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<p><font style='font-size:1.2em;font-weight:bold;'>Java常用类</font></p>
<a id="more"></a>
<ul>
<li>String StringBuffer StringBuilder</li>
<li>System Date SimpleDateFormate Calendar</li>
<li>Math BigInteger BigDecimal</li>
</ul>
<h2 id="一-字符串">一.字符串</h2>
<p>String: 代表不可变的字符序列. 底层使用char[]存放. String 是final的.</p>
<p>StringBuffer: 代表可变序列的字符序列, 可以对字符串内容修改. 底层也是char[]存放</p>
<h3 id="1-String内存解析">1.String内存解析</h3>
<details>
<summary>
One More Thing 👇
</summary>
<figure class="highlight java"><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span></span>{</span><br><span class="line"> String str1 = <span class="string">"JavaEE"</span>;</span><br><span class="line"> String str2 = <span class="string">"JavaEE"</span>;</span><br><span class="line"> String str3 = <span class="keyword">new</span> String(<span class="string">"JavaEE"</span>);</span><br><span class="line"> String str4 = <span class="string">"JavaEE"</span> + <span class="string">"Android"</span>;</span><br><span class="line"> String str5 = <span class="string">"Android"</span>;</span><br><span class="line"> String str6 = str1 + str5;</span><br><span class="line"> str5 = str5 + <span class="string">"Handoop"</span>;</span><br><span class="line"> String str7 = str6.intern();</span><br><span class="line"> String str8 = <span class="string">"JavaEEAndroid"</span>;</span><br><span class="line"> System.out.println(str1 == str2);<span class="comment">//true</span></span><br><span class="line"> System.out.println(str1 == str3);<span class="comment">//false</span></span><br><span class="line"> System.out.println(str1.equals(str3));<span class="comment">//true</span></span><br><span class="line"></span><br><span class="line"> System.out.println(str4 == str6);<span class="comment">//false</span></span><br><span class="line"> System.out.println(str4.equals(str6));<span class="comment">//true</span></span><br><span class="line"> System.out.println(str7 == str4);<span class="comment">//true</span></span><br><span class="line"> System.out.println(str4 == str8);<span class="comment">//true</span></span><br><span class="line"></span><br><span class="line"> Person p1 = <span class="keyword">new</span> Person(<span class="string">"AA"</span>);</span><br><span class="line"> Person p2 = <span class="keyword">new</span> Person(<span class="string">"AA"</span>);</span><br><span class="line"> System.out.println(<span class="string">"^_^"</span>+ (p1.name == p2.name));<span class="comment">//true</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</details>
<p><img src="https://cdn.jsdelivr.net/gh/crainyday/blog@master/lazyload.svg" data-original="https://cdn.jsdelivr.net/gh/crainyday/blog@master/imgs/Java/String%E5%86%85%E5%AD%98.png" alt="String内存"></p>
<h3 id="2-String常用方法">2.String常用方法</h3>
<figure class="highlight java"><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">length</span><span class="params">()</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">char</span> <span class="title">charAt</span><span class="params">(<span class="keyword">int</span> index)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">equals</span><span class="params">(Object anObject)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">compareTo</span><span class="params">(String anotherString)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">indexOf</span><span class="params">(String s)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">indexOf</span><span class="params">(String s ,<span class="keyword">int</span> startpoint)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">lastIndexOf</span><span class="params">(String s)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">lastIndexOf</span><span class="params">(String s ,<span class="keyword">int</span> startpoint)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">startsWith</span><span class="params">(String prefix)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">endsWith</span><span class="params">(String suffix)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">regionMatches</span><span class="params">(<span class="keyword">int</span> firstStart,String other,<span class="keyword">int</span> otherStart ,<span class="keyword">int</span> length)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> String <span class="title">substring</span><span class="params">(<span class="keyword">int</span> startpoint)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> String <span class="title">substring</span><span class="params">(<span class="keyword">int</span> start,<span class="keyword">int</span> end)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> String <span class="title">replace</span><span class="params">(<span class="keyword">char</span> oldChar,<span