };</span><br><span class="line">ArrayList<<span class="keyword">String</span>> arrayList = <span class="keyword">new</span> ArrayList<<span class="keyword">String</span>>(Arrays.asList(stringArray));</span><br><span class="line"><span class="comment">//创建一个和list的大小一致的空的数组</span></span><br><span class="line"><span class="keyword">String</span>[] stringArr = <span class="keyword">new</span> <span class="keyword">String</span>[arrayList.<span class="built_in">size</span>()];</span><br><span class="line"><span class="comment">//list 2 array</span></span><br><span class="line">arrayList.toArray(stringArr);</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">String</span> s : stringArr)</span><br><span class="line"> System.out.<span class="built_in">println</span>(s);</span><br></pre></td></tr></table></figure></p>
<h2 id="7-_Array转换Set">7. Array转换Set</h2><p>set的构造函数可传入一个集合<br> array to list ; new HsahSet(list)<br><figure class="highlight dart"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//set是无序的</span></span><br><span class="line"><span class="built_in">Set</span><<span class="built_in">String</span>> <span class="literal">set</span> = <span class="keyword">new</span> HashSet<<span class="built_in">String</span>>(Arrays.asList(stringArray));</span><br><span class="line">System.out.println(<span class="literal">set</span>);</span><br><span class="line"><span class="comment">//[d, e, b, c, a]</span></span><br></pre></td></tr></table></figure></p>
<h2 id="8-_反转Array">8. 反转Array</h2><figure class="highlight gradle"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span>[] intArray = { <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span> };</span><br><span class="line">ArrayUtils.<span class="keyword">reverse</span>(intArray);</span><br><span class="line">System.out.<span class="keyword">println</span>(Arrays.toString(intArray));</span><br><span class="line"><span class="comment">//[5, 4, 3, 2, 1]</span></span><br></pre></td></tr></table></figure>
<h2 id="9-_数组中删除一个元素">9. 数组中删除一个元素</h2><figure class="highlight cs"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span>[] intArray = { <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span> };</span><br><span class="line"><span class="comment">//create a new array</span></span><br><span class="line"><span class="keyword">int</span>[] removed = ArrayUtils.removeElement(intArray, <span class="number">3</span>);</span><br><span class="line">System.<span class="keyword">out</span>.println(Arrays.toString(removed));</span><br></pre></td></tr></table></figure>
<h2 id="10-_检查数组中是否包含某个值">10. 检查数组中是否包含某个值</h2><figure class="highlight gradle"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">String[] stringArray = { <span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>, <span class="string">"d"</span>, <span class="string">"e"</span> };</span><br><span class="line"><span class="keyword">boolean</span> b = Arrays.<span class="keyword">asList</span>(stringArray).contains(<span class="string">"a"</span>);</span><br><span class="line">System.out.<span class="keyword">println</span>(b);</span><br><span class="line"><span class="comment">//return true</span></span><br><span class="line"><span class="keyword">int</span>[] numbers = {<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>,<span class="number">4</span>,<span class="number">5</span>,<span class="number">6</span>,<span class="number">7</span>,<span class="number">8</span>,<span class="number">9</span>};</span><br><span class="line"><span class="keyword">boolean</span> a = Arrays.<span class="keyword">asList</span>(numbers).contains(<span class="number">1</span>);</span><br><span class="line">System.out.<span class="keyword">println</span>();</span><br><span class="line"><span class="comment">//return false;</span></span><br></pre></td></tr></table></figure>
<p>上述String[]返回true，然而int[]却返回false。原因是Arrays.asList(array)这个方法的返回值类型。<br>当array是int类型的时候，返回的是List<int[]>，而不是List<integer>。所以传入的值不会和int[]相等的。<br>此方法 不适用于int[]。</integer></int[]></p>
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Java中equals()和hashCode()的契约
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发表于 2015-07-12
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<p><img src="/avatar/java-hashcode.png" alt="Alt text"><br>Java的父类 java.lang.Object 有两个很重要的方法被定义如下：<br><figure class="highlight stylus"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">public boolean <span class="function"><span class="title">equals</span><span class="params">(Object obj)</span></span></span><br><span class="line">public int <span class="function"><span class="title">hashCode</span><span class="params">()</span></span></span><br></pre></td></tr></table></figure></p>
<p>它们已经被证明非常重要去理解，特别当用户自定义的对象被add到Maps中。然后，即使是高级开发者有时候也不知道如何合理的使用这两个方法。这篇文章中，我首先举一个经常犯的错误的例子，并解释hashCode和equals是如何起作用的。</p>
<h2 id="1-_一个常见的错误">1. 一个常见的错误</h2><p>常见的错误如下例子<br><figure class="highlight processing"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> java.util.<span class="keyword">HashMap</span>;</span><br><span class="line"> </span><br><span class="line"><span class="keyword">public</span> class Apple {</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">String</span> <span class="built_in">color</span>;</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> Apple(<span class="keyword">String</span> <span class="built_in">color</span>) {</span><br><span class="line"> <span class="keyword">this</span>.<span class="built_in">color</span> = <span class="built_in">color</span>;</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="built_in">boolean</span> equals(<span class="keyword">Object</span> obj) {</span><br><span class="line"> <span class="keyword">if</span> (!(obj <span class="keyword">instanceof</span> Apple))</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">false</span>; </span><br><span class="line"> <span class="keyword">if</span> (obj == <span class="keyword">this</span>)</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">this</span>.<span class="built_in">color</span>.equals(((Apple) obj).<span class="built_in">color</span>);</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> main(<span class="keyword">String</span>[] args) {</span><br><span class="line"> Apple a1 = <span class="keyword">new</span> Apple(<span class="string">"green"</span>);</span><br><span class="line"> Apple a2 = <span class="keyword">new</span> Apple(<span class="string">"red"</span>);</span><br><span class="line"> </span><br><span class="line"> <span class="comment">//hashMap stores apple type and its quantity</span></span><br><span class="line"> <span class="keyword">HashMap</span><Apple, Integer> m = <span class="keyword">new</span> <span class="keyword">HashMap</span><Apple, Integer>();</span><br><span class="line"> m.put(a1, <span class="number">10</span>);</span><br><span class="line"> m.put(a2, <span class="number">20</span>);</span><br><span class="line"> System.out.<span class="built_in">println</span>(m.<span class="built_in">get</span>(<span class="keyword">new</span> Apple(<span class="string">"green"</span>)));</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>在这个例子中，一个green苹果对象被成功的存在一个HashMap中，但是当在map中查询此对象的时候，这个对象没有被找到。程序结果是null。然而我们可以确定的是在debugger中该对象确实存在HashMap中。<br><img src="/avatar/hashCode-and-equals-contract.png" alt="Alt text"></p>
<h2 id="2-_问题是由hashCode()引起的">2. 问题是由hashCode()引起的</h2><p>这个问题是由于没有重写hashCode()方法。hashCode和equals中的契约是这样的：</p>
<ol>
<li>如果两个对象相等，它们必须有相同的hashcode值。</li>
<li>如果两个对象有相同的hashcode值，它们可能相等也可能不相等。</li>
</ol>
<p>Map背后的思想是能找比线性搜索更快的找到一个对象。使用散列键值来定位对象有两步过程。map内部存储对象作为一个数组的数组。第一个数组的索引就是键的hashCode的值。位于第二个数组通过线性搜索根据equals()来判断该对象是否被找到。</p>
<p>对象类的默认实现了hashcode()方法并返回不同的整数对于不同的对象来说。因此，在上面的例子中，不同的对象（即使同一类型）都有不同的hashCode值。</p>
<p>hashcode就像是一个序列的仓库存储，不同的东西可以存储在不同的仓库中。如果你把材料组织到不同的地方，而不是同一个仓库，那就更有效了。所以这是平均分配的hashCode值的一个很好的实践。（虽然不是主要的观点）。</p>
<p>解决方案就是在类中添加hashCode()方法，这里我只是使用color字符串的长度做示范。<br><figure class="highlight aspectj"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="annotation">@Override</span></span><br><span class="line"><span class="keyword">public</span> <span class="function"><span class="keyword">int</span> <span class="title">hashCode</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">this</span>.color.length();</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p><a href="http://www.programcreek.com/2011/07/java-equals-and-hashcode-contract/" target="_blank" rel="external">原文链接</a></p>
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Top关于Java中Map的9种常见问题
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<p>总的来说，Map是一种集合的数据结构有key-value对组成，每一个key可以在Map中出现一次。这篇文章总结了Java中Map最常用的9种使用方法。为了简单起见，我将在例子中使用泛型。因此我只使用Map而不使用具体的Map。但是你可以假设K和V都实现了Comparable接口。</p>
<h2 id="0-_Map2List_Map转化成List">0. Map2List Map转化成List</h2><p>Java中，Map接口提供三种集合：key集合、value结合、和key-value结合。它们都可以转化成一个List通过一个构造器或者addAll()方法。下面的代码片段展示了如何使用构造器创建通过Map创建一个ArrayList。</p>
<figure class="highlight lasso"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//keyList</span></span><br><span class="line"><span class="built_in">List</span> keyList <span class="subst">=</span> ArrayList(<span class="built_in">map</span><span class="built_in">.</span>keySet());</span><br><span class="line"><span class="comment">//valueList</span></span><br><span class="line"><span class="built_in">List</span> valueList <span class="subst">=</span> ArrayList(<span class="built_in">map</span><span class="built_in">.</span>valueSet());</span><br><span class="line"><span class="comment">//entryList key-value set</span></span><br><span class="line"><span class="built_in">List</span> entryList <span class="subst">=</span> ArrayList(<span class="built_in">map</span><span class="built_in">.</span>entrySet());</span><br></pre></td></tr></table></figure>
<h2 id="1-_在Map中遍历每一个Entry">1. 在Map中遍历每一个Entry</h2><p>通过一个键值对迭代遍历一个Map是最基本的操作。在Java，键值对存储在Map中被称为Map.Entry。Map.entrySet()返回一个键值对的集合，因此要通过一个Map的每一个entry是最有效的方式来遍历map</p>
<figure class="highlight armasm"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="label">for</span>(<span class="preprocessor">Entry</span> <span class="preprocessor">entry</span> : <span class="preprocessor">map</span>.entrySet()) {</span><br><span class="line"> K key = <span class="preprocessor">entry</span>.getKey()<span class="comment">;</span></span><br><span class="line"> V value = <span class="preprocessor">entry</span>.getValue()<span class="comment">;</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>在JDK1.5以前还可以一下方式遍历Map</p>
<figure class="highlight armasm"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">Iterator </span><span class="keyword">it </span>= <span class="preprocessor">map</span>.entrySet().<span class="keyword">iterator();</span><br><span class="line"></span><span class="label">while</span>(<span class="keyword">it.hasNext()) </span>{</span><br><span class="line"> <span class="preprocessor">Entry</span> <span class="preprocessor">entry</span> = <span class="keyword">it.next();</span><br><span class="line"></span> K key = <span class="preprocessor">entry</span>.getKey()<span class="comment">;</span></span><br><span class="line"> V value = <span class="preprocessor">entry</span>.getValue()<span class="comment">;</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="2-_根据key在Map中排序">2. 根据key在Map中排序</h2><p>根据key排序Map是一个经常用到的操作。一个方法是将Map.Entry放进List中，使用一个比较器Comparator来排序value。<br><figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">List <span class="built_in">list</span> = <span class="keyword">new</span> ArrayList(<span class="built_in">map</span>.entrySet());</span><br><span class="line">Collections.sort(<span class="built_in">list</span>,<span class="keyword">new</span> Comparator() {</span><br><span class="line"> @<span class="function">Override</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">int</span> <span class="title">compare</span><span class="params">(Entry e1, Entry e2)</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> e1.getKey().compareTo(e2.getKey());</span><br><span class="line"> }</span><br><span class="line">});</span><br></pre></td></tr></table></figure></p>
<p>另外一种方法是使用排好序的Map，可以根据key值进行自动排序。因此所有的key值必须要实现Comparable接口或者可以被一个Comparator接受。<br>一种实现了SortMap的类是TreeMap。它的构造器可以接受一个Comparator。下面的代码展示了如何将一个map转换成一个排好序的map的。</p>
<figure class="highlight aspectj"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">SortedMap sortMap = <span class="keyword">new</span> TreeMap(<span class="keyword">new</span> Comparator() {</span><br><span class="line"> <span class="annotation">@Override</span></span><br><span class="line"> <span class="keyword">public</span> <span class="function"><span class="keyword">int</span> <span class="title">compare</span><span class="params">(K k1, K k2)</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">return</span> k1.<span class="title">compareTo</span><span class="params">(k2)</span></span>;</span><br><span class="line"> }</span><br><span class="line">});</span><br><span class="line">sortMap.putAll(map);</span><br></pre></td></tr></table></figure>
<h2 id="3-_根据value在Map中排序">3. 根据value在Map中排序</h2><p>将Map变成一个list然后排序也是可以的，不同的是我们比较Entry.getValue().下面的代码和上面几乎一样。<br><figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">List <span class="built_in">list</span> = <span class="keyword">new</span> ArrayList(<span class="built_in">map</span>.entrySet());</span><br><span class="line">Collections.sort(<span class="built_in">list</span>,<span class="keyword">new</span> Comparator() {</span><br><span class="line"> @<span class="function">Override</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">int</span> <span class="title">compare</span><span class="params">(Entry e1, Entry e2)</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> e1.getValue().compareTo(e2.getValue());</span><br><span class="line"> }</span><br><span class="line">});</span><br></pre></td></tr></table></figure></p>
<p>我们仍然可以使用一个排序的Map来解决这个问题，但只有当value是唯一的时候。在这样的条件下，你可以把key-value的值反倒。这个解决方案有很强的限制，因此不推荐使用。</p>