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<div id="pf1" class="pf w0 h0" data-page-no="1"><div class="pc pc1 w0 h0"><img class="bi x0 y0 w1 h1" alt="" src="https://static.pudn.com/prod/directory_preview_static/6259192dbe9ad24cfab34349/bg1.jpg"><div class="c x0 y1 w2 h0"><div class="t m0 x1 h2 y2 ff1 fs0 fc0 sc0 ls0 ws0">第七章</div><div class="t m0 x1 h3 y3 ff1 fs1 fc0 sc0 ls1 ws0">常用<span class="_ _0"></span>系统<span class="_ _0"></span>类</div></div></div><div class="pi" data-data='{"ctm":[1.333333,0.000000,0.000000,1.333333,0.000000,0.000000]}'></div></div>
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<div id="pf2" class="pf w0 h0" data-page-no="2"><div class="pc pc2 w0 h0"><img class="bi x0 y0 w1 h1" alt="" src="https://static.pudn.com/prod/directory_preview_static/6259192dbe9ad24cfab34349/bg2.jpg"><div class="c x0 y1 w2 h0"><div class="t m0 x2 h4 y4 ff1 fs2 fc0 sc0 ls2 ws0">第七章<span class="_ _1"> </span><span class="ls0">常用系统类</span></div><div class="t m0 x3 h5 y5 ff2 fs3 fc1 sc1 ls3 ws0">主要任务：</div><div class="t m0 x3 h6 y6 ff2 fs4 fc1 sc1 ls1 ws0">介绍<span class="ff3">Java</span>常用的系统类</div><div class="t m0 x3 h6 y7 ff4 fs5 fc0 sc1 ls1 ws0"><span class="_ _2"> </span><span class="ff3 fs4 fc1">Java Applet<span class="ff2">、</span></span></div><div class="t m0 x3 h7 y8 ff4 fs6 fc0 sc1 ls1 ws0"><span class="_ _2"> </span><span class="ff2 fs7 fc1 ls3">字符串类、</span></div><div class="t m0 x3 h6 y9 ff4 fs5 fc0 sc1 ls1 ws0"><span class="_ _2"> </span><span class="ff2 fs4 fc1">输入输出流类、</span></div><div class="t m0 x3 h6 ya ff4 fs5 fc0 sc1 ls1 ws0"><span class="_ _2"> </span><span class="ff2 fs4 fc1">数学函数类、</span></div><div class="t m0 x3 h7 yb ff4 fs6 fc0 sc1 ls1 ws0"><span class="_ _2"> </span><span class="ff2 fs7 fc1 ls3">日期类、</span></div><div class="t m0 x3 h6 yc ff4 fs5 fc0 sc1 ls1 ws0"><span class="_ _2"> </span><span class="ff2 fs4 fc1">随机数类</span></div><div class="t m0 x3 h6 yd ff4 fs5 fc0 sc1 ls1 ws0"><span class="_ _2"> </span><span class="ff2 fs4 fc1">向量类</span></div></div></div><div class="pi" data-data='{"ctm":[1.333333,0.000000,0.000000,1.333333,0.000000,0.000000]}'></div></div>
<div id="pf3" class="pf w0 h0" data-page-no="3"><div class="pc pc3 w0 h0"><img class="bi x0 y0 w1 h1" alt="" src="https://static.pudn.com/prod/directory_preview_static/6259192dbe9ad24cfab34349/bg3.jpg"><div class="c x0 y1 w2 h0"><div class="t m0 x4 h8 ye ff5 fs8 fc0 sc1 ls1 ws0">7.1 Applet<span class="ff1 sc0">类</span></div><div class="t m0 x5 h9 yf ff4 fs9 fc0 sc1 ls1 ws0"><span class="_ _3"> </span><span class="ff3 fsa fc2 ls4">Applet<span class="ff2 ls3">是一种<span class="fc3 ls1">特殊的</span></span><span class="fc3">Java<span class="ff2 ls1">程序<span class="fc2 ls3">，相当于被</span></span></span>HTML<span class="ff2 ls1">文件调用的</span>Java<span class="ff2 ls3">程序，</span></span></div><div class="t m0 x6 ha y10 ff2 fsb fc2 sc1 ls1 ws0">由<span class="ff3">Web</span><span class="ls5">浏览器内置的</span><span class="ff3">Java</span><span class="ls5">解释器执行。