使用JOptionPane进行输入验证

Question

I'm doing input validation on a program using JOptionPane boxes. I'm trying to have the input box repeat after the error message every time the user enters in a non-double. How would I do this?

    try {
        lengthResult = Double.parseDouble(JOptionPane.showInputDialog("What is the length of your garage in square feet?"));
    }
    catch (NumberFormatException e) {
        JOptionPane.showMessageDialog(null, "Please enter a number in digit format.","Inane error",JOptionPane.ERROR_MESSAGE);
    }

Answer 1

If you want to repeat the message box until the user enters something valid, I'd go like this:

Double lengthResult = null; //Init to null, which is invalid
String title = "Please anter a number";
int initialType = JOptionPane.QUESTION_MESSAGE;
do {
  try {
    lengthResult = Double.parseDouble(
      JOptionPane.showInputDialog(null,
         "What is the length of your garage in square feet?",
         title, initialType));
  } catch (NumberFormatException e) {
    initialType = JOptionPane.ERROR_MESSAGE;
    title = "Error: Please enter a number!";
  }
} while(lengthResult == null); //Iterate as long as no valid input found

Note that this check relies on lengthResult being an Object of type Double, not a primitive type double. With primitive double you'd need some extra flag as you cannot check on lengthResult value this way.