從 JVM 層面理解 i++ 和 ++i 的真正區別！

原創

2021-09-02 09:33

前言

如果只用普通的知識解釋i++和++i的話

i++ 先將i賦值再++
++i 先++再賦值

但是這簡單的回答並不能入吸引面試官的眼球,如果用java字節碼指令分析則效果完全不同。

代碼實現

public class OperandStackTest {
/**
    程序員面試過程中， 常見的i++和++i 的區別
     */
    public static void add(){
        //第1類問題：
        int i1 = 10;
        i1++;
        System.out.println(i1);//11

        int i2 = 10;
        ++i2;
        System.out.println(i2);//11

        //第2類問題：
        int i3 = 10;
        int i4 = i3++;
        System.out.println(i3);//11
        System.out.println(i4);//10

        int i5 = 10;
        int i6 = ++i5;
        System.out.println(i5);//11
        System.out.println(i6);//11

        //第3類問題：
        int i7 = 10;
        i7 = i7++;
        System.out.println(i7);//10

        int i8 = 10;
        i8 = ++i8;
        System.out.println(i8);//11

        //第4類問題：
        int i9 = 10;
        int i10 = i9++ + ++i9;//10+12
        System.out.println(i9);//12
        System.out.println(i10);//22
    }

    public static void main(String[] args) {
        add();
    }
}

運行結果

字節碼指令

通過javap -v out目錄下的class文件名在終端運行得到如下結果

 public static void add();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=10, args_size=0
         0: bipush        10
         2: istore_0
         3: iinc          0, 1
         6: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
         9: iload_0
        10: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
        13: bipush        10
        15: istore_1
        16: iinc          1, 1
        19: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
        22: iload_1
        23: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
        26: bipush        10
        28: istore_2
        29: iload_2
        30: iinc          2, 1
        33: istore_3
        34: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
        37: iload_2
        38: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
        41: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
        44: iload_3
        45: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
        48: bipush        10
        50: istore        4
        52: iinc          4, 1
        55: iload         4
        57: istore        5
        59: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
        62: iload         4
        64: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
        67: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
        70: iload         5
        72: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
        75: bipush        10
        77: istore        6
        79: iload         6
        81: iinc          6, 1
        84: istore        6
        86: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
        89: iload         6
        91: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
        94: bipush        10
        96: istore        7
        98: iinc          7, 1
       101: iload         7
       103: istore        7
       105: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
       108: iload         7
       110: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
       113: bipush        10
       115: istore        8
       117: iload         8
       119: iinc          8, 1
       122: iinc          8, 1
       125: iload         8
       127: iadd
       128: istore        9
       130: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
       133: iload         8
       135: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
       138: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
       141: iload         9
       143: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
       146: return