Java 装箱与拆箱详解及实例代码

java 装箱与拆箱详解

前言：

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/integer."<init>":(i)v
  9:  astore_1
  10: bipush 100
  12: invokestatic  #49; //method java/lang/integer.valueof:(i)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
</init></init></init></init>
　

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/la
ng/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/la
ng/stringbuilder;
  45: invokevirtual  #70; //method java/lang/stringbuilder.tostring:()ljava/l
ang/string;
  48: invokevirtual  #74; //method java/io/printstream.println:(ljava/lang/st
ring;)v
  51: return

public 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/la
ng/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/la
ng/stringbuilder;
  47: invokevirtual  #70; //method java/lang/stringbuilder.tostring:()ljava/l
ang/string;
  50: invokevirtual  #74; //method java/io/printstream.println:(ljava/lang/st
ring;)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/in
teger;
  5:  astore_1
  6:  bipush 100
  8:  invokestatic  #49; //method java/lang/integer.valueof:(i)ljava/lang/in
teger;
  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/la
ng/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/la
ng/stringbuilder;
  37: invokevirtual  #70; //method java/lang/stringbuilder.tostring:()ljava/l
ang/string;
  40: invokevirtual  #74; //method java/io/printstream.println:(ljava/lang/st
ring;)v
  43: return

public void m32();
 code:
  0:  sipush 128
  3:  invokestatic  #49; //method java/lang/integer.valueof:(i)ljava/lang/in
teger;
  6:  astore_1
  7:  sipush 128
  10: invokestatic  #49; //method java/lang/integer.valueof:(i)ljava/lang/in
teger;
  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/la
ng/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/la
ng/stringbuilder;
  39: invokevirtual  #70; //method java/lang/stringbuilder.tostring:()ljava/l
ang/string;
  42: invokevirtual  #74; //method java/io/printstream.println:(ljava/lang/st
ring;)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/in
teger;
  5:  astore_1
  6:  bipush 100
  8:  invokestatic  #49; //method java/lang/integer.valueof:(i)ljava/lang/in
teger;
  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/la
ng/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/la
ng/stringbuilder;
  37: invokevirtual  #70; //method java/lang/stringbuilder.tostring:()ljava/l
ang/string;
  40: invokevirtual  #74; //method java/io/printstream.println:(ljava/lang/st
ring;)v
  43: return

public void m42();
 code:
  0:  sipush 128
  3:  invokestatic  #49; //method java/lang/integer.valueof:(i)ljava/lang/in
teger;
  6:  astore_1
  7:  sipush 128
  10: invokestatic  #49; //method java/lang/integer.valueof:(i)ljava/lang/in
teger;
  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/la
ng/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/la
ng/stringbuilder;
  39: invokevirtual  #70; //method java/lang/stringbuilder.tostring:()ljava/l
ang/string;
  42: invokevirtual  #74; //method java/io/printstream.println:(ljava/lang/st
ring;)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。

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