Java 包装类拆箱装箱笔记

简单来说：装箱就是把值类型转变为引用类型，拆箱就是把引用类型转变为值类型

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。

示例代码：

  1 /** 

  2  * @author  hellosure 

  3  * @time 2011-7-27 上午8:10:46 

  4  * @description：装箱拆箱例子

  5  */ 

  6 public class Test { 

  7  

  8     public static void main(String arg[]) { 

  9         int v1 = 100; 

 10         int v2 = 100; 

 11         //自动装箱 

 12         Integer autovalue1  =   100 ; 

 13         Integer autovalue2  =   100 ; 

 14         //手动装箱两种方式 

 15         Integer value1 = Integer.valueOf(v1); 

 16         Integer value2 = Integer.valueOf(v2); 

 17         Integer va1 = new Integer(v1); 

 18         Integer va2 = new Integer(v2); 

 19         //自动拆箱 

 20         int autov1 = autovalue1; 

 21         int autov2 = autovalue2; 

 22         //手动拆箱 

 23         int vv1 = value1.intValue(); 

 24         int vv2 = value2.intValue(); 

 25          

 26          

 27         System.out.println(" v1 == v2 is "+ (v1 == v2)); 

 28         System.out.println(" autovalue1 == autovalue2 is "+ (autovalue1 == autovalue2)); 

 29         System.out.println(" value1 == value2 is  " + (value1 == value2)); 

 30         System.out.println(" va1 == va2 is "+ (va1 == va2)); 

 31         System.out.println(" va1 equals va2 is "+ (va1.equals(va2))); 

 32         System.out.println(" autov1 == autov2 is "+ (autov1 == autov2)); 

 33         System.out.println(" vv1 == vv2 is "+ (vv1 == vv2)); 

 34  

 35         System.out.println("-----------------------------------------------------"); 

 36          

 37         String strv1 = "100"; 

 38         String strv2 = "100"; 

 39         String stringv1 = new String("100"); 

 40         String stringv2 = new String("100"); 

 41         Integer strvalue1 = Integer.parseInt(strv1); 

 42         Integer strvalue2 = Integer.parseInt(strv2); 

 43         Integer stringvalue1 = Integer.parseInt(stringv1); 

 44         Integer stringvalue2 = Integer.parseInt(stringv2); 

 45         Integer newstrv1 = new Integer(strv1); 

 46         Integer newstrv2 = new Integer(strv2); 

 47  

 48         System.out.println(" strv1 == strv2 is "+ (strv1 == strv2)); 

 49         System.out.println(" stringv1 == stringv2 is "+ (stringv1 == stringv2)); 

 50         System.out.println(" stringv1 equals stringv2 is "+ (stringv1.equals(stringv2))); 

 51         System.out.println(" strvalue1 == strvalue2 is "+ (strvalue1 == strvalue2)); 

 52         System.out.println(" stringvalue1 == stringvalue2 is "+ (stringvalue1 == stringvalue2)); 

 53         System.out.println(" newstrv1 == newstrv2 is "+ (newstrv1 == newstrv2)); 

 54         System.out.println(" newstrv1 equals newstrv2 is "+ (newstrv1.equals(newstrv2))); 

 55          

 56         System.out.println("-----------------------------------------------------"); 

 57          

 58         int v3 = 200; 

 59         int v4 = 200; 

 60         //自动装箱 

 61         Integer autovalue3  =   200 ; 

 62         Integer autovalue4  =   200 ; 

 63         //手动装箱两种方式 

 64         Integer value3 = Integer.valueOf(v3); 

 65         Integer value4 = Integer.valueOf(v4); 

 66         Integer va3 = new Integer(v3); 

 67         Integer va4 = new Integer(v4); 

 68         //自动拆箱 

 69         int autov3 = autovalue3; 

 70         int autov4 = autovalue4; 

 71         //手动拆箱 

 72         int vv3 = value3.intValue(); 

 73         int vv4 = value4.intValue(); 

