尝试 - 捕获异常问题

Question

I'm trying to make a desktop calculator with different options (Hex, Binary, Octal, Decimal) and if a value is entered that is not of that mode, an error will appear saying, for example "BinaryInteger expected, the user entered 909382" and prompt again. Right now the only error that appears is "Invalid option" due to my catch(Exception e). How do I write 4 exceptions/errors for invalid inputs.

import java.util.*;

public class IntDriver
{
   //declared fields
   private static LongInteger num1;
   private static LongInteger num2;
   private static String opr;
   private static int mode = 0;
   // Operators that will be used
   private final static String[] Operators = { "", "+", "-", "*", "/" };

   public static void main(String[] args)
   {
      Scanner kb = new Scanner(System.in);

      boolean run = true;
      while(run)
      {
         displayMenu();

         while(true)
         {
            try
            {
               System.out.print("Option or value --> ");
               String option = kb.nextLine();

               switch(option.toLowerCase())
               {
               //modes

                  // decimal mode
                  case "decimal":
                  case "dcm":
                  case "dec":
                     mode = 0;
                     break;

                  // binary mode
                  case "binary":
                  case "bin":
                     mode = 1;
                     break;

                  //octal mode
                  case "octal":
                  case "oct":
                     mode = 2;
                     break;

                  //hex mode
                  case "hexadecimal":
                  case "hex":
                     mode = 3;
                     break;

                  //quit
                  case "q":
                  case "quit":
                     System.out.println("Thank you. Have a nice day.");
                     run = false;
                     break;

               //Operators
                  //add
                  case "+":
                     opr = "+";
                     break;

                  //subtract   
                  case "-":
                     opr = "-";
                     break;

                  //multiply   
                  case "*":
                     opr = "*";
                     break;

                  //divide
                  case "/":
                     opr = "/";
                     break;

                  //equals
                  case "=":
                     operate();
                     break;

                  default:
                     LongInteger temp;
                     //with mode it's in:
                     switch (mode)
                     {
                        case 1:
                           temp = new BinaryInteger(option);
                           break;
                        case 2:
                           temp = new OctalInteger(option);
                           break;
                        case 3:
                           temp = new HexInteger(option);
                           break;
                        default:
                           temp = new DecInteger(option);
                           break;
                     }
                     // if no num1 = LongInteger temp.
                     if (num1 == null)
                     {
                        num1 = temp;
                     }
                     else
                     {
                        if (opr == null)
                        {
                           throw new UnsupportedOperationException();
                        }
                        num2 = temp;
                     }

                     break;
               }
               break;
            }
            catch (UnsupportedOperationException e)
            {
               System.out.println("Invalid option; operator not specified.\n");
            }
            // Invalid option entered.
            catch (Exception e)
            {
               System.out.println("Invalid option.\n");
            }
         }
      }
   }
   //Menu Display
   private static void displayMenu()
   {
      System.out.println();

      switch (mode)
      {
         case 1:
            System.out.println("Binary Mode");
            break;
         case 2:
            System.out.println("Octal Mode");
            break;
         case 3:
            System.out.println("Hexadecimal Mode");
            break;
         default:
            System.out.println("Decimal Mode");
            break;
      }

      if (num1 != null)
      {
         System.out.print(num1 + "\t");
      }

      if (opr != null)
      {
         System.out.print(opr + "\t");
      }

      if (num2 != null)
      {
         System.out.print(num2 + "\n");
      }
      System.out.println("\n\n\tModes:\t\t    Operators:");
      System.out.println("\tBin - Binary\t\t+");
      System.out.println("\tOct - Octal\t\t-");
      System.out.println("\tDcm - Decimal\t\t*");
      System.out.println("\tHex - Hexadecimal\t/");
      System.out.println("\tQ   - Quit\t\t=\n");
   }

   private static void operate() throws Exception
   {

      if (num1 == null || num2 == null || opr == null)
      {
         throw new Exception("Not enough numbers.");
      }

      switch (opr)
      {
         case "+":
            num1 = num1.calcValue(opr, num2, mode);
            break;
         case "-":
            num1 = num1.calcValue(opr, num2, mode);
            break;
         case "*":
            num1 = num1.calcValue(opr, num2, mode);
            break;
         case "/":
            num1 = num1.calcValue(opr, num2, mode);
            break;
         default:
            throw new Exception("Invalid operator.");
      }

      num2 = null;
      opr = "";
   }


}

Answer 1

You could go and extend java.lang.Exception 4 times for each case :

public class YourCustomException extends Exception {

  //copy paste and modify whatever constructors are relevant in your case.

}

.. and then go on and throw those 4 exceptions when you have invalid input for your modes. This is rather inefficient in your case, though. I think your case is simple enough that you can simply do something like:

public class DecInteger {

    public DecInteger (String option)
    {
        if (isNotDecimal(option))
            throw new Exception("Input option " + option + " is not a decimal");
    }

}

.. then in your catch block you will need to do something like this:

catch(Exception ex) {
    System.out.println(ex.getMessage);
}

Technically speaking it is not good practice to throw exceptions of type Exception , it is better to create subtypes that are more descriptive to the actual exception, but your case is simple enough where this doesn't really matter.