位运算（练习）

1. 最有效率的方式算出 2乘以 8等于几

   2 << 3 = 2 * 2^3 = 2 * 8 = 16

2. 对于两个整数变量的值进行互换（不需要第三方变量）

   class Test
   {
       public static void main(String[] args)
       {
           int n = 3, m = 4;
           System.out.println("n = "+n+ ", m = "+m);
    
           //1、通过定义第三方变量来实现
           	int temp;
           	temp = n;
           	n = m;
           	m = temp;       
           System.out.println("n = "+n+ ", m = "+m);
           
   //2-1、不借助第三方变量实现
           n = n + m;  // n = 3 + 4 = 7;
           m = n - m;  // m = 7 - 4 = 3; 
           n = n - m;  // n = 7 - 3 = 4;
           System.out.println("n = "+n+ ", m = "+m);
           //这种方法有局限性，如果n和m的值比较大，相加容易超出           int范围，造成损失精度
   
   //2-2、不借助三方变量实现
           n = n ^ m; //此时 n =(n ^ m)
           m = n ^ m; // m = (n ^ m) ^ m == n = 3;
           
           n = m ^ n; // n = n ^ m == n ^ (n ^ m);
           System.out.println("n = "+n+ ", m = "+m);
           //这种方式属于技巧性
           
           /*
             或者直接打印：
             System.out.println("n = "+m+ ", m = "+n);
               ……
           */
       }
   }
   

   获取60的最低4位，通过 60 & 15;

   class Text
   {
       public static void main(String[] args)
       {
           int num = 60;
           int n1 = num & 15;
           
           System.out.println((char) (n1 - 10 + \'A\'));
           
           //要获取下一组四位，将 60 右移 4 位
           int temp = 60 >> 4; 
           
           //对temp的值进行最低4位的获取
           int n2 = temp & 15;
           System.out.println( n2 );
           /*
                0 — 9 \'A\' \'B\' \'C\' \'D\' \'E\' \'F\'        
                       65  66  67
                       10  11  12  13  14  15
                  12 - 10 = 2 + \'A\' = (char)67;
           */
       }
   }