Java装箱和拆箱的基本概念及使用

要理解装箱和拆箱的概念，就要理解Java数据类型

装箱：把基本类型用它们相应的引用类型包装起来，使其具有对象的性质。int包装成Integer、float包装成Float

拆箱：和装箱相反，将引用类型的对象简化成值类型的数据

Integer a = 100;                  这是自动装箱  (编译器调用的是static Integer valueOf(int i))
int     b = new Integer(100); 这是自动拆箱

看下面一段代码

m1

public class DataType {

 

    public static void main(String args[]) {

        DataType dt = new DataType();

        dt.m11();

        dt.m12();

         

    }

 

    public void m11() {

        Integer a = new Integer(100);

        Integer b = 100;

        System.out.println("m11 result " + (a == b));

    }

 

    public void m12() {

        Integer a = new Integer(128);

        Integer b = 128;

        System.out.println("m12 result " + (a == b));

    }

 

     

}

　　打印结果是什么？

m11 result false
m12 result false

“==”比较的是地址，而a和b两个对象的地址不同，即是两个对象，所以都是false

通过javap解析字节码，内容如下

public void m11();

  Code:

   0:   new     #44; //class java/lang/Integer

   3:   dup

   4:   bipush  100

   6:   invokespecial   #46; //Method java/lang/<span style="color: #ff0000;">Integer."<init>":(I)V</span>

   9:   astore_1

   10:  bipush  100

   12:  invokestatic    #49; //Method java/lang/<span style="color: #ff0000;">Integer.valueOf:(I)</span>Ljava/lang/In

teger;

   15:  astore_2

   16:  getstatic       #53; //Field java/lang/System.out:Ljava/io/PrintStream;

   19:  new     #59; //class java/lang/StringBuilder

   22:  dup

   23:  ldc     #61; //String m11 result

   25:  invokespecial   #63; //Method java/lang/StringBuilder."<init>":(Ljava/la

ng/String;)V

   28:  aload_1

   29:  aload_2

   30:  if_acmpne       37

   33:  iconst_1

   34:  goto    38

   37:  iconst_0

   38:  invokevirtual   #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la

ng/StringBuilder;

   41:  invokevirtual   #70; //Method java/lang/StringBuilder.toString:()Ljava/l

ang/String;

   44:  invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/St

ring;)V

   47:  return

 

public void m12();

  Code:

   0:   new     #44; //class java/lang/Integer

   3:   dup

   4:   sipush  128

   7:   invokespecial   #46; //Method java/lang/Integer."<init>":(I)V

   10:  astore_1

   11:  sipush  128

   14:  invokestatic    #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In

teger;

   17:  astore_2

   18:  getstatic       #53; //Field java/lang/System.out:Ljava/io/PrintStream;

   21:  new     #59; //class java/lang/StringBuilder

   24:  dup

   25:  ldc     #82; //String m12 result

   27:  invokespecial   #63; //Method java/lang/StringBuilder."<init>":(Ljava/la

ng/String;)V

   30:  aload_1

   31:  aload_2

   32:  if_acmpne       39

   35:  iconst_1

   36:  goto    40

   39:  iconst_0

   40:  invokevirtual   #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la

ng/StringBuilder;

   43:  invokevirtual   #70; //Method java/lang/StringBuilder.toString:()Ljava/l

ang/String;

   46:  invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/St

ring;)V

   49:  return

　m2

public class DataType {

 

    public static void main(String args[]) {

        DataType dt = new DataType();

        dt.m21();

        dt.m22();

    }

 

    public void m21() {

        Integer a = new Integer(100);

        Integer b = new Integer(100);

        System.out.println("m21 result " + (a == b));

    }

 

    public void m22() {

        Integer a = new Integer(128);

        Integer b = new Integer(128);

        System.out.println("m22 result " + (a == b));

    }

 

     

}

　　打印结果是

m21 result false
m22 result false

a和b仍是两个对象

javap解析内容

public void m21();  Code:   0:   new     #44; //class java/lang/Integer   3:   dup   4:   bipush  100   6:   invokespecial   #46; //Method java/lang/Integer."<init>":(I)V   9:   astore_1   10:  new     #44; //class java/lang/Integer   13:  dup   14:  bipush  100   16:  invokespecial   #46; //Method java/lang/Integer."<init>":(I)V   19:  astore_2   20:  getstatic       #53; //Field java/lang/System.out:Ljava/io/PrintStream;   23:  new     #59; //class java/lang/StringBuilder   26:  dup   27:  ldc     #84; //String m21 result   29:  invokespecial   #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V   32:  aload_1   33:  aload_2   34:  if_acmpne       41   37:  iconst_1   38:  goto    42   41:  iconst_0   42:  invokevirtual   #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;   45:  invokevirtual   #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;   48:  invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   51:  returnpublic void m22();  Code:   0:   new     #44; //class java/lang/Integer   3:   dup   4:   sipush  128   7:   invokespecial   #46; //Method java/lang/Integer."<init>":(I)V   10:  astore_1   11:  new     #44; //class java/lang/Integer   14:  dup   15:  sipush  128   18:  invokespecial   #46; //Method java/lang/Integer."<init>":(I)V   21:  astore_2   22:  getstatic       #53; //Field java/lang/System.out:Ljava/io/PrintStream;   25:  new     #59; //class java/lang/StringBuilder   28:  dup   29:  ldc     #86; //String m22 result   31:  invokespecial   #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V   34:  aload_1   35:  aload_2   36:  if_acmpne       43   39:  iconst_1   40:  goto    44   43:  iconst_0   44:  invokevirtual   #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;   47:  invokevirtual   #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;   50:  invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   53:  return

m3

public class DataType {

 

    public static void main(String args[]) {

        DataType dt = new DataType();

        dt.m31();

        dt.m32();

