从 JVM 层面一次性理解 i++ 和 ++i 的真正区别！

前言

如果只用普通的知识解释i++和++i的话

i++ 先将i赋值再++
++i 先++再赋值

但是这简单的回答并不能入吸引面试官的眼球,如果用java字节码指令分析则效果完全不同。

代码实现

    public class OperandStackTest {
    /**
        程序员面试过程中， 常见的i++和++i 的区别
         */
        public static void add(){
            //第1类问题：
            int i1 = 10;
            i1++;
            System.out.println(i1);//11
    
            int i2 = 10;
            ++i2;
            System.out.println(i2);//11
    
            //第2类问题：
            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类问题：
            int i7 = 10;
            i7 = i7++;
            System.out.println(i7);//10
    
            int i8 = 10;
            i8 = ++i8;
            System.out.println(i8);//11
    
            //第4类问题：
            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();
        }
    }

运行结果

字节码指令

通过javap -v out目录下的class文件名在终端运行得到如下结果

     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