Java装箱和拆箱的基本概念及使用

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）调用valueOf 是装箱)

int b = new Integer(100); 这是自动拆箱
看下面一段代码
m1

　　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

　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。　　
【参考】/article/4175353.html
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