    }

 

    public void m31() {

        Integer a = 100;

        Integer b = 100;

        System.out.println("m31 result " + (a == b));

    }

 

    public void m32() {

        Integer a = 128;

        Integer b = 128;

        System.out.println("m32 result " + (a == b));

    }

 

 

}

　　打印结果

m31 result true
m32 result false

为什么有第一个是true，第二个是false呢？观察javap解析的数据

javap解析内容

public void m31();  Code:   0:   bipush  100   2:   invokestatic    #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;   5:   astore_1   6:   bipush  100   8:   invokestatic    #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;   11:  astore_2   12:  getstatic       #53; //Field java/lang/System.out:Ljava/io/PrintStream;   15:  new     #59; //class java/lang/StringBuilder   18:  dup   19:  ldc     #88; //String m31 result   21:  invokespecial   #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V   24:  aload_1   25:  aload_2   26:  if_acmpne       33   29:  iconst_1   30:  goto    34   33:  iconst_0   34:  invokevirtual   #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;   37:  invokevirtual   #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;   40:  invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   43:  returnpublic void m32();  Code:   0:   sipush  128   3:   invokestatic    #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;   6:   astore_1   7:   sipush  128   10:  invokestatic    #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;   13:  astore_2   14:  getstatic       #53; //Field java/lang/System.out:Ljava/io/PrintStream;   17:  new     #59; //class java/lang/StringBuilder   20:  dup   21:  ldc     #90; //String m32 result   23:  invokespecial   #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V   26:  aload_1   27:  aload_2   28:  if_acmpne       35   31:  iconst_1   32:  goto    36   35:  iconst_0   36:  invokevirtual   #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;   39:  invokevirtual   #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;   42:  invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   45:  return

m4

public class DataType {

 

    public static void main(String args[]) {

        DataType dt = new DataType();

        dt.m41();

        dt.m42();

    }

 

 

    public void m41() {

        Integer a = Integer.valueOf(100);

        Integer b = 100;

        System.out.println("m41 result " + (a == b));

    }

     

    public void m42() {

        Integer a = Integer.valueOf(128);

        Integer b = 128;

        System.out.println("m42 result " + (a == b));

    }

}

　　打印结果

m41 result true
m42 result false

javap解析内容

public void m41();  Code:   0:   bipush  100   2:   invokestatic    #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;   5:   astore_1   6:   bipush  100   8:   invokestatic    #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;   11:  astore_2   12:  getstatic       #53; //Field java/lang/System.out:Ljava/io/PrintStream;   15:  new     #59; //class java/lang/StringBuilder   18:  dup   19:  ldc     #92; //String m41 result   21:  invokespecial   #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V   24:  aload_1   25:  aload_2   26:  if_acmpne       33   29:  iconst_1   30:  goto    34   33:  iconst_0   34:  invokevirtual   #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;   37:  invokevirtual   #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;   40:  invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   43:  returnpublic void m42();  Code:   0:   sipush  128   3:   invokestatic    #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;   6:   astore_1   7:   sipush  128   10:  invokestatic    #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;   13:  astore_2   14:  getstatic       #53; //Field java/lang/System.out:Ljava/io/PrintStream;   17:  new     #59; //class java/lang/StringBuilder   20:  dup   21:  ldc     #94; //String m42 result   23:  invokespecial   #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V   26:  aload_1   27:  aload_2   28:  if_acmpne       35   31:  iconst_1   32:  goto    36   35:  iconst_0   36:  invokevirtual   #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;   39:  invokevirtual   #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;   42:  invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   45:  return}

分析

javap是Java自带的一个工具，可以反编译，也可以查看Java编译器生成的字节码(上面代码只使用了javap -c DataType)，是分析代码的一个好工具，具体怎么使用请Google一下

先看一下m4，为什么运行结果中出现了“true”呢，true说明a、b是同一个对象。　

但a对象是调用Integer.valueOf()生成的，b是通过自动装箱生成的对象，为什么会是同一个对象呢？再看一下字节码吧，毕竟Java程序是依靠虚拟机运行字节码实现的。

m41这个方法只适用了一次valueOf()，但字节码中出现了两次，说明自动装箱时也调用了valueOf()。

下面是valueOf()具体实现

/**

 * Returns a <tt>Integer</tt> instance representing the specified

 * <tt>int</tt> value.

 * If a new <tt>Integer</tt> instance is not required, this method

 * should generally be used in preference to the constructor

 * {@link #Integer(int)}, as this method is likely to yield

 * significantly better space and time performance by caching

 * frequently requested values.

 *

 * @param  i an <code>int</code> value.

 * @return a <tt>Integer</tt> instance representing <tt>i</tt>.

 * @since  1.5

 */

public static Integer valueOf(int i) {

final int offset = 128;

if (i >= -128 && i <= 127) { // must cache

    return IntegerCache.cache[i + offset];

}

    return new Integer(i);

}

在【-128，127】之间的数字，valueOf返回的是缓存中的对象，所以两次调用返回的是同一个对象。　　

【参考】http://jzinfo.iteye.com/blog/450590

　　　　http://xiaoyu1985ban.iteye.com/blog/1384191