Determine if a String is a valid date before parsing

Question

I have this situation where I am reading about 130K records containing dates stored as String fields. Some records contain blanks (nulls), some contain strings like this: \'

Answer 1

On one hand I see nothing wrong with your use of try/catch for the purpose, it’s the option I would use. On the other hand there are alternatives:

1. Take a taste from the string before deciding how to parse it.
2. Use optional parts of the format pattern string.

For my demonstrations I am using java.time, the modern Java date and time API, because the Date class used in the question was always poorly designed and is now long outdated. For a date without time of day we need a java.time.LocalDate.

try-catch

Using try-catch with java.time looks like this:

    DateTimeFormatter ddmmmuuFormatter = DateTimeFormatter.ofPattern("dd-MMM-uu", Locale.ENGLISH);
    DateTimeFormatter ddmmuuuuFormatter = DateTimeFormatter.ofPattern("dd/MM/uuuu");

    String dateString = "07-Jun-09";

    LocalDate result;
    try {
        result = LocalDate.parse(dateString, ddmmmuuFormatter);
    } catch (DateTimeParseException dtpe) {
        result = LocalDate.parse(dateString, ddmmuuuuFormatter);
    }
    System.out.println("Date: " + result);

Output is:

Date: 2009-06-07

Suppose instead we defined the string as:

    String dateString = "07/06/2009";

Then output is still the same.

Take a taste

If you prefer to avoid the try-catch construct, it’s easy to make a simple check to decide which of the formats your string conforms to. For example:

    if (dateString.contains("-")) {
        result = LocalDate.parse(dateString, ddmmmuuFormatter);
    } else {
        result = LocalDate.parse(dateString, ddmmuuuuFormatter);
    }

The result is the same as before.

Use optional parts in the format pattern string

This is the option I like the least, but it’s short and presented for some measure of completeness.

    DateTimeFormatter dateFormatter
            = DateTimeFormatter.ofPattern("[dd-MMM-uu][dd/MM/uuuu]", Locale.ENGLISH);
    LocalDate result = LocalDate.parse(dateString, dateFormatter);

The square brackets denote optional parts of the format. So Java first tries to parse using dd-MMM-uu. No matter if successful or not it then tries to parse the remainder of the string using dd/MM/uuuu. Given your two formats one of the attempts will succeed, and you have parsed the date. The result is still the same as above.

Link

Oracle tutorial: Date Time explaining how to use java.time.