Blackberry only Alpha Numeric characters in text field

EditField eField = new EditField("Enter letters and digits only: ", "")
 {
    protected boolean keyChar(char ch, int status, int time) 
  {
        if (CharacterUtilities.isLetter(ch) || CharacterUtilities.isDigit(ch) || (ch == Characters.BACKSPACE)) 
  {
            return super.keyChar(ch, status, time);
        }
        return true;
    }
};