</span></div><div class="t m0 x5 ha y11 ff4 fsc fc0 sc1 ls1 ws0"><span class="_ _3"> </span><span class="ff3 fsb fc2">Java<span class="ff2">的</span>Applet<span class="ff2">与</span><span class="ls5">Application</span><span class="ff2">有一个明显的区别：</span></span></div><div class="t m0 x7 hb y12 ff4 fsd fc0 sc1 ls1 ws0"><span class="_ _4"> </span><span class="ff3 fse fc2">Application<span class="ff2">是独立程序，可直接通过</span>Java<span class="ff2">解释器来解释运行；</span></span></div><div class="t m0 x7 hb y13 ff4 fsd fc0 sc1 ls1 ws0"><span class="_ _4"> </span><span class="ff3 fse fc2">Applet<span class="ff2">只能在</span><span class="ls5">Web</span><span class="ff2">浏览器上运行，必须通过</span><Applet><span class="ff2">标签将编译后的字节</span></span></div><div class="t m0 x8 hc y14 ff2 fsf fc2 sc1 ls3 ws0">码文件嵌入到<span class="ff3 ls6">Web</span>页中。</div><div class="t m0 x5 h9 y15 ff4 fsc fc0 sc1 ls1 ws0"><span class="_ _3"> </span><span class="ff2 fsa fc2 ls3">所有<span class="ff3 ls4">Applet</span>都继承自同一个类：<span class="ff3 ls4">java.applet.Applet</span>，这个类是</span></div><div class="t m0 x6 ha y16 ff3 fsb fc2 sc1 ls1 ws0">Java<span class="ff2 ls5">语言的基础类，有关<span class="ff3">Applet<span class="_ _5"></span><span class="ff2 ls1">的所有特性都被定义在这个类中。</span></span></span></div></div></div><div class="pi" data-data='{"ctm":[1.333333,0.000000,0.000000,1.333333,0.000000,0.000000]}'></div></div>
<div id="pf4" class="pf w0 h0" data-page-no="4"><div class="pc pc4 w0 h0"><img class="bi x0 y0 w1 h1" alt="" src="https://static.pudn.com/prod/directory_preview_static/6259192dbe9ad24cfab34349/bg4.jpg"><div class="c x0 y1 w2 h0"><div class="t m0 x9 h8 y17 ff5 fs8 fc0 sc1 ls1 ws0">7.1.2 applet<span class="ff1 sc0 ls2">的特点</span></div><div class="t m0 xa ha y18 ff3 fsc fc0 sc1 ls1 ws0">1.<span class="_ _6"> </span><span class="fsb fc1">Applet <span class="ff2">类<span class="fc3">不定义<span class="_"> </span><span class="ff3">main()</span></span>，所以一个<span class="_"> </span></span>Applet <span class="ff2">程序不会调用<span class="_"> </span></span>main</span></div><div class="t m0 xb ha y19 ff3 fsb fc1 sc1 ls1 ws0">() <span class="ff2 ls5">方法。</span></div><div class="t m0 xa ha y1a ff3 fsc fc0 sc1 ls1 ws0">2.<span class="_ _6"> </span><span class="ff2 fsb fc1">当用户浏览包含<span class="_"> </span><span class="ff3">Applet </span>的<span class="_"> </span><span class="ff3">HTML </span><span class="ls5">页面，</span><span class="ff3">Applet </span>的代码就被<span class="fc3">下载</span></span></div><div class="t m0 xb ha y1b ff2 fsb fc3 sc1 ls1 ws0">到用户的机器<span class="fc1 ls5">上。</span></div><div class="t m0 xa ha y1c ff3 fsc fc0 sc1 ls1 ws0">3.