How to Check whether the date is valid or invalid?

Question

I’m learning Java on my own and I came across an exercise that I don’t know where to start researching. I found something about datetime, Try, catch. But I couldn’t quite understand why the examples were much more complex than my problem.

Could someone give me some direction on datetime and some method I might know that will help me with this exercise?

My statement from the exercise is:

   //Faça um Programa que peça uma data no formato dd/mm/aaaa 
   //e determine se a mesma é uma data válida.

Answer 1

Complementing the response of Muril, despite validating the format, it turns out that the method passes some dates considered invalid (such as 30 February).

Using ResolverStyle on-mode Strict, you end up forcing Formatter to validate if the date, even having a valid date format, is actually a valid date:

public static void main (String[] args) {

        System.out.println("29/02/2016 eh uma data valida? " + isDateValid("29/02/2016"));
        System.out.println("29/02/2017 eh uma data valida? " + isDateValid("29/02/2017"));
        System.out.println("31/06/2017 eh uma data valida? " + isDateValid("30/01/2017"));
        System.out.println("31/04/2017 eh uma data valida? " + isDateValid("31/04/2017"));

}

public static boolean isDateValid(String strDate) {
    String dateFormat = "dd/MM/uuuu";

    DateTimeFormatter dateTimