Topic: int i =1; i=i++ 为什么结果是 i =1?

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1.int i =1; i=i++ 为什么结果是 i =1? Copy to clipboard
Posted by: xiaopan
Posted on: 2003-03-18 15:15

这是JVM中的一个问题，在C和C++中结果是符合语意的2，这是由于在C中编译会把这样的语句 i = i++ 优化成 i++, 而JVM却不会，所以结果是1
2.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: rainman
Posted on: 2003-03-18 15:23

i=++i 才是i = 2啊。
3.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: rainman] Copy to clipboard
Posted by: ditty
Posted on: 2003-03-18 15:26

如果你是非计算机专业的人，可以理解；否则，这种问题都问的话，真该好好反省一下自己了！
4.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: ccic134302
Posted on: 2003-03-18 15:54

如果你是非计算机专业的人的话我告诉你：
i++是 i 赋给 i 后在加1；++i是 i 加1后赋给i是2
明白否，好好加油！！！
5.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: snowbug
Posted on: 2003-03-18 23:13

xiaopan wrote:
这是JVM中的一个问题，在C和C++中结果是符合语意的2，这是由于在C中编译会把这样的语句 i = i++ 优化成 i++, 而JVM却不会，所以结果是1

i = i++ is the same as the following set of statements:
1). int temp = i;
2). i++;
3). i = temp;

Therefore, the original i value is assigned back to the variable i after (i++) statement.
6.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: snowbug] Copy to clipboard
Posted by: mitnickcbc
Posted on: 2003-03-18 23:37

snowbug wrote:
i = i++ is the same as the following set of statements:
1). int temp = i;
2). i++;
3). i = temp;

Therefore, the original i value is assigned back to the variable i after (i++) statement.


that's clear.
7.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: mitnickcbc] Copy to clipboard
Posted by: rainman
Posted on: 2003-03-19 01:41

Java Source

public class Test {
  public static void main(String[] args) {
    int i=1;
    i = i++;
  }
}


JVM Assembler code:

Compiled from Test.java
public class Test extends java.lang.Object {
    public Test();
    public static void main(java.lang.String[]);
}

Method Test()
   0 aload_0
   1 invokespecial #1 <Method java.lang.Object()>
   4 return


Method void main(java.lang.String[])
   0 iconst_1
   1 istore_1
   2 iload_1
   3 iinc 1 1
   6 istore_1
   7 return

8.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: rainman
Posted on: 2003-03-19 01:45


public class Test {
  public static void main(String[] args) {
    int i=1;    
    i = i+1;
  }
}



Compiled from Test.java
public class Test extends java.lang.Object {
    public Test();
    public static void main(java.lang.String[]);
}

Method Test()
   0 aload_0
   1 invokespecial #1 <Method java.lang.Object()>
   4 return

Method void main(java.lang.String[])
   0 iconst_1
   1 istore_1
   2 iload_1
   3 iconst_1
   4 iadd
   5 istore_1
   6 return

9.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: rainman
Posted on: 2003-03-19 01:46


public class Test {
  public static void main(String[] args) {
    int i=1;    
    i = ++i;
  }
}



Compiled from Test.java
public class Test extends java.lang.Object {
    public Test();
    public static void main(java.lang.String[]);
}

Method Test()
   0 aload_0
   1 invokespecial #1 <Method java.lang.Object()>
   4 return

Method void main(java.lang.String[])
   0 iconst_1
   1 istore_1
   2 iinc 1 1
   5 iload_1
   6 istore_1
   7 return

10.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: rainman
Posted on: 2003-03-19 02:19

指令解释：

iconst_1 ; push 1 onto the stack

istore_1 ;store integer in local variable 1

iload_1 ;push integer in local variable 1 onto the stack

iinc <varnum> <n> ; increments the int held in the local variable <varnum> by <n>.

iadd ; Pops two integers from the operand stack, adds them, and pushes the integer result back onto the stack
11.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: rainman
Posted on: 2003-03-19 02:35

i = 1;i = i++
相当于下面的汇编码
0 iconst_1 ； 把1放到堆栈
1 istore_1 ；从栈顶取出1并存在局部变量1中（也就是变量 i ) 这时候i=1；
2 iload_1 ； 把i的值放到堆栈里去，这时候栈顶为1
3 iinc 1 1 ；把变量1（也就是 i）加1，这时候i = 2咯
6 istore_1 ；呵呵，从栈顶取出1存到i里去，呵呵i又回到1了。
7 return ； 返回。

i = 1; i = i+1
也来看一下：
0 iconst_1 ; stack = [1];
1 istore_1 ; stack =[] ; i = 1;
2 iload_1 ; stack = [1]; i = 1;
3 iconst_1 ; stack = [1,1] ; i =1;
4 iadd ; stack =[2];i = 1;
5 istore_1 ; stack = [];i =2;
6 return

i = 1 ; i=++i
0 iconst_1 ; stack=[1];
1 istore_1 ; stack=[] ; i =1;
2 iinc 1 1 ; stack=[];i=2;
5 iload_1 ; stack=[2];i=2;
6 istore_1 ; stack=[];i=2;
7 return

Done.
12.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: cmslovehxh
Posted on: 2003-03-19 21:10

snowbug wrote:
i = i++ is the same as the following set of statements:
1). int temp = i;
2). i++;
3). i = temp;

Therefore, the original i value is assigned back to the variable i after (i++) statement.
我是这样理解的，因为++的优先级高于=；所以按照snowbug的步骤，既能够满足++的优先级高于=,又能够满足i++的先赋值再加一；
13.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: scottding
Posted on: 2003-03-20 09:19

hoho，解释的这么详细。
14.Re:int i =1; i=i++ 为什么结果是 i =1? [Re: xiaopan] Copy to clipboard
Posted by: scottding
Posted on: 2003-03-20 09:21

不过在C＋＋中，＋＋和－－的运算在不同的编译器中得到的结果也是不一样的，不同的编译器的处理不同。在java中，也就一种处理了。对于这种问题，不需要深究了，已经有了太多无意义的讨论了。

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