我的代码有什么问题？ 它崩溃了

Question

When the user enters an input like 0 or 6, the program enters an endless loop.

I tried to create new while loops, break and continue statements. However, none of them worked.

public static final Pattern p12345 = Pattern.compile("[1-5]");

public Counter() {
          this.counter = 0;
          // System.in is like System.out, however, for input and not for output.
          Scanner s = new Scanner(System.in);
          /*
           * can take value 1 (add) 2 (subtract) 3 (show counter) 4 (set counter) 5 (exit)
           */
          int topChoice = 0; // Can be anything but 5 to enter the loop
          while (topChoice != 5) {
               System.out.println("Please enter:\n" + "1 to add to the total\n" + "2 to subtract from the total\n"
                         + "3 to show the total\n" + "4 to set the total\n" + "5 to exit the program");

               try {
                    topChoice = Integer.parseInt(s.next(p12345));
               } catch (InputMismatchException e) {
                    System.out.println("You need to enter an integer between 1-5.");
               }
               switch (topChoice) {
               case 1:
                    add(s);
                    break;
               case 2:
                    subtract(s);
                    break;
               case 3:
                    show();
                    break;
               case 4:
                    set(s);
                    break;
               case 5:
                    System.out.println("Finally there are " + counter + " items available.");
                    break;
               default:
                    throw new IllegalArgumentException();
               }
          }
     }

When the user enters an integer which is not in range of 1-5, I expected to turn back and ask again, but it crashes.

Please enter:
1 to add to the total
2 to subtract from the total
3 to show the total
4 to set the total
5 to exit the program
6
You need to enter an integer between 1-5.
Exception in thread "main" java.lang.IllegalArgumentException
        at Counter.<init>(Counter.java:87)

Answer 1

That is because of this line of code:

 default:
                throw new IllegalArgumentException();

I assume you are getting an error in the console for IllegalArgumentException right? Having not declared a case for 0-6, the switch statement will by default go to the default case, where you throw an exception

Answer 2

You can simplify your code much more. What I mean is you can use a simple if-else statement checking for the range of 1 to 5 instead of the try-catch that would throw an IllegalArgumentException with the Pattern class. The else statement can be a console print saying that the number was out of range. Also, I'm assuming you're supposed to increment your counter variable by 1 for each iteration of the while loop? I don't see that in your code.

Revised:

while (topChoice != 5) {
   counter++;
   try {
      topChoice = Integer.parseInt(s.next());
   }
   catch (NumberFormatException er)
   {
      System.out.println("Error: You must enter in an integer.");
   }
   catch (Exception er)
   {
      System.out.println("Error: " + er.getMessage());
   }
   if (topChoice >= 1 && topChoice <= 5)
   {
       switch (topChoice) {
          case 1:
              add(s);
              break;
          case 2:
              subtract(s);
              break;
          case 3:
              show();
              break;
          case 4:
              set(s);
              break;
          case 5:
              System.out.println("Finally there are " + counter + " items available.");
              break;
       }
   }
   else
       System.out.println("Error. Number must be between 1 and 5.");
}

Answer 3

Your program is throwing an exception because you told it to:

throw new IllegalArgumentException();

If you want it to loop back to the top, just don't do anything in the default case! You can delete the whole default case all together.

The other thing you need to do is to actually read the input that you failed to read. Also, you should reset topChoice back to 0, otherwise it will end up doing whatever it did last time the loop ran.

try {
    topChoice = Integer.parseInt(s.next("[1-5]"));
} catch (InputMismatchException e) {
    System.out.println("You need to enter an integer between 1-5.");
    s.nextLine(); // <----- here
    topChoice = 0; // <---- and here
}

Otherwise in the next iteration of the loop, the scanner is going to try to read the same input that it couldn't read before, and fail again, creating an infinite loop.

---

Even though the code works after the modifications above, you shouldn't really do "control flow" with exceptions. You should read a line, then check whether it matches a pattern:

String line = s.nextLine();
if (line.matches("[1-5]")) {
    topChoice = Integer.parseInt(line);
} else {
    System.out.println("You need to enter an integer between 1-5.");
    topChoice = 0;
}