一键复制
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java的装箱和拆箱篇一
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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
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;
n("m11 result " + (a == b));
}
public void m12() {
integer a = new integer(128);
integer b = 128;
n("m12 result " + (a == b));
}
}
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."
9: astore_1
10: bipush 100
12: invokestatic #49; //method java/lang/f:(i)ljava/lang/in
teger;
15: astore_2
16: getstatic #53; //field java/lang/: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."
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/:(z)ljava/la
ng/stringbuilder;
41: invokevirtual #70; //method java/lang/ng:()ljava/l
ang/string;
44: invokevirtual #74; //method java/io/n:(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."
10: astore_1
11: sipush 128
14: invokestatic #49; //method java/lang/f:(i)ljava/lang/in
teger;
17: astore_2
18: getstatic #53; //field java/lang/: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."
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/:(z)ljava/la
ng/stringbuilder;
43: invokevirtual #70; //method java/lang/ng:()ljava/l
ang/string;
46: invokevirtual #74; //method java/io/n:(ljava/lang/st
ring;)v
49: return
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);
n("m21 result " + (a == b));
}
public void m22() {
integer a = new integer(128);
integer b = new integer(128);
n("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."
9: astore_1
10: new #44; //class java/lang/integer
13: dup
14: bipush 100
16: invokespecial #46; //method java/lang/integer."
19: astore_2
20: getstatic #53; //field java/lang/: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."
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/:(z)ljava/la
ng/stringbuilder;
45: invokevirtual #70; //method java/lang/ng:()ljava/l
ang/string;
48: invokevirtual #74; //method java/io/n:(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."
10: astore_1
11: new #44; //class java/lang/integer
14: dup
15: sipush 128
18: invokespecial #46; //method java/lang/integer."
21: astore_2
22: getstatic #53; //field java/lang/: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."
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/:(z)ljava/la
ng/stringbuilder;
47: invokevirtual #70; //method java/lang/ng:()ljava/l
ang/string;
50: invokevirtual #74; //method java/io/n:(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;
n("m31 result " + (a == b));
}
public void m32() {
integer a = 128;
integer b = 128;
n("m32 result " + (a == b));
}
}
m31 result true
m32 result false