<span class="_ _6"> </span><span class="fsb fc1">Applet <span class="ff2">需要的其他类可以用<span class="_"> </span></span>Java <span class="ff2">归档（</span>JAR<span class="ff2">）文件的形式下载</span></span></div><div class="t m0 xb ha y1d ff2 fsb fc1 sc1 ls1 ws0">下来。</div><div class="t m0 xa ha y1e ff3 fsc fc0 sc1 ls1 ws0">4.<span class="_ _6"> </span><span class="ff2 fsb fc1">要查看一个<span class="_"> </span><span class="ff3">Applet </span>需要<span class="_"> </span><span class="ff3">JVM</span>。<span class="_"> </span><span class="ff3 ls5">JVM </span>可以是<span class="_"> </span><span class="ff3">Web </span>浏览器的一个</span></div><div class="t m0 xb ha y1f ff2 fsb fc1 sc1 ls1 ws0">插件，或一个独立的运行时环境。</div><div class="t m0 xa ha y20 ff3 fsc fc0 sc1 ls1 ws0">5.<span class="_ _6"> </span><span class="ff2 fsb fc1">用户机器上的<span class="_"> </span><span class="ff3">JVM </span>创建一个<span class="_"> </span><span class="ff3">Applet </span>类的实例，并调用<span class="_"> </span><span class="ff3">Applet </span></span></div><div class="t m0 xb ha y21 ff2 fsb fc1 sc1 ls1 ws0">生命周期过程中的各种方法。</div><div class="t m0 xa ha y22 ff3 fsc fc0 sc1 ls1 ws0">6.<span class="_ _6"> </span><span class="fsb fc1">Applet <span class="ff2">有<span class="_"> </span></span>Web <span class="ff2">浏览器强制执行的严格的安全规则。</span></span></div></div></div><div class="pi" data-data='{"ctm":[1.333333,0.000000,0.000000,1.333333,0.000000,0.000000]}'></div></div>
<div id="pf5" class="pf w0 h0" data-page-no="5"><div class="pc pc5 w0 h0"><img class="bi x0 y0 w1 h1" alt="" src="https://static.pudn.com/prod/directory_preview_static/6259192dbe9ad24cfab34349/bg5.jpg"><div class="c x0 y1 w2 h0"><div class="t m0 xc hd y23 ff5 fs7 fc0 sc1 ls1 ws0">Applet <span class="ff1 sc0 ls0">的安全限制：</span></div><div class="t m0 xd h9 y24 ff4 fs9 fc0 sc1 ls1 ws0"><span class="_ _3"> </span><span class="ff2 fsa fc2">·<span class="ls3">不能加载库函数或定义本地方法。</span></span></div><div class="t m0 xd ha y25 ff4 fsc fc0 sc1 ls1 ws0"><span class="_ _3"> </span><span class="ff2 fsb fc2">·不能读写本地计算机上的文件系统。</span></div><div class="t m0 xd ha y26 ff4 fsc fc0 sc1 ls1 ws0"><span class="_ _3"> </span><span class="ff2 fsb fc2">·除了下载它的服务器外不能和其它主机建立网络通讯。</span></div><div class="t m0 xd h9 y27 ff4 fs9 fc0 sc1 ls1 ws0"><span class="_ _3"> </span><span class="ff2 fsa fc2">·<span class="ls3">不能运行本地计算机上的任何程序（木马）。</span></span></div><div class="t m0 xd ha y28 ff4 fsc fc0 sc1 ls1 ws0"><span class="_ _3"> </span><span class="ff2 fsb fc2">·不能读取某些系统特性。</span></div><div class="t m0 xd ha y29 ff4 fsc fc0 sc1 ls1 ws0"><span class="_ _3"> </span><span class="ff2 fsb fc2">·Applet打开的窗口和<span class="ff3">Application</span><span class="ls5">打开的窗口在外观上有所不同。</span></span></div></div></div><div class="pi" data-data='{"ctm":[1.333333,0.000000,0.000000,1.333333,0.000000,0.000000]}'></div></div>
