public enum Operation
{
  PLUS
  {
   public double eval(double x , double y)
   {
     return x + y;
   }
  },
  MINUS
  {
   public double eval(double x , double y)
   {
     return x - y;
   }
  },
  TIMES
  {
   public double eval(double x , double y)
   {
     return x * y;
   }
  },
  DIVIDE
  {
   public double eval(double x , double y)
   {
     return x / y;
   }
  };
  // 为枚举类定义一个抽象方法
  // 这个抽象方法由不同的枚举值提供不同的实现
  public abstract double eval(double x, double y);
  public static void main(String[] args)
  {
   System.out.println(Operation.PLUS.eval(3, 4));
   System.out.println(Operation.MINUS.eval(5, 4));
   System.out.println(Operation.TIMES.eval(5, 4));
   System.out.println(Operation.DIVIDE.eval(5, 4));
  }
}