class="keyword">char</span> newChar)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> String <span class="title">replaceAll</span><span class="params">(String oldStr,String newStr)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> String <span class="title">trim</span><span class="params">()</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> String <span class="title">concat</span><span class="params">(String str)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> String[] <span class="title">split</span><span class="params">(String regex)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">char</span>[] <span class="title">toCharArray</span><span class="params">()</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">contains</span><span class="params">(String str)</span></span></span><br></pre></td></tr></table></figure>
<h3 id="3-String与其他类型间的转换">3.String与其他类型间的转换</h3>
<ol>
<li>String、基本数据类型、包装类之间的转换</li>
</ol>
<ul>
<li>①String —> 基本数据类型、包装类: 调用相应包装类的parseXxx(String str)方法</li>
<li>②基本数据类型、包装类 —> String: 调用字符串重载的valueOf()方法</li>
</ul>
<ol start="2">
<li>String与byte[]之间的转换</li>
</ol>
<ul>
<li>①String —> byte[]: 调用字符串的getBytes()方法</li>
<li>②byte[] —> String: 调用字符串的构造器</li>
</ul>
<ol start="3">
<li>String与char[]之间的转换</li>
</ol>
<ul>
<li>①String —> char[]: 调用字符串的toCharArray()方法</li>
<li>②char[] —> String: 调用字符串的构造器</li>
</ul>
<details>
<summary>
One More Thing 👇
</summary>
<figure class="highlight java"><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="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span></span>{</span><br><span class="line"> <span class="comment">//1.String、基本数据类型、包装类</span></span><br><span class="line"> String str1 = <span class="string">"123"</span>;</span><br><span class="line"> <span class="keyword">int</span> i = Integer.parseInt(str1);</span><br><span class="line"> System.out.println(i);</span><br><span class="line"> </span><br><span class="line"> String str2 = i + <span class="string">""</span>;</span><br><span class="line"> str2 = String.valueOf(i);</span><br><span class="line"> System.out.println(str2);</span><br><span class="line"> <span class="comment">//2.String与byte[]之间的转换</span></span><br><span class="line"> String str3 = <span class="string">"abcdefg"</span>;</span><br><span class="line"> <span class="keyword">byte</span>[] b = str.getBytes();</span><br><span class="line"> <span class="keyword">for</span>(<span class="keyword">int</span> j = <span class="number">0</span>;j < b.length; j++){</span><br><span class="line"> System.out.println((<span class="keyword">char</span>)b[j]);<span class="comment">//不能正确打印出汉字</span></span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> String str4 = <span class="keyword">new</span> String(b);</span><br><span class="line"> System.out.println(str4);</span><br><span class="line"> <span class="comment">//3.String与char[]之间的转换</span></span><br><span class="line"> String str5 = <span class="string">"love中国"</span>;</span><br><span class="line"> <span class="keyword">char</span>[] c = str5.toCharArray();</span><br><span class="line"> <span class="keyword">for</span>(<span class="keyword">int</span> j = <span class="number">0</span>;j < c.length; c++){</span><br><span class="line"> System.out.println(c[j]);</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> String str6 = <span class="keyword">new</span> String(c);</span><br><span class="line"> System.out.println(str6);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</details>
<h3 id="font-style-color-black-4-StringBuffer类-font"><font style='color:black;'>4.StringBuffer类</font></h3>
<p>StringBuffer(相当于一个容器)类有三个构造方法:</p>
<ol>
<li>StringBuffer()初始容量为16的字符串缓冲区</li>
<li>StringBuffer(int size)构造指定容量的字符串缓冲区</li>
<li>StringBuffer(String str)将内容初始化为指定字符串内容</li>
</ol>
<h3 id="5-StringBuffer常用方法">5.StringBuffer常用方法</h3>
<p>StringBuffer很多方法与String相同</p>
<figure class="highlight java"><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></pre></td><td class="code"><pre><span class="line"><span class="function">StringBuffer <span class="title">append</span><span class="params">(String s)</span><span class="comment">//参数可以为:int n、Object o、char n、long n、boolean n</span></span></span><br><span class="line"><span class="function">StringBuffer <span class="title">insert</span><span class="params">(<span class="keyword">int</span> index, String str)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> StringBuffer <span class="title">reverse</span><span class="params">()</span><span class="comment">//调用此方法的StringBuffer也会反转</span></span></span><br><span class="line"><span class="function">StringBuffer <span class="title">delete</span><span class="params">(<span class="keyword">int</span> startIndex, <span