Error checking with jTextFields

Question

I've built a GUI application with several jTextFields. The user input from the text fields is parsed into doubles when the user clicks a jButton. The problem that I'm encountering is dealing with the cases of empty text fields and inputs that are not numbers. For example, if I enter an A into a text field and hit the button, I get a :
Exception occurred during event dispatching:
java.lang.NumberFormatException: a
Any thoughts on how to correct this problem?
Colin Spencely

Answer 1

Well I think is obvious: before parsing the strings as numbers make a check to see if you have indeed a number.
Or build your own text field which accepts only digits:
public class DigitField extends JTextField{
  public DigitField(int cols){
    super(cols);
  //other constructors here
  protected Document createDefaultModel(){
    return new NumberDocument();
  class NumberDocument extends PlainDocument{
    public void insertString(int offs, String str, AttributeSet a) throws
      BadLocationException{
      if(str == null || !isNumber(str)){
        //error message here, if you wish
        return;
      super.insertString(offs, str, a);
  private final boolean isNumber(String number){
    if(number == null || number.equals("") || number.trim().equals("")){
      return false;
    char[] data = number.toCharArray();
    char[] digits = {
      '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.'};
    boolean isNumber = true;
    int dotCounter = 0;
    for(int i = 0; i < data.length; i++){
      int matchCounter = digits.length;
      for(int j = 0; j < digits.length; j++){
        if(data[i] != digits[j]){
          matchCounter--;
      if(data[i] == '.'){
        dotCounter++;
      if(matchCounter == 0 || (matchCounter >= 0 && dotCounter > 1)){
        isNumber = false;
        break;
    if(dotCounter == 1 && number.length() == 1){
      isNumber = false;
    return isNumber;
}