java 自动装箱与拆箱

？？什么是自动装箱拆箱

基本数据类型的自动装箱(autoboxing)、拆箱(unboxing)是自J2SE 5.0开始提供的功能。 

一般我们要创建一个类的对象的时候，我们会这样：

 Class a = new Class(parameter);

  当我们创建一个Integer对象时，却可以这样：

 Integer i = 100; (注意：不是 int i = 100; )

实际上，执行上面那句代码的时候，系统为我们执行了：Integer i = new Integer(100); 此即基本数据类型的自动装箱功能。

？？基本数据类型与对象的差别 

基本数据类型不是对象，也就是使用int、double、boolean等定义的变量、常量。

基本数据类型没有可调用的方法。

eg：  int t = 1；     t.  后面是没有方法滴。

 Integer t =1； t.  后面就有很多方法可让你调用了。

？？什么时候自动装箱

　　例如：Integer i = 100;

     相当于编译器自动为您作以下的语法编译：Integer i = new Integer(100);

？？什么时候自动拆箱

　　自动拆箱(unboxing)，也就是将对象中的基本数据从对象中自动取出。如下可实现自动拆箱：

<div class="cnblogs_code" style="margin: 5px 0px; padding: 5px; background-color: rgb(245, 245, 245); border: 1px solid rgb(204, 204, 204); overflow: auto; font-family: 'Courier New' !important; font-size: 12px !important;"><pre style="margin-top: 0px; margin-bottom: 0px; padding: 0px; white-space: pre-wrap; word-wrap: break-word; font-family: 'Courier New' !important;"><div style="margin: 0px; padding: 0px; background-color: rgb(245, 245, 245);"><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">1</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">Integer i </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">10</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">;        </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">装箱  </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"><br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">2</span> <span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 0, 255);">int</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> t </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> i;             </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">拆箱</span></div>

　　在进行运算时，也可以进行自动装箱与拆箱。 

1 Integer i = 10;    
2 System.out.println(i++); 

？？Integer的自动装箱

<div class="cnblogs_code" style="margin: 5px 0px; padding: 5px; background-color: rgb(245, 245, 245); border: 1px solid rgb(204, 204, 204); overflow: auto; font-family: 'Courier New' !important; font-size: 12px !important;"><div class="cnblogs_code_toolbar" style="margin: 5px 0px 0px; padding: 0px;"><span class="cnblogs_code_copy" style="margin: 0px; padding: 0px 5px 0px 0px; line-height: 1.5;"><a target=_blank title="复制代码" style="margin: 0px; padding: 0px; color: rgb(7, 93, 179); text-decoration: underline; border: none !important;"><img src="http://common.cnblogs.com/images/copycode.gif" alt="复制代码" style="margin: 0px; padding: 0px; border: none !important;" /></a></span></div><pre style="margin-top: 0px; margin-bottom: 0px; padding: 0px; white-space: pre-wrap; word-wrap: break-word; font-family: 'Courier New' !important;"><div style="margin: 0px; padding: 0px; background-color: rgb(245, 245, 245);"><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">在-128~127 之外的数</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"><br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> Integer i1 </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">200</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">;  <br style="margin: 0px; padding: 0px;" /> Integer i2 </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">200</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">;          <br style="margin: 0px; padding: 0px;" /> System.out.println(</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">i1==i2: </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">+</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">(i1</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">==</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">i2));                   <br style="margin: 0px; padding: 0px;" /> </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"> 在-128~127 之内的数</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"><br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> Integer i3 </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">100</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">;  <br style="margin: 0px; padding: 0px;" /> Integer i4 </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">100</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">;  <br style="margin: 0px; padding: 0px;" /> System.out.println(</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">i3==i4: </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">+</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">(i3</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">==</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">i4));</span></div>

 　　 输出的结果是：

<div class="cnblogs_code" style="margin: 5px 0px; padding: 5px; background-color: rgb(245, 245, 245); border: 1px solid rgb(204, 204, 204); overflow: auto; font-family: 'Courier New' !important; font-size: 12px !important;"><pre style="margin-top: 0px; margin-bottom: 0px; padding: 0px; white-space: pre-wrap; word-wrap: break-word; font-family: 'Courier New' !important;"><div style="margin: 0px; padding: 0px; background-color: rgb(245, 245, 245);"><span style="margin: 0px; padding: 0px; line-height: 1.5;">    i1</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">==</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">i2: </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 0, 255);">false</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"><br style="margin: 0px; padding: 0px;" />    i3</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">==</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">i4: </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 0, 255);">true</span></div>

