来道不一样的 Java 面试题

你的名字 2022-12-20 06:11 162阅读 0赞

### 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][IDEA _ Java] 。

### 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);
        }
    }

1.  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);
        }
    }

1.  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);
        }
    }

1.  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);
        }
    }

1.  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);
        }
    }

1.  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。

[IDEA _ Java]: https://blog.csdn.net/qq_24489717/article/details/53837493