Java中变量自增：a++和++a的区别

157 阅读 0 评论 104 点赞

我是靠谱客的博主辛勤万宝路，这篇文章主要介绍Java中变量自增：a++和++a的区别，现在分享给大家，希望可以做个参考。

复制代码

1
2
3
4
5
public static void main(String[] args) {
int a=10;
int b=a++;
int c=++a;
}

在最开始学习Java时我们都知道，第二行代码：int b=a++，是先把变量赋值给b，然后再执行自增。第三行代码：int b=++a；是先对a执行自增，再赋值给c。

下面我们看Java编译的字节码文件来看一下在底层，栈内部这个区别是具体怎么体现的。

经过编译之后会生成.class的字节码文件，我们可以使用以下命令查看字节码文件

复制代码

1
javap -v *.class

得到以下结果

复制代码

Classfile /Users/mengao/IdeaProjects/JavaBook/target/classes/_05_jvm相关/_02_.class
Last modified 2022-2-12; size 454 bytes
MD5 checksum 5c2f282df67cce412e133bdf41f398da
Compiled from "_02_.java"
public class _05_jvm相关._02_
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref
#3.#21
// java/lang/Object."<init>":()V
#2 = Class
#22
// _05_jvm相关/_02_
#3 = Class
#23
// java/lang/Object
#4 = Utf8
<init>
#5 = Utf8
()V
#6 = Utf8
Code
#7 = Utf8
LineNumberTable
#8 = Utf8
LocalVariableTable
#9 = Utf8
this
#10 = Utf8
L_05_jvm相关/_02_;
#11 = Utf8
main
#12 = Utf8
([Ljava/lang/String;)V
#13 = Utf8
args
#14 = Utf8
[Ljava/lang/String;
#15 = Utf8
a
#16 = Utf8
I
#17 = Utf8
b
#18 = Utf8
c
#19 = Utf8
SourceFile
#20 = Utf8
_02_.java
#21 = NameAndType
#4:#5
// "<init>":()V
#22 = Utf8
_05_jvm相关/_02_
#23 = Utf8
java/lang/Object
{
public _05_jvm相关._02_();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1
// Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 9: 0
LocalVariableTable:
Start
Length
Slot
Name
Signature
0
5
0
this
L_05_jvm相关/_02_;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=4, args_size=1
0: bipush
10
2: istore_1
3: iload_1
4: iinc
1, 1
7: istore_2
8: iinc
1, 1
11: iload_1
12: istore_3
13: return
LineNumberTable:
line 11: 0
line 12: 3
line 13: 8
line 18: 13
LocalVariableTable:
Start
Length
Slot
Name
Signature
0
14
0
args
[Ljava/lang/String;
3
11
1
a
I
8
6
2
b
I
13
1
3
c
I
}
SourceFile: "_02_.java"

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
Classfile /Users/mengao/IdeaProjects/JavaBook/target/classes/_05_jvm相关/_02_.class
Last modified 2022-2-12; size 454 bytes
MD5 checksum 5c2f282df67cce412e133bdf41f398da
Compiled from "_02_.java"
public class _05_jvm相关._02_
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref
#3.#21
// java/lang/Object."<init>":()V
#2 = Class
#22
// _05_jvm相关/_02_
#3 = Class
#23
// java/lang/Object
#4 = Utf8
<init>
#5 = Utf8
()V
#6 = Utf8
Code
#7 = Utf8
LineNumberTable
#8 = Utf8
LocalVariableTable
#9 = Utf8
this
#10 = Utf8
L_05_jvm相关/_02_;
#11 = Utf8
main
#12 = Utf8
([Ljava/lang/String;)V
#13 = Utf8
args
#14 = Utf8
[Ljava/lang/String;
#15 = Utf8
a
#16 = Utf8
I
#17 = Utf8
b
#18 = Utf8
c
#19 = Utf8
SourceFile
#20 = Utf8
_02_.java
#21 = NameAndType
#4:#5
// "<init>":()V
#22 = Utf8
_05_jvm相关/_02_
#23 = Utf8
java/lang/Object
{
public _05_jvm相关._02_();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1
// Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 9: 0
LocalVariableTable:
Start
Length
Slot
Name
Signature
0
5
0
this
L_05_jvm相关/_02_;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=4, args_size=1
0: bipush
10
2: istore_1
3: iload_1
4: iinc
1, 1
7: istore_2
8: iinc
1, 1
11: iload_1
12: istore_3
13: return
LineNumberTable:
line 11: 0
line 12: 3
line 13: 8
line 18: 13
LocalVariableTable:
Start
Length
Slot
Name
Signature
0
14
0
args
[Ljava/lang/String;
3
11
1
a
I
8
6
2
b
I
13
1
3
c
I
}
SourceFile: "_02_.java"

其中的关键部分是main方法里执行的代码：

复制代码

1
2
3
4
5
6
7
8
9
10
11
12
13

0: bipush
10
2: istore_1
3: iload_1
4: iinc
1, 1
7: istore_2
8: iinc
1, 1
11: iload_1
12: istore_3
13: return

添加注释：

复制代码

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22

0: bipush
10
# 把变量10放入操作数栈
2: istore_1
# 把操作数栈里的数存储到槽位编号1的变量上，也就是赋值给变量a，此后a=10
3: iload_1
# 把变量a的值放入操作数栈
4: iinc
1, 1
# 对槽位编号1的变量执行+1，此后a=11
7: istore_2
# 把操作数栈里的内容赋值给槽位编号2的变量上，也就是赋值给变量b，此后b=10
8: iinc
1, 1
# 对槽位编号1的变量执行+1，此后a=12
11: iload_1
# 把槽位编号为1的内容，存储到操作数栈
12: istore_3
# 把操作数栈里栈顶的值赋值给槽位编号为2的变量，也就是赋值给变量c，此后c=12
13: return
# 结束