Java Strict vs. Lenient Date Parsing: What You Need to Know

Imagine a scenario where you added a date validator. It correctly rejects integer inputs by throwing an exception, but it still accepts valid dates even when they are not in the expected format.

The Problem: A Date Nightmare

Date Format used here is lenient and not strict.

package com;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * A utility class to validate and parse date strings.
 */
public class DateFormatChecker {

 public static void main(String[] args) {
  System.out.println("Date is " + validateDate("123") + "\n");

  System.out.println("Date is " + validateDate("12-12-2025") + "\n");

  System.out.println("Date is " + validateDate("2025-21-12") + "\n");

  System.out.println("Date is " + validateDate("2025-12-24") + "\n");

 }

 private static Date validateDate(String dateStr) {
  if (dateStr != null) {
   try {
    System.out.println("Date Format Set is yyyy-MM-dd");
    SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
    Date date = dateFormat.parse(dateStr);
    System.out.println("Date " + dateStr + " Parsed Successfully");
    return date;
   } catch (ParseException e) {
    System.out.println("Exception -> " + e.getMessage());
   }
  }
  return null;
 }

}

Output:

Date Format is not Strict.

The Hero:

We now use SimpleDateFormat with setLenient(false) to allow only valid dates. This strictly enforces the expected date format, rejecting any other formats.

 dateFormat.setLenient(false); // Strict validation

Always remember to add dateFormat.setLenient(false) when parsing dates for validation to prevent potential bugs in the future.

 /**
  * Validates and parses a given date string.
  * <p>
  * The method only accepts dates in the format {@code yyyy-MM-dd}. If the input
  * does not match the expected format or is invalid, an exception message is
  * printed and {@code null} is returned.
  * </p>
  *
  * @param dateStr the date string to validate
  * @return the parsed {@code Date} object if valid, otherwise {@code null}
  */
 private static Date validateDate(String dateStr) {
  if (dateStr != null) {
   try {
    System.out.println("Date Format Set is yyyy-MM-dd");
    SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
    dateFormat.setLenient(false); // Strict validation
    Date date = dateFormat.parse(dateStr);
    System.out.println("Date " + dateStr + " Parsed Successfully");
    return date;
   } catch (ParseException e) {
    System.out.println("Exception -> " + e.getMessage());
   }
  }
  return null;
 }

Output:

setLenient saved us

Happy Coding :)