代码

public class Test{

public static void main(String args[]){

int i=0;

i=i++;

System.out.println(i);

}

}

输出 0 , 说实话我第一看也打错了，看看字节码

public class Test {

public Test();

Code:

0: aload_0

1: invokespecial #1 // Method java/lang/Object."<init>":

()V

4: return

public static void main(java.lang.String[]);

Code:

0: iconst_0

1: istore_1

2: iload_1

3: iinc 1, 1

6: istore_1

7: getstatic #2 // Field java/lang/System.out:Ljava/

io/PrintStream;

10: iload_1

11: invokevirtual #3 // Method java/io/PrintStream.printl

n:(I)V

14: return

}

指令3 iinc 1,1 直接+1到第二个变量i 中，这时候i=1;

再执行istore_1 把操作数栈写回到变量i, 这时候i又变成0

 

我们把代码改改

public class Test{

public static void main(String args[]){

int i=0;

//i=i++;

i=i+1;

System.out.println(i);

}

}

字节码指令

Compiled from "Test.java"

public class Test {

public Test();

Code:

0: aload_0

1: invokespecial #1 // Method java/lang/Object."<init>":

()V

4: return

public static void main(java.lang.String[]);

Code:

0: iconst_0

1: istore_1

2: iload_1

3: iconst_1

4: iadd

5: istore_1

6: getstatic #2 // Field java/lang/System.out:Ljava/

io/PrintStream;

9: iload_1

10: invokevirtual #3 // Method java/io/PrintStream.printl

n:(I)V

13: return

}

i=i+1 执行的是iadd指令，这是操作数栈里的字节码指令,把新值1压入到操作数栈，再执行istore_1 把操作数栈回写到变量i, 于是i变成了1