class="keyword">int</span> endIndex)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">char</span> <span class="title">charAt</span><span class="params">(<span class="keyword">int</span> n)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setCharAt</span><span class="params">(<span class="keyword">int</span> n ,<span class="keyword">char</span> ch)</span></span></span><br><span class="line"><span class="function">StringBuffer <span class="title">replace</span><span class="params">( <span class="keyword">int</span> startIndex ,<span class="keyword">int</span> endIndex, String str)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">indexOf</span><span class="params">(String str)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> String <span class="title">substring</span><span class="params">(<span class="keyword">int</span> start,<span class="keyword">int</span> end)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">length</span><span class="params">()</span></span></span><br></pre></td></tr></table></figure>
<h3 id="6-StringBuilder类">6.StringBuilder类</h3>
<p>也是可变的字符序列,而且方法也一样, JDK5.0后加的, 线程不安全, 但效率高于StringBuffer</p>
<p><font style='color:red;font-size:1.5em;font-weight:bold;'>使用String的一个弊端</font></p>
<blockquote>
<p>string s=“a”; //创建了一个字符串<br>
s=s+“b”; //实际上原来的"a"字符串对象已经丢弃了, 现在又产生了一个字符串s+“b”(也就是"ab").如果多次执行这些改变串内容的操作, 会导致大量副本字符串对象存留在内存中, 降低效率. 如果这样的操作放到循环中, 会极大影响程序的性能.</p>
</blockquote>
<h3 id="7-String、StringBuffer、StringBuilder性能比较">7.String、StringBuffer、StringBuilder性能比较</h3>
<details>
<summary>
One More Thing 👇
</summary>
<figure class="highlight java"><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span></span>{</span><br><span class="line"> String text = <span class="string">""</span>;</span><br><span class="line"> <span class="keyword">long</span> startTime = <span class="number">0L</span>;</span><br><span class="line"> <span class="keyword">long</span> endTime = <span class="number">0L</span>;</span><br><span class="line"> StringBuffer buffer = <span class="keyword">new</span> StringBuffer(<span class="string">""</span>);</span><br><span class="line"> StringBuilder builder = <span class="keyword">new</span> StringBuilder(<span class="string">""</span>);</span><br><span class="line"> startTime = System.currentTimeMillis();</span><br><span class="line"> <span class="keyword">for</span>(<span class="keyword">int</span> i = <span class="number">0</span>;i<<span class="number">20000</span>;i++){</span><br><span class="line"> buffer.append(String.valueOf(i));</span><br><span class="line"> }</span><br><span class="line"> endTime = System.currentTimeMillis();</span><br><span class="line"> System.out.println(<span class="string">"StringBuffer的执行时间："</span>+(endTime-startTime));</span><br><span class="line"> </span><br><span class="line"> startTime = System.currentTimeMillis();</span><br><span class="line"> <span class="keyword">for</span>(<span class="keyword">int</span> i = <span class="number">0</span>;i<<span class="number">20000</span>;i++){</span><br><span class="line"> builder.append(String.valueOf(i));</span><br><span class="line"> }</span><br><span class="line"> endTime = System.currentTimeMillis();</span><br><span class="line"> System.out.println(<span class="string">"StringBuilder的执行时间："</span>+(endTime-startTime));</span><br><span class="line"> </span><br><span class="line"> startTime = System.currentTimeMillis();</span><br><span class="line"> <span class="keyword">for</span>(<span class="keyword">int</span> i = <span class="number">0</span>;i<<span class="number">20000</span>;i++){</span><br><span class="line"> text = text + i;</span><br><span class="line"> }</span><br><span class="line"> endTime = System.currentTimeMillis();</span><br><span class="line"> System.out.println(<span class="string">"String的执行时间："</span>+(endTime-startTime));</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</details>
<h2 id="二-时间相关类">二.时间相关类</h2>
<h3 id="1-System类">1.System类</h3>
<p>currentTimeMillis(): 返回当前时间的long型值. 此long值是从1970年1月1日0点0分00秒开始到当前的毫秒数.此方法常用来计算时间差</p>
<p>计算世界时间的主要标准:</p>
<ul>
<li>UTC(Universal Time Coordinated)</li>
<li>GMT(Greenwich Mean Time)—格林威治时间</li>
<li>CST(Central Standard Time)—标准时间(四个国家)</li>
</ul>
<h3 id="2-Date类">2.Date类</h3>
<p>有两个: java.util.Date类及它的子类 java.sql.Date<br>
表示特定的瞬间, 精确到毫秒</p>
<figure class="highlight java"><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">import</span> java.util.Date;</span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">testDate</span><span class="params">()</span></span>{</span><br><span class="line"> Date date = <span class="keyword">new</span> Date();<span class="comment">//两个构造方法, 无参和long型参数</span></span><br><span class="line"> System.out.println(date);</span><br><span class="line"> System.out.println(System.currentTimeMillis());</span><br><span class="line"> System.out.println(date.getTime());</span><br><span class="line"> System.out.println(date.toString());</span><br><span class="line"> </span><br><span class="line"> java.sql.Date d = <span class="keyword">new</span> java.sql.Date(date.getTime());</span><br><span class="line"> System.out.println(d);<span class="comment">//格式:year-month-day</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>Date类的API不易于国际化, 大部分方法被废弃了, java.text.SimpleDateFormat类是一个不与语言环境有关的方式来格式化和解析日期的具体类.</p>