 74          

 75          

 76         System.out.println(" v3 == v4 is "+ (v3 == v4)); 

 77         System.out.println(" autovalue3 == autovalue4 is "+ (autovalue3 == autovalue4)); 

 78         System.out.println(" value3 == value4 is  " + (value3 == value4)); 

 79         System.out.println(" va3 == va4 is "+ (va3 == va4)); 

 80         System.out.println(" va3 equals va4 is "+ (va3.equals(va4))); 

 81         System.out.println(" autov3 == autov4 is "+ (autov3 == autov4)); 

 82         System.out.println(" vv3 == vv4 is "+ (vv3 == vv4)); 

 83  

 84         System.out.println("-----------------------------------------------------"); 

 85          

 86         String strv3 = "200"; 

 87         String strv4 = "200"; 

 88         String stringv3 = new String("200"); 

 89         String stringv4 = new String("200"); 

 90         Integer strvalue3 = Integer.parseInt(strv3); 

 91         Integer strvalue4 = Integer.parseInt(strv4); 

 92         Integer stringvalue3 = Integer.parseInt(stringv3); 

 93         Integer stringvalue4 = Integer.parseInt(stringv4); 

 94         Integer newstrv3 = new Integer(strv3); 

 95         Integer newstrv4 = new Integer(strv4); 

 96  

 97         System.out.println(" strv3 == strv4 is "+ (strv3 == strv4)); 

 98         System.out.println(" stringv3 == stringv4 is "+ (stringv3 == stringv4)); 

 99         System.out.println(" stringv3 equals stringv4 is "+ (stringv3.equals(stringv4))); 

100         System.out.println(" strvalue3 == strvalue4 is "+ (strvalue3 == strvalue4)); 

101         System.out.println(" stringvalue3 == stringvalue4 is "+ (stringvalue3 == stringvalue4)); 

102         System.out.println(" newstrv3 == newstrv4 is "+ (newstrv3 == newstrv4)); 

103         System.out.println(" newstrv3 equals newstrv4 is "+ (newstrv3.equals(newstrv4))); 

104          

105         System.out.println("-----------------------------------------------------"); 

106          

107     } 

108 } 

运行结果：

v1 == v2 is true 

autovalue1 == autovalue2 is true 

value1 == value2 is  true 

va1 == va2 is false 

va1 equals va2 is true 

autov1 == autov2 is true 

vv1 == vv2 is true 

---------------------------------------------------- 

strv1 == strv2 is true 

stringv1 == stringv2 is false 

stringv1 equals stringv2 is true 

strvalue1 == strvalue2 is true 

stringvalue1 == stringvalue2 is true 

newstrv1 == newstrv2 is false 

newstrv1 equals newstrv2 is true 

---------------------------------------------------- 

v3 == v4 is true 

autovalue3 == autovalue4 is false 

value3 == value4 is  false 

va3 == va4 is false 

va3 equals va4 is true 

autov3 == autov4 is true 

vv3 == vv4 is true 

---------------------------------------------------- 

strv3 == strv4 is true 

stringv3 == stringv4 is false 

stringv3 equals stringv4 is true 

strvalue3 == strvalue4 is false 

stringvalue3 == stringvalue4 is false 

newstrv3 == newstrv4 is false 

newstrv3 equals newstrv4 is true 

---------------------------------------------------- 

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，对于new创建出的两个对象，用==比较肯定是不同的，因为指向的不是同一内存。

Integer的自动装箱

// 在-128~127 之外的数
Integer i1 = 200 ;
Integer i2 = 200 ;
System.out.println( " i1==i2: " + (i1 == i2));
// 在-128~127 之内的数
Integer i3 = 100 ;
Integer i4 = 100 ;
System.out.println( " i3==i4: " + (i3 == i4));

 　　 输出的结果是：

i1 == i2: false
i3 == i4: true

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。

Integer装箱过程中调用的是valueOf方法，而 valueOf方法对值在-128到127之间的数值缓存了，源代码如下：

public static Integer valueOf(int i) { 

        if(i >= -128 && i <= IntegerCache.high) 

            return IntegerCache.cache[i + 128]; 

        else 

            return new Integer(i); 

 } 

可见，Integer缓存中有一个静态的Integer数组，在类加载时就将-128 到 127 的Integer对象创建了，并保存在cache数组中，一旦程序调用valueOf 方法，如果i的值是在-128 到 127 之间就直接在cache缓存数组中去取Integer对象。