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<div align="center" class="title">JAVA程序員必讀：基礎(chǔ)篇(5)---類(lèi)和繼承</div>
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<div align="center"><font color="#666666">編譯：ZSC/太平洋網(wǎng)絡(luò)學(xué)院</font></div>
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<div align="right" class="gray"><a href="mailto:tntst@pconline.com.cn">［責(zé)編:爆破手］</a></div>
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<p></p>
<p><b> <span class="article">5.1.7 理解實(shí)例和類(lèi)成員</span></b></p>
<p></p>
<p></p>
<p class="article"> 為了指定方法為一個(gè)類(lèi)方法，你可以在方法聲明的地方使用static關(guān)鍵字。現(xiàn)在我們改變一下AnIntegerNamedX類(lèi)，比如它的成員變量x為實(shí)例變量，以及它的兩個(gè)方法為類(lèi)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"> 當(dāng)我們編譯這個(gè)修改過(guò)的AnInterNamedX的時(shí)候，編譯器就給出以下的錯(cuò)誤： </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"> 出現(xiàn)這個(gè)錯(cuò)誤的原因是類(lèi)方法不能訪(fǎng)問(wèn)實(shí)例變量，除非方法首先創(chuàng)建AnIntergerNamedX的實(shí)例并且通過(guò)它來(lái)訪(fǎng)問(wèn)變量。
</p>
<p class="article"> 下面再修改AnIntegerNamedX，使x變量為一個(gè)類(lèi)變量： </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"> 現(xiàn)在類(lèi)就可以編譯了，它是在創(chuàng)建AnIntegerNamedX兩個(gè)實(shí)例之前設(shè)置了它們的x數(shù)值并打印出來(lái)：</p>
<p class="article">myX.x = 2</p>
<p class="article">anotherX.x = 2</p>
<p class="article"> 實(shí)例成員和類(lèi)成員之間的另外不同點(diǎn)是類(lèi)成員可以在類(lèi)本身中訪(fǎng)問(wèn)。你不必實(shí)例化一個(gè)類(lèi)來(lái)訪(fǎng)問(wèn)類(lèi)成員。下面再對(duì)上面的代碼進(jìn)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"> 這里注意你不再需要?jiǎng)?chuàng)建myX和anotherX。你可以直接從AnIntegerNamedX中設(shè)置x并檢索x。而你不能這樣來(lái)處理實(shí)例成員，你可以從對(duì)象中只調(diào)用實(shí)例方法。你可以從類(lèi)的實(shí)例或者從類(lèi)本身訪(fǎng)問(wèn)類(lèi)變量和方法。</p>
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