<h3 id="3-SimpleDateFormat类">3.SimpleDateFormat类</h3>
<ul>
<li>格式化: 日期 —> 文本 使用SimpleDateFormat 的format()方法</li>
<li>解析: 文本 —> 日期 使用SimpleDateFormat的parse()方法</li>
</ul>
<figure class="highlight java"><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="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span></span>{</span><br><span class="line"> <span class="comment">//1.默认模式格式化</span></span><br><span class="line"> SimpleDateFormat sdf = <span class="keyword">new</span> SimpleDateFormat();</span><br><span class="line"> String date = sdf.format(<span class="keyword">new</span> Date());</span><br><span class="line"> System.out.println(date);<span class="comment">//19-3-26 下午4:50</span></span><br><span class="line"> <span class="comment">//2.自定义模式格式化</span></span><br><span class="line"> SimpleDateFormat sdf1 = <span class="keyword">new</span> SimpleDateFormat(<span class="string">"EEE, d MMM yyyy HH:mm:ss Z"</span>);</span><br><span class="line"> date = sdf1.format(<span class="keyword">new</span> Date());</span><br><span class="line"> System.out.println(date);<span class="comment">//星期二, 26 三月 2019 16:55:16 +0800</span></span><br><span class="line"> <span class="comment">//3.解析:</span></span><br><span class="line"> Date date1 = sdf.parse(<span class="string">"19-3-26 下午4:50"</span>);<span class="comment">//要和构造器的格式字符串对应</span></span><br><span class="line"> System.out.println(date1);</span><br><span class="line"> date1 = sdf1.parse(<span class="string">"星期二, 26 三月 2019 16:55:16 +0800"</span>);</span><br><span class="line"><span class="comment">// date1 = sdf1.parse("19-3-26 下午4:50");//出异常</span></span><br><span class="line"> System.out.println(date1);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="4-Calendar类">4.Calendar类</h3>
<p>Calendar是一个抽象基类, 主用用于完成日期字段之间相互操作的功能.</p>
<p>使用Calendar.getInstance()方法得到一个Calendar的实例, 一个Calendar的实例是系统时间的抽象表示</p>
<figure class="highlight java"><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">//常用方法</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">get</span><span class="params">(<span class="keyword">int</span> field)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">set</span><span class="params">(<span class="keyword">int</span> field,<span class="keyword">int</span> value)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">add</span><span class="params">(<span class="keyword">int</span> field,<span class="keyword">int</span> amount)</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">final</span> Date <span class="title">getTime</span><span class="params">()</span></span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">setTime</span><span class="params">(Date date)</span></span></span><br></pre></td></tr></table></figure>
<h2 id="三-计算相关类">三.计算相关类</h2>
<h3 id="1-Math类">1.Math类</h3>
<p>java.lang.Math提供了一系列静态方法用于科学计算; 其方法的参数和返回值类型一般为double型.</p>
<blockquote>
<p>abs 绝对值<br>
acos,asin,atan,cos,sin,tan 三角函数<br>
sqrt 平方根<br>
pow(double a,doble b) a的b次幂<br>
log 自然对数<br>
exp e为底指数<br>
max(double a,double b)<br>
min(double a,double b)<br>
random() 返回0.0到1.0的随机数<br>
long round(double a) double型数据a转换为long型（四舍五入）<br>
toDegrees(double angrad) 弧度—>角度<br>
toRadians(double angdeg) 角度—>弧度</p>
</blockquote>
<h3 id="2-BigInteger类">2.BigInteger类</h3>
<p>Integer类作为int的包装类. 能存储的最大整型值为2^31−1, BigInteger类的数字范围较Integer类的数字范围要大得多, 可以支持任意精度的整数.</p>
<blockquote>
<p>构造器<br>
BigInteger(String val)<br>
常用方法<br>
public BigInteger abs()<br>
public BigInteger add(BigInteger val)<br>
public BigInteger subtract(BigInteger val)<br>
public BigInteger multiply(BigInteger val)<br>
public BigInteger divide(BigInteger val)<br>
public BigInteger remainder(BigInteger val)<br>
public BigInteger pow(int exponent)<br>
public BigInteger[] divideAndRemainder(BigInteger val)</p>
</blockquote>
<h3 id="3-BigDecimal类">3.BigDecimal类</h3>
<p>一般的Float类和Double类可以用来做科学计算或工程计算, 但在商业计算中, 要求数字精度比较高, 故用到java.math.BigDecimal类. BigDecimal类支持任何精度的定点数.</p>
<blockquote>
<p>构造器<br>
public BigDecimal(double val)<br>
public BigDecimal(String val)<br>
常用方法<br>
public BigDecimal add(BigDecimal augend)<br>
public BigDecimal subtract(BigDecimal subtrahend)<br>
public BigDecimal multiply(BigDecimal multiplicand)<br>
public BigDecimal divide(BigDecimal divisor, int scale, int roundingMode)</p>
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