Java：i++问题（详细分析）

i++问题引发的知识链

例子

public class SumPlusTest {

    public static void main(String[] args){
        int a = 1, b = 1, c = 1, d =1;
        a++;
        ++b;
        c = c++;
        d = ++d;
        System.out.println("a : " + a + "\nb : " + b + "\nc : " + c + "\nd : " + d);
    }
}

输出结果

a : 2
b : 2
c : 1
d : 2

分析

解法一

tmp = c;
c = c++;
c = tmp;

解法二

javap

c=c++

[root@z1 classdir]# javap -verbose SumPlusTest
Classfile /mnt/workspace/java/i++issue/classdir/SumPlusTest.class
  Last modified Sep 3, 2015; size 794 bytes
  MD5 checksum 3a406dc81fe69722d53ce74ef96516a4
public class SumPlusTest
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #14.#30        //  java/lang/Object."<init>":()V
   #2 = Fieldref           #31.#32        //  java/lang/System.out:Ljava/io/PrintStream;

  ......

  #54 = Utf8               println
  #55 = Utf8               (Ljava/lang/String;)V
{
  public SumPlusTest();
    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        
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
               0       5     0  this   LSumPlusTest;

  public static void main(java.lang.String[]);
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=5, args_size=1
         0: iconst_1      
         1: istore_1      #0,1相当于是执行a=1
         2: iconst_1      
         3: istore_2      #2,3相当于是执行b=1
         4: iconst_1      
         5: istore_3      #4,5相当于是执行c=1
         6: iconst_1      
         7: istore_4      #6,7相当于是执行d=1
         9: iinc          1, 1      #相当于执行a的值+1
        12: iinc          2, 1      #相当于执行b的值+1
        15: iload_3                 #将c的值放入栈顶
        16: iinc          3, 1      #执行c的值+1
        19: istore_3                #再将栈顶的值赋值给c，此时即c=1
        20: iinc          4, 1      #相当于是执行d的值+1
        23: iload         4         #把d的值放置于栈顶
        25: istore        4         #然后将栈顶的值付给d这个变量，此时d=2

        ......

      LocalVariableTable:
        Start  Length  Slot  Name   Signature
               0      81     0  args   [Ljava/lang/String;
               2      79     1     a   I
               4      77     2     b   I
               6      75     3     c   I
               9      72     4     d   I
}

i++问题例子补充

例一

程序

int i = 0;
i = (i++) + (++i);
System.out.println("i : " + i);

结果

i=2

分析

0: iconst_0      
1: istore_1      
2: iload_1       #将数值0推入栈顶
3: iinc          1, 1
6: iinc          1, 1   ##自加两次
9: iload_1       #将数值2推入栈顶
10: iadd          #将0和2相加，并将结果放入栈顶
11: istore_1      将i赋值为2

tmp1=i++

++

++

例二

程序

int i = 0;
i = (i++) + (++i) + (i++);
System.out.println("i : " + i);

结果

分析

0: iconst_0      
1: istore_1      
2: iload_1       
3: iinc          1, 1
6: iinc          1, 1
9: iload_1       
10: iadd          
11: iload_1       
12: iinc          1, 1
15: iadd          
16: istore_1

例三

程序

int i = 0;
i = (++i) + (i++) + (++i);
System.out.println("i : " + i);

结果

分析

0: iconst_0      
1: istore_1      
2: iinc          1, 1
5: iload_1       
6: iload_1       
7: iinc          1, 1
10: iadd          
11: iinc          1, 1
14: iload_1       
15: iadd          
16: istore_1

i++

总结

++

++

++

疑问

c = c++

c = c++

d = ++d

i = 0; i = i++