Java Date to String解析/验证需要帮助

Question

I'm having trouble changing my date field in java to a string. It works in my output toString but the validation piece doesn't recognize. What is wrong with my code? I cannot have users posting anything but a true "M/d/yyyy" date but my validation isn't working. Thoughts?

I've posted the start date code below, the "end date" code is the same but thought you should know it is present.

Many thanks in advance.

        String stringStartDate = this.txtStartDate.getText();
        SimpleDateFormat formatter = new SimpleDateFormat("M/d/yyyy");
        Date pStartDate = null;

        if (stringStartDate.length() > 0)
        {
            try {
                pStartDate = formatter.parse(stringStartDate);
            } catch (ParseException e1) {
                e1.printStackTrace();
            }

          if (pStartDate == null)
          {
            this.textArea.setText(this.textArea.getText() + "\n"
              + "Error! Please enter a valid start date in the format M/d/yyyy (e.g. 1/2/2015)!");
            return;
          }
        }
        else
        {
          this.textArea.setText(this.textArea.getText() + "\n"
            + "Error! Please enter a valid start date in the format M/d/yyyy (e.g. 1/2/2015)!");
          return;
        }

Here is my toString()... this is working perfectly.

    @Override 
    public String toString(){
    return "Pay Period Start Date = " +        simpleDateFormatMonthDayYear.format(this.pStartDate) + "\n" 
            + "Pay Period End Date = " + simpleDateFormatMonthDayYear.format(this.pEndDate) + "\n";

    }

From the comments and my own trial and error I've change a few things around but now neither work. What is the deal?

        SimpleDateFormat simpleDateFormatMonthDayYear = new SimpleDateFormat("M/d/yyyy");
        String stringStartDate = simpleDateFormatMonthDayYear.format(this.txtStartDate.getText());
        Date pStartDate = null;

        if (stringStartDate.length() > 0)
        {
            try {
                pStartDate = simpleDateFormatMonthDayYear.parse(stringStartDate);
            } catch (ParseException e1) {
                e1.printStackTrace();
            }

          if (pStartDate == null)
          {
            this.textArea.setText(this.textArea.getText() + "\n"
              + "Error! Please enter a valid start date in the format M/d/yyyy (e.g. 1/2/2015)!");
            return;
          }
        }
        else
        {
          this.textArea.setText(this.textArea.getText() + "\n"
            + "Error! Please enter a valid start date in the format M/d/yyyy (e.g. 1/2/2015)!");
          return;
        }

Answer 1

You are trying to use a SimpleDateFormat to format a String ...

SimpleDateFormat simpleDateFormatMonthDayYear = new SimpleDateFormat("M/d/yyyy");
String stringStartDate = simpleDateFormatMonthDayYear.format(field.getText());

which simply doesn't make sense. You want to parse the String value to a Date

The first thing you want to do is parse the String value to a Date , this will check to see if the String is in the correct format. Once you've done that, you want to check to see if the formatted version of the parsed Date is equal to the text in the field, to see if the date is actually valid...

SimpleDateFormat simpleDateFormatMonthDayYear = new SimpleDateFormat("M/d/yyyy");
try {
    Date date = simpleDateFormatMonthDayYear.parse(field.getText());

    if (date != null) {

        String formatted = simpleDateFormatMonthDayYear.format(date);
        if (!formatted.equals(field.getText())) {

            System.out.println("Date is not valid");

        }

    } else {

        System.out.println("Error! Please enter a valid start date in the format M/d/yyyy (e.g. 1/2/2015)!");

    }
} catch (ParseException exp) {

    System.out.println("Error! Please enter a valid start date in the format M/d/yyyy (e.g. 1/2/2015)!");

}

But I'd just use JSpinner or JFormattedTextField as these can auto valid the text themselves, but that's just me (or JXDatePicker from SwingLabs SwingX library, because I'm just that lazy)

Answer 2

You did not tell us what tests the method is failing on. My usual method of finding errors in code like this is to test, find a test input that fails, step through it with a debugger and see where it is failing.

But, here is a hint. I note that you are using "M/d/yyyy" for a format. This is problematic because some months have two digits, and some days have two digits. Why not use "MM/dd/YYYY"?