  说明：

equals() 比较的是两个对象的值（内容）是否相同。

"==" 比较的是两个对象的引用（内存地址）是否相同，也用来比较两个基本数据类型的变量值是否相等。

 

在自动装箱时对于值从–128到127之间的值，它们被装箱为Integer对象后，会存在内存中被重用，

所以范例中，i3 与 i4实际上参考至同一个对象。

如果超过了从–128到127之间的值，被装箱后的Integer对象并不会被重用，

即相当于每次装箱时都新建一个 Integer对象，所以范例中，i1与i2参考的是不同的对象。

另外，当不使用自动装箱功能的时候，情况与普通类对象一样，请看下例：

1 Integer i3 = new Integer(100); 
2 Integer i4 = new Integer(100); 
3 System.out.println("i3==i4: "+(i3==i4));//显示false

（感谢易之名的提醒O(∩_∩)O~）

 

？？String 的拆箱装箱

先看个例子：

<div class="cnblogs_code" style="margin: 5px 0px; padding: 5px; background-color: rgb(245, 245, 245); border: 1px solid rgb(204, 204, 204); overflow: auto; font-family: 'Courier New' !important; font-size: 12px !important;"><div class="cnblogs_code_toolbar" style="margin: 5px 0px 0px; padding: 0px;"><span class="cnblogs_code_copy" style="margin: 0px; padding: 0px 5px 0px 0px; line-height: 1.5;"><a target=_blank title="复制代码" style="margin: 0px; padding: 0px; color: rgb(7, 93, 179); text-decoration: underline; border: none !important;"><img src="http://common.cnblogs.com/images/copycode.gif" alt="复制代码" style="margin: 0px; padding: 0px; border: none !important;" /></a></span></div><pre style="margin-top: 0px; margin-bottom: 0px; padding: 0px; white-space: pre-wrap; word-wrap: break-word; font-family: 'Courier New' !important;"><div style="margin: 0px; padding: 0px; background-color: rgb(245, 245, 245);"><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">1</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">           String str1 </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">abc</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">;<br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">2</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">           String str2 </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">abc</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">;<br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">3</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">           System.out.println(str2</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">==</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">str1);  </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">输出为 true </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"><br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">4</span> <span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">           System.out.println(str2.equals(str1));  </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">输出为 true </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"><br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">5</span> <span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">           <br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">6</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">           String str3 </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 0, 255);">new</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> String(</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">abc</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">);<br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">7</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">           String str4 </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 0, 255);">new</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> String(</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">abc</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">); <br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">8</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">           System.out.println(str3</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">==</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">str4);  </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">输出为 false </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"><br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">9</span> <span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">           System.out.println(str3.equals(str4));  </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">输出为 true </span></div>

  　　这个怎么解释呢？貌似看不出什么。那再看个例子。

<div class="cnblogs_code" style="margin: 5px 0px; padding: 5px; background-color: rgb(245, 245, 245); border: 1px solid rgb(204, 204, 204); overflow: auto; font-family: 'Courier New' !important; font-size: 12px !important;"><pre style="margin-top: 0px; margin-bottom: 0px; padding: 0px; white-space: pre-wrap; word-wrap: break-word; font-family: 'Courier New' !important;"><div style="margin: 0px; padding: 0px; background-color: rgb(245, 245, 245);"><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">1</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">        String d </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">2</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">;     <br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">2</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">        String e </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">23</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">"</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">;<br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">3</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">        e </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">=</span><span style="margin: 0px; padding: 0px; line-height: 1.5;"> e.substring(</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">0</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">, </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">1</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">);<br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">4</span> <span style="margin: 0px; padding: 0px; line-height: 1.5;">        System.out.println(e.equals(d));       </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">输出为 true      </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"><br style="margin: 0px; padding: 0px;" /></span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 128);">5</span> <span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);"> </span><span style="margin: 0px; padding: 0px; line-height: 1.5;">        System.out.println(e</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">==</span><span style="margin: 0px; padding: 0px; line-height: 1.5;">d);           </span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">//</span><span style="margin: 0px; padding: 0px; line-height: 1.5; color: rgb(0, 128, 0);">输出为 false  </span></div>

  第二个例子中，e的初始值与d并不同，因此e与d是各自创建了个对象，（e==d）为false 。

同理可知，第一个例子中的str3与str4也是各自new了个对象，而str1与str2却是引用了同一个对象