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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>Wrapper 类 </b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr> 第 3 页（共5 页）</nobr></font></td>
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<p>Wrapper 类对应于 Java 语言中的基本数据类型。这些类将基本值表示为对象。除了 <code>Character</code> 所有 wrapper 类都有两个构造函数 -- 一个以基本类型值为参数，另一个以这个值的 <code>String</code> 表示为参数。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> 类构造函数以 <code>char type </code> 元素作为参数： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Character c = new Character('A');
</code>
</pre>
<p>Wrapper 对象是不可变的。这意味着为 wrapper 对象指定了一个值后，这个值就不会改变。 </p>
<p>
<b>
<code>valueOf()</code> 方法 </b>
<br>
所有 wrapper 类(除了 <code>Character</code> )都定义了名为 <code>valueOf()</code> 的静态方法，它返回对应于 <code>String</code> 参数所表示的基本类型值的 wrapper 对象。例如： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Float f1 = Float.valueOf("1.5f");
</code>
</pre>
<p>这个方法的重载形式取第一个参数的表示基础(二进制、八进制或者十六进制)作为第二个参数。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>将 wrapper 对象转换为基本类型 </b>
<br>
所有数字 wrapper 类都有六种方法，可以用于将数字 wrapper 转换为任意一种基本类型。这些方法是 <code>byteValue</code>、 <code>doubleValue</code>、 <code>floatValue</code>、 <code>intValue</code>、 <code>longValue</code> 和 <code>shortValue</code>。下面是一个例子： </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>
六种解析器方法是 <code>parseInt</code>、 <code>parseDouble</code>、 <code>parseFloat</code>、 <code>parseLong</code>、 <code>parseByte</code> 和 <code>parseShort</code> 它们以 <code>String</code> 作为参数并将它转换为相应的基本类型。如果这个 <code>String</code> 格式不对，它们将抛出 <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>它还可以取 radix(base) 作为第二个参数： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
int i = Integer.parseInt("10011110",2);
</code>
</pre>
<p>
<b>进制转换 </b>
<br>
<code>Integer</code> 和 <code>Long</code> 类具有像 <code>toBinaryString()</code> 和 <code>toOctalString()</code> 这样的方法，它们将十进制数字转换为其他进制的数字。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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