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Dibbie

Tue May 01, 2012 4:57 pm


Newbish question - Exception loop?

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First off, hello, im new to this forums, so hello to everyone! =D
Secondly, i know this is not the right area to post this, since this is a Java related issue, but it wont let me post in Java section, sais i dont have the rights to. Anyway, on to my question:

So, I made a program thats a simple calculator, and this is my first year programming (iv been useing Java and programming for a good 2 months now, im not in school for it)

Anyway, so the problem is that I donno how to loop it, so it works properly, heres whats suposto happen:
Enter your first number to add: g
That's not a number.
Enter your first number to add: 

Heres what happens:
Enter your first number to add: g
That's not a number.
+
Enter your second number: 

Here is my code, can someone please tell me what im doing wrong here:

if (op == 'A' || op == 'a')
                {
                    while (exception==null)
                    {
                        write ("Enter your first number to add: ");
                        try 
                        {
                            num1 = readInt ();
                        }
                        catch(Exception e) 
                        { 
                            System.out.println("That's not a number.");
                            exception=e.toString();
                        }
                    }
                    writeLine ("+");
                    write ("Enter your second number: ");
                    num2 = readInt ();
                    sum = num1 + num2;
                    writeLine (num1 + " + " + num2 + " = " + sum);
                    writeLine ("");
                    writeLine ("Back to menu?        (Y|N)");
                    writeLine ("");
                    op = readChar ();
                }

(Also, just so you understand the program, its a simple calculator that adds, subtracts, multiplys, or devides)
Dibbie Simple Calculator selected!

A (Add)      S (Subtract)      M (Multiply)      D (Divide)

^ Above you pick wich option you want, in the error shown above, i picked A, add


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DemonWasp

Tue May 01, 2012 5:13 pm


RE:Newbish question - Exception loop?

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What's happening is that when you enter a non-number, the readInt() call fails and throws an exception (hopefully a NumberFormatException). Then, the 'exception' variable gets set to a string representing that exception, then exception is not null, so the loop does not continue. The more straightforward way to write this kind of thing is:


while ( true ) {
    try {
        num1 = readInt();
        break;  // leaves the outer while loop immediately
    } catch ( NumberFormatException e ) {
        System.out.println ( "That is not a number." );
    }
}