i++ 和 ++i的字节码指令

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