电脑/java/java语言学习笔记.html

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<title>java语言学习笔记</title>
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<meta  name="author" content="万泽(德山书生)">
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<h1 class="title">java语言学习笔记</h1>
</header><nav id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#orgheadline3">1. 前言</a>
<ul>
<li><a href="#orgheadline1">1.1. 命令行脚本的参数</a></li>
<li><a href="#orgheadline2">1.2. 脚本接受管道输入</a></li>
</ul>
</li>
<li><a href="#orgheadline4">2. 符号常量</a></li>
<li><a href="#orgheadline7">3. 附录</a>
<ul>
<li><a href="#orgheadline5">3.1. eclipse编辑器</a></li>
<li><a href="#orgheadline6">3.2. 参考资料</a></li>
</ul>
</li>
</ul>
</div>
</nav>


<div class="outline-2">
<h2 id="orgheadline3">前言</h2>
<div class="outline-text-2" id="text-1">
<p>
虽然java开发主要是在eclipse这样的编辑器下进行，但还是要简单了解下命令行编译的细节:
</p>

<p>
新建一个文本 Hello.java （注意这里的文件名必须是Hello对应里面的类Hello）
</p>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">Hello</span> <span class="o">{</span>
    <span class="kd">public</span> <span class="kd">static</span> <span class="kt">void</span> <span class="nf">main</span><span class="o">(</span><span class="n">String</span><span class="o">[]</span> <span class="n">args</span><span class="o">){</span>
        <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">&quot;hello java&quot;</span><span class="o">);</span>
    <span class="o">}</span>
<span class="o">}</span>
</pre></div>

<p>
然后运行:
</p>
<div class="highlight"><pre><span></span>javac Hello.java
</pre></div>

<p>
这样你就得到 <code>Hello.class</code> 这个编译码文件了，这个编译码文件能够直接在java虚拟机上运行:
</p>
<div class="highlight"><pre><span></span>java Hello
</pre></div>

<p>
注意后面不要写 <code>.class</code> 了。如果不出意外的话应该会打印一段字符串信息。这算是你的第一个java小程序了吧。
</p>

<p>
本文的假设是读者已经懂得一门编程语言了，所以行文会越来越言简意赅，点到为止，觉得该说的才说一下。看上面的例子，具体就是定义了一个类 Hello，然后在这个类里面定义了一个main方法，main方法前面和括号里的东西暂时先不管，main方法是一个特殊的方法，大体程序的入口就在这里。然后就是调用 <code>System.out.println</code> 函数来打印一段字符串，然后记得行结尾用分号 <code>;</code> 。
</p>
</div>

<div class="outline-3">
<h3 id="orgheadline1">命令行脚本的参数</h3>
<div class="outline-text-3" id="text-1-1">
<p>
你可以用java语言写点命令行小脚本之类的东西，然后我们将前面的Hello.java程序改造一下:
</p>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">Hello</span> <span class="o">{</span>
    <span class="kd">public</span> <span class="kd">static</span> <span class="kt">void</span> <span class="nf">main</span><span class="o">(</span><span class="n">String</span><span class="o">[]</span> <span class="n">args</span><span class="o">){</span>
        <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">&quot;hello, &quot;</span> <span class="o">+</span> <span class="n">System</span><span class="o">.</span><span class="na">getenv</span><span class="o">(</span><span class="s">&quot;USER&quot;</span><span class="o">));</span>
            <span class="k">for</span> <span class="o">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="o">;</span> <span class="n">i</span><span class="o">&lt;</span><span class="n">args</span><span class="o">.</span><span class="na">length</span><span class="o">;</span><span class="n">i</span><span class="o">++){</span>
        <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="n">args</span><span class="o">[</span><span class="n">i</span><span class="o">]);</span>
        <span class="o">}</span>
    <span class="o">}</span>
<span class="o">}</span>
</pre></div>

<p>
这里main方法接受的 <code>args</code> 是一个字符串数组，其类似于c语言的 <code>argv</code> ，里面装填着你往这个java小脚本里塞进去的一些参数。然后c语言的 <code>argc</code> 可以通过 <code>args.length</code> 来获得。然后上面演示的简单的for迭代语言结构，然后可以通过 <code>System.getenv()</code> 函数来获取当前脚本运行的一些背景环境参数，类似于c语言的 <code>envp</code> ，但显然更好用了。
</p>
</div>
</div>

<div class="outline-3">
<h3 id="orgheadline2">脚本接受管道输入</h3>
<div class="outline-text-3" id="text-1-2">
<p>
在linux系统下脚本接受管道输入，最关键的一点就是判断 <code>istty</code> ，这个java的支持并不是很好，只能通过 <code>System.console()</code> 来达到某种近似的判断，请参看 <a href="http://stackoverflow.com/questions/1403772/how-can-i-check-if-a-java-programs-input-output-streams-are-connected-to-a-term">这个网页</a> 。
</p>

<p>
下面这个小脚本更加的大型了:
</p>
<div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">java.util.Scanner</span><span class="o">;</span>

<span class="kd">public</span> <span class="kd">class</span> <span class="nc">Echo</span><span class="o">{</span>
    <span class="kd">public</span> <span class="kd">static</span> <span class="kt">void</span> <span class="nf">main</span><span class="o">(</span><span class="n">String</span><span class="o">[]</span> <span class="n">args</span><span class="o">){</span>
        <span class="kt">boolean</span> <span class="n">istty</span><span class="o">;</span>
        <span class="k">if</span> <span class="o">(</span><span class="n">System</span><span class="o">.</span><span class="na">console</span><span class="o">()</span> <span class="o">==</span> <span class="kc">null</span><span class="o">){</span>
            <span class="n">istty</span> <span class="o">=</span> <span class="kc">true</span><span class="o">;</span>
        <span class="o">}</span><span class="k">else</span><span class="o">{</span>
            <span class="n">istty</span> <span class="o">=</span> <span class="kc">false</span><span class="o">;</span>
        <span class="o">}</span>

        <span class="k">if</span> <span class="o">(</span><span class="n">args</span><span class="o">.</span><span class="na">length</span> <span class="o">!=</span> <span class="mi">0</span><span class="o">){</span>
            <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">print</span><span class="o">(</span><span class="s">&quot;some parameters:&quot;</span><span class="o">);</span>
            <span class="k">for</span> <span class="o">(</span><span class="kt">int</span> <span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="o">;</span><span class="n">i</span><span class="o">&lt;</span><span class="n">args</span><span class="o">.</span><span class="na">length</span><span class="o">;</span><span class="n">i</span><span class="o">++){</span>
                <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">print</span><span class="o">(</span><span class="n">args</span><span class="o">[</span><span class="n">i</span><span class="o">]</span> <span class="o">+</span> <span class="s">&quot; &quot;</span><span class="o">);</span>
            <span class="o">}</span>
            <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">();</span>
        <span class="o">}</span>

        <span class="k">if</span> <span class="o">(</span><span class="n">istty</span><span class="o">){</span>
            <span class="n">Scanner</span> <span class="n">sc</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Scanner</span><span class="o">(</span><span class="n">System</span><span class="o">.</span><span class="na">in</span><span class="o">);</span>
            <span class="k">while</span> <span class="o">(</span><span class="n">sc</span><span class="o">.</span><span class="na">hasNext</span><span class="o">()){</span>
                <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="n">sc</span><span class="o">.</span><span class="na">nextLine</span><span class="o">());</span>
            <span class="o">}</span>
        <span class="o">}</span>
    <span class="o">}</span>
<span class="o">}</span>
</pre></div>
<p>
首先引入 <code>Scanner</code> 类，其对于输入有很好的支持，相似细节后面再细讲。然后 <code>System.console()</code> 用来判断是否输入流是终端，并不是很完美的解决方案。Scanner类的 <code>hasNext()</code> 方法用于判断下面还有内容没有， <code>nextLine()</code> 方法用于取出输入流的下面的一行内容。
</p>

<p>
实际写java程序都在eclipse编辑器下，一般安装好就设置好了，点击那个运行按钮即可。一些eclipse的细节知识放在本文的附录那里了。
</p>
</div>
</div>
</div>


<div class="outline-2">
<h2 id="orgheadline4">符号常量</h2>
<div class="outline-text-2" id="text-2">
<div class="highlight"><pre><span></span><span class="kd">final</span> <span class="kt">double</span> <span class="n">PI</span> <span class="o">=</span> <span class="mf">3.1415926</span><span class="o">;</span>
</pre></div>
</div>
</div>


<div class="outline-2">
<h2 id="orgheadline7">附录</h2>
<div class="outline-text-2" id="text-3">
</div><div class="outline-3">
<h3 id="orgheadline5">eclipse编辑器</h3>
</div>


<div class="outline-3">
<h3 id="orgheadline6">参考资料</h3>
<div class="outline-text-3" id="text-3-2">
<ol class="org-ol">
<li>java语言程序设计基础篇; Author: Y.Daniel Liang ; 李娜译 ;version:第8版</li>
<li>Learn Java for Android Development; Author: Jeff Friesen; version:third</li>
</ol>
</div>
</div>
</div>
</div>
</body>
</html>