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<div align="center" class="title">JAVA程序员必读：基础篇(5)---类和继承</div>
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<div align="center"><font color="#666666">编译：ZSC/太平洋网络学院</font></div>
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<div align="right" class="gray"><a href="mailto:tntst@pconline.com.cn">［责编:爆破手］</a></div>
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<p></p>
<p><b> <span class="article">5.1.7 理解实例和类成员</span></b></p>
<p></p>
<p></p>
<p class="article"> 为了指定方法为一个类方法，你可以在方法声明的地方使用static关键字。现在我们改变一下AnIntegerNamedX类，比如它的成员变量x为实例变量，以及它的两个方法为类方法：</p>
<p class="article"> class AnIntegerNamedX {</p>
<p class="article"> int x;</p>
<p class="article"> static public int x() {</p>
<p class="article"> return x;</p>
<p class="article"> }</p>
<p class="article"> static public void setX(int newX) {</p>
<p class="article"> x = newX;</p>
<p class="article"> }</p>
<p class="article">}</p>
<p class="article"> 当我们编译这个修改过的AnInterNamedX的时候，编译器就给出以下的错误： </p>
<p class="article">AnIntegerNamedX.java:4: Can't make a static reference
to</p>
<p class="article">nonstatic variable x in class AnIntegerNamedX.</p>
<p class="article"> return x;</p>
<p class="article"> ^ </p>
<p class="article"> 出现这个错误的原因是类方法不能访问实例变量，除非方法首先创建AnIntergerNamedX的实例并且通过它来访问变量。
</p>
<p class="article"> 下面再修改AnIntegerNamedX，使x变量为一个类变量： </p>
<p class="article">class AnIntegerNamedX {</p>
<p class="article"> static int x;</p>
<p class="article"> static public int x() {</p>
<p class="article"> return x;</p>
<p class="article"> }</p>
<p class="article"> static public void setX(int newX) {</p>
<p class="article"> x = newX;</p>
<p class="article"> }</p>
<p class="article">}</p>
<p class="article"> 现在类就可以编译了，它是在创建AnIntegerNamedX两个实例之前设置了它们的x数值并打印出来：</p>
<p class="article">myX.x = 2</p>
<p class="article">anotherX.x = 2</p>
<p class="article"> 实例成员和类成员之间的另外不同点是类成员可以在类本身中访问。你不必实例化一个类来访问类成员。下面再对上面的代码进行修改：
</p>
<p class="article">. . .</p>
<p class="article">AnIntegerNamedX.setX(1);</p>
<p class="article">System.out.println("AnIntegerNamedX.x = " + AnIntegerNamedX.x());</p>
<p class="article">. . .</p>
<p class="article"> 这里注意你不再需要创建myX和anotherX。你可以直接从AnIntegerNamedX中设置x并检索x。而你不能这样来处理实例成员，你可以从对象中只调用实例方法。你可以从类的实例或者从类本身访问类变量和方法。</p>
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