Can Java Parse Dates as Flexibly as Python's dateutil?

Parse Any Date in Java

Question:

Despite its acclaim, Joda time requires users to manually specify date formats before parsing. Is there a Java library that can parse a wide range of date formats effortlessly, similar to python-dateutil?

Answer:

While Python's dateutil library excels in flexible date parsing, achieving comparable results in Java requires a pragmatic approach:

1. Regular Expression Matching:

Regular expressions provide a powerful means to identify date format patterns. By mapping common date formats to their corresponding regular expressions, it's possible to determine the correct format for a given date string.

private static final Map<String, String> DATE_FORMAT_REGEXPS = new HashMap<>() {{
    put("^\d{8}$", "yyyyMMdd");
    put("^\d{1,2}-\d{1,2}-\d{4}$", "dd-MM-yyyy");
    put("^\d{4}-\d{1,2}-\d{1,2}$", "yyyy-MM-dd");
    // … (other format patterns)
}};

public static String determineDateFormat(String dateString) {
    for (String regexp : DATE_FORMAT_REGEXPS.keySet()) {
        if (dateString.toLowerCase().matches(regexp)) {
            return DATE_FORMAT_REGEXPS.get(regexp);
        }
    }
    return null; // Unknown format.
}

2. Brute Forcing:

If the date format cannot be determined via regex, brute force can be employed as a last resort. Iterate through a collection of common date formats and attempt to parse the date string with each one.

public static Date parseDate(String dateString) {
    String dateFormat = determineDateFormat(dateString);
    if (dateFormat != null) {
        return new SimpleDateFormat(dateFormat).parse(dateString);
    }
    
    // Brute force using common formats
    for (String format : COMMON_DATE_FORMATS) {
        try {
            return new SimpleDateFormat(format).parse(dateString);
        } catch (ParseException e) {
            // Ignore parsing error
        }
    }
    
    throw new IllegalArgumentException("Invalid date format: " + dateString);
}

By combining regex matching and brute force, it's possible to parse a wide range of date formats in Java, albeit with slightly more effort than Python's dateutil library.

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