Thread: Problems with Try/Catch Catching Wrong Exception

This is a test build for a new version of the calculator application in which I am trying to implement dynamic mode switching (fairly standard... subtract your total while in addition mode). I'm basically rewriting the entire Arithmetic.java class so that all of the methods have return types rather than declaring them void.

This current test build implements only addition and subtraction with a temporary main method (for testing purposes). Mathematically it works fine. The problem arises with the Try/Catch layout.

[CODE]
public class Arithmetic {
  public Double Add(double totNum) {
    boolean calc = true;
    while(calc == true) {
      try {
        CalcInput user = new CalcInput();
        String userInput = user.getUserInput(" ");
        if(userInput.equals("-")) {
          System.out.println(totNum + "-");
          double result = Subtract(totNum);
          totNum = result;
        } else {}
        double addNum = Double.parseDouble(userInput);
        if(totNum == 0) {
          System.out.println(addNum + "+");
        } else {
          System.out.println(totNum + "+" + addNum);
        }
        totNum = totNum + addNum;
      } catch(NumberFormatException e) {
        System.out.println("Invalid input!  Check to make sure you typed it correctly and try again!");
        System.out.println("-DO NOT use operators.  Enter your NUMERICAL input and press ENTER.");
        System.out.println("-You may use shortcuts to call other calculation modes.  (+, -, x, etc.");
      } catch(NullPointerException e) {
        calc = false;
      }
    }
    return totNum;
  }
  public Double Subtract(double totNum) {
    boolean calc = true;
    while(calc == true) {
      try {
        CalcInput user = new CalcInput();
        String userInput = user.getUserInput(" ");
        if(userInput.equals("+")) {
          System.out.println(totNum + "+");
          double result = Add(totNum);
          totNum = result;
        } else {}
        double addNum = Double.parseDouble(userInput);
        if(totNum == 0) {
          System.out.println(addNum + "-");
          totNum = addNum;
        } else {
          System.out.println(totNum + "-" + addNum);
          totNum = totNum - addNum;
        }
      } catch(NumberFormatException e) {
        System.out.println("Invalid input!  Check to make sure you typed it correctly and try again!");
        System.out.println("-DO NOT use operators.  Enter your NUMERICAL input and press ENTER.");
        System.out.println("-You may use shortcuts to call other calculation modes.  (+, -, x, etc.");
      } catch(NullPointerException e) {
        calc = false;
      }
    }
    return totNum;
  }
  public static void main(String[] args) {
    Arithmetic a = new Arithmetic();
    double result = a.Add(0);
    System.out.println(result);
  }
}
[/CODE]

If you call the subtraction method while in the addition mode and press ENTER with nothing in the input field (which I would assume to be a NullPointerException), it displays the error message I added for the NumberFormatException.

Why is that? Is it because you are operating within a call? In that case, how do I get out of the called method so that the person can hit ENTER and get the result?

If it isn't clear yet what the problem is, I will try to clarify a bit more:
While in the program....
//I enter 5. The system prints the response as usual.
5.0+
//I enter 5 again. Still working fine.
5.0+5.0
//I enter - The call works fine.
10.0-
//I enter 5. Still lookin good.
10.0-5
//I hit ENTER (no input). Here is where it goes to hell.
It displays the error message I implemented in the NumberFormatException catch.
//I hit ENTER again. NOW it gets it.
5.0 <----- It got the right answer!

But it's recognizing a NullPointerException as a NumberFormatException while inside the method call.

Any help would be appreciated.

EDIT: I did just realize there are two loops within the try/catch. So the problem probably comes from the fact that it's trying to end the first loop before the inner loop is done executing. But, how do I fix that and/or why is it displaying the defined catch for NumberFormatException?

Well, I found at least 2 errors in this code that should be fixed:

public Double Subtract(double totNum) {
    boolean calc = true;
    while(calc == true) {
      try {
        CalcInput user = new CalcInput();
        String userInput = user.getUserInput(" ");
        if(userInput.equals("+")) {
          System.out.println(totNum + "+");
          double result = Add(totNum);
          totNum = result;
        } else {}
        double addNum = Double.parseDouble(userInput);
        if(totNum == 0) {
          System.out.println(addNum + "-");
          totNum = addNum;
        } else {
          System.out.println(totNum + "-" + addNum);
          totNum = totNum - addNum;
        }
      } catch(NumberFormatException e) {
        System.out.println("Invalid input!  Check to make sure you typed it correctly and try again!");
        System.out.println("-DO NOT use operators.  Enter your NUMERICAL input and press ENTER.");
        System.out.println("-You may use shortcuts to call other calculation modes.  (+, -, x, etc.");
      } catch(NullPointerException e) {
        calc = false;
      }
    }
    return totNum;
  }

First off, calc appears to never be set to false unless a Null Pointer Exception is thrown, so unless one is thrown, your while loop should go forever. Was that done on purpose?

It would be better if calc were set to false at the end of each of the if and else things in your try block or something. That way it will exit only if the Exceptions aren't thrown, not just if a Null Pointer Exception is thrown.

Second, those two brackets after that first else statement serve no purpose, except to probably mess up your code by making sure that everything after it was not inside your else block like you probably intended.

Ok, so you are getting NumberFormatException.

Yeah,

"-" isn't very helpful to a compiler.

First off negative what? Second, to the compiler, without any number attached to it, it will think that you entered in a String and it wants you to use a double.

That's why it said:

For input string: "-"

I think I see what your program is trying to do however. Correct me if I'm wrong, but you want the user to enter like "+" or "-" or something and then user that operator to perform some calculation.

Well, what you're doing might work if you fixed those two errors I pointed out above, particularly the bracket error, though the other one needs fixing too to avoid a potential infinite loop.

If the user inputs nothing it throws a NPE. That is the only reason the catch is there. If nothing is put in the input, then it tries to pass a null value to the parsing method which throws a NPE. Seems logical to have a catch to prevent the program from crashing upon this event. It's there in the interest of user stupidity (or in the most recent build it's how you get the output).