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

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