How to Validate a Date String Format in Java without Parsing to a Date Object?

Java: Checking the Date Format of a String against a Specified Requirement

Question:

How can I determine if a given string follows a specific date format in Java? I want to avoid converting the string into a date object and focus solely on validating its format.

Answer:

Java 8 :

Utilize the LocalDateTime, LocalDate, and LocalTime classes from the Java 8 date and time API for comprehensive format validation.

Code:

<code class="java">public static boolean isValidFormat(String format, String value, Locale locale) {
    LocalDateTime ldt = null;
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern(format, locale);

    try {
        ldt = LocalDateTime.parse(value, formatter);
        String result = ldt.format(formatter);
        return result.equals(value);
    } catch (DateTimeParseException e) {
        try {
            LocalDate ld = LocalDate.parse(value, formatter);
            String result = ld.format(formatter);
            return result.equals(value);
        } catch (DateTimeParseException exp) {
            try {
                LocalTime lt = LocalTime.parse(value, formatter);
                String result = lt.format(formatter);
                return result.equals(value);
            } catch (DateTimeParseException e2) {
                // Handle exceptions for debugging purposes
            }
        }
    }

    return false;
}</code>

Usage Example:

<code class="java">System.out.println("isValid - dd/MM/yyyy with 20130925 = " + isValidFormat("dd/MM/yyyy", "20130925", Locale.ENGLISH));
System.out.println("isValid - dd/MM/yyyy with 25/09/2013 = " + isValidFormat("dd/MM/yyyy", "25/09/2013", Locale.ENGLISH));
System.out.println("isValid - dd/MM/yyyy with 25/09/2013 12:13:50 = " + isValidFormat("dd/MM/yyyy", "25/09/2013  12:13:50", Locale.ENGLISH));
System.out.println("isValid - yyyy-MM-dd with 2017-18--15 = " + isValidFormat("yyyy-MM-dd", "2017-18--15", Locale.ENGLISH));</code>

Output:

isValid - dd/MM/yyyy with 20130925 = false
isValid - dd/MM/yyyy with 25/09/2013 = true
isValid - dd/MM/yyyy with 25/09/2013 12:13:50 = false
isValid - yyyy-MM-dd with 2017-18--15 = false

Pre-Java 8:

For earlier versions of Java, use SimpleDateFormat to validate the format.

Code:

<code class="java">public static boolean isValidFormat(String format, String value) {
    Date date = null;
    try {
        SimpleDateFormat sdf = new SimpleDateFormat(format);
        date = sdf.parse(value);
        if (!value.equals(sdf.format(date))) {
            date = null;
        }
    } catch (ParseException ex) {
        ex.printStackTrace();
    }
    return date != null;
}</code>

Usage Example and Output:

<code class="java">System.out.println("isValid - dd/MM/yyyy with 20130925 = " + isValidFormat("dd/MM/yyyy", "20130925"));
System.out.println("isValid - dd/MM/yyyy with 25/09/2013 = " + isValidFormat("dd/MM/yyyy", "25/09/2013"));
System.out.println("isValid - dd/MM/yyyy with 25/09/2013 12:13:50 = " + isValidFormat("dd/MM/yyyy", "25/09/2013  12:13:50"));</code>

isValid - dd/MM/yyyy with 20130925 = false
isValid - dd/MM/yyyy with 25/09/2013 = true
isValid - dd/MM/yyyy with 25/09/2013 12:13:50 = false

Both solutions return true if the string matches the specified format and false otherwise, providing a reliable method for validating date formats in Java.

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