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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>Wrapper 類 </b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr> 第 3 頁（共5 頁）</nobr></font></td>
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<p>Wrapper 類對應于 Java 語言中的基本數據類型。這些類將基本值表示為對象。除了 <code>Character</code> 所有 wrapper 類都有兩個構造函數 -- 一個以基本類型值為參數，另一個以這個值的 <code>String</code> 表示為參數。例如： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Integer i1 = new Integer(50);
Integer i2 = new Integer("50");
</code>
</pre>
<p>
<code>Character</code> 類構造函數以 <code>char type </code> 元素作為參數： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Character c = new Character('A');
</code>
</pre>
<p>Wrapper 對象是不可變的。這意味著為 wrapper 對象指定了一個值后，這個值就不會改變。 </p>
<p>
<b>
<code>valueOf()</code> 方法 </b>
<br>
所有 wrapper 類(除了 <code>Character</code> )都定義了名為 <code>valueOf()</code> 的靜態方法，它返回對應于 <code>String</code> 參數所表示的基本類型值的 wrapper 對象。例如： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Float f1 = Float.valueOf("1.5f");
</code>
</pre>
<p>這個方法的重載形式取第一個參數的表示基礎(二進制、八進制或者十六進制)作為第二個參數。例如： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Integer I = Integer.valueOf("10011110",2);
</code>
</pre>
<p>
<b>將 wrapper 對象轉換為基本類型 </b>
<br>
所有數字 wrapper 類都有六種方法，可以用于將數字 wrapper 轉換為任意一種基本類型。這些方法是 <code>byteValue</code>、 <code>doubleValue</code>、 <code>floatValue</code>、 <code>intValue</code>、 <code>longValue</code> 和 <code>shortValue</code>。下面是一個例子： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Integer i = new Integer(20);
byte b = i.byteValue();
</code>
</pre>
<p>
<b>解析器方法 </b>
<br>
六種解析器方法是 <code>parseInt</code>、 <code>parseDouble</code>、 <code>parseFloat</code>、 <code>parseLong</code>、 <code>parseByte</code> 和 <code>parseShort</code> 它們以 <code>String</code> 作為參數并將它轉換為相應的基本類型。如果這個 <code>String</code> 格式不對，它們將拋出 <code>NumberFormatException</code> 。例如： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
double d = Double.parseDouble("4.23");
</code>
</pre>
<p>它還可以取 radix(base) 作為第二個參數： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
int i = Integer.parseInt("10011110",2);
</code>
</pre>
<p>
<b>進制轉換 </b>
<br>
<code>Integer</code> 和 <code>Long</code> 類具有像 <code>toBinaryString()</code> 和 <code>toOctalString()</code> 這樣的方法，它們將十進制數字轉換為其他進制的數字。例如： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
String s = Integer.toHexString(25);
</code>
</pre>
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