来道不一样的 Java 面试题

Posted 2022-09-10 泰 戈 尔

问题描述

int a = 2;
int b = a++ << ++a + ++a;

问：此时a、b 的值分别是多少？

先来发一下答案：1024 。是不是很惊讶？接下来一点点来看下，为什么会得到这个答案。

知识储备：
1 运算符优先级 + ++ <<
参考链接 https://www.cnblogs.com/gw811/archive/2012/10/13/2722752.html
2 查看字节码
参考链接 https://segmentfault.com/a/1190000003871183

本次借助 IDEA 查看 Java 字节码 。

逐步攻克

1. 来一版最简单的

public class HelloWorld 
    public static void main(String[] args) 
        int a = 2;
        int b = a++;
        System.out.println(b);  // 2
    

字节码反编译结果

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

public class HelloWorld 
    public HelloWorld() 
    

    public static void main(String[] args) 
        int a = 2;
        byte var10000 = a;
        int var3 = a + 1;
        int b = var10000;
        System.out.println(b);
    

2. 运算符编译优化处理

public class HelloWorld 
    public static void main(String[] args) 
        int a = 2;
        int b = ++a + ++a;
        System.out.println(b);  // 7
    

字节码反编译结果

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

public class HelloWorld 
    public HelloWorld() 
    

    public static void main(String[] args) 
        int a = 2;
        int a = a + 1;
        int b = a++ + a;
        System.out.println(b);
    

3. 朝着题头问题前进

public class HelloWorld 
    public static void main(String[] args) 
        int a = 2;
        int b = 1 << ++a + ++a;
        System.out.println(b);  // 128
    

字节码反编译结果

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

public class HelloWorld 
    public HelloWorld() 
    

    public static void main(String[] args) 
        int a = 2;
        int a = a + 1;
        int b = 1 << a++ + a;
        System.out.println(b);
    

4. 题头问题复现

public class HelloWorld 
    public static void main(String[] args) 
        int a = 2;
        int b = a++ << ++a + ++a;
        System.out.println(b);  // 1024
    

字节码反编译结果

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

public class HelloWorld 
    public HelloWorld() 
    

    public static void main(String[] args) 
        int a = 2;
        int a = a + 1;
        ++a;
        int b = a << a++ + a;
        System.out.println(b);
    

5. 拓展左移操作符

public class HelloWorld 
    public static void main(String[] args) 
        int a = 2;
        int b = ++a << ++a;
        System.out.println(a);  // 4
        System.out.println(b);  // 48
    

字节码反编译结果

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

public class HelloWorld 
    public HelloWorld() 
    

    public static void main(String[] args) 
        int a = 2;
        int a = a + 1;
        int b = a++ << a;
        System.out.println(a);
        System.out.println(b);
    

6. 再遇难题

public class HelloWorld 
    public static void main(String[] args) 
        int a = 2;
        int b = ++a << (++a + ++a);
        System.out.println(a);  // 5
        System.out.println(b);  // 1536 == 6 << 8
    

字节码反编译结果

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

public class HelloWorld 
    public HelloWorld() 
    

    public static void main(String[] args) 
        int a = 2;
        int a = a + 1;
        int b = a++ << a++ + a;
        System.out.println(a);
        System.out.println(b);
    

面试过程中，遇到此类问题，可能没啥争议，但是工作时组内谁要是写这样的代码，恐怕会被吐槽的吧。