从原理上弄清楚i++和++i

怼烎@ 2022-11-05 12:52 122阅读 0赞

**代码**

public class TestMain {
        
        public static void main(String[] args) {
        
            int a=0,b=0;
            a=a++;
            b=++b;
            System.out.println("a="+a);
            System.out.println("b="+b);
        }
    }

**字节码**

public static void main(java.lang.String[]);
        Code:
           0: iconst_0
           1: istore_1
           2: iconst_0
           3: istore_2
           4: iload_1
           5: iinc          1, 1
           8: istore_1
           9: iinc          2, 1
          12: iload_2
          13: istore_2
          14: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
          17: new           #3                  // class java/lang/StringBuilder
          20: dup
          21: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
          24: ldc           #5                  // String a=
          26: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
          29: iload_1
          30: invokevirtual #7                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
          33: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
          36: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
          39: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
          42: new           #3                  // class java/lang/StringBuilder
          45: dup
          46: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
          49: ldc           #10                 // String b=
          51: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
          54: iload_2
          55: invokevirtual #7                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
          58: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
          61: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
          64: return

主要看这块

4: iload_1                  #将局部变量表下标1的数压入栈顶
           5: iinc          1, 1       #将局部变量表自增1
           8: istore_1	               #将栈顶值存到局部变量表1的位置
           9: iinc          2, 1       #局部变量表2位置自增1
          12: iload_2                  #将局部变量表2位置压入栈顶
          13: istore_2	               #将栈顶的数存到局部变量表的2位置

**i=i++，是先入栈，再自增  
i=++i ，是先自增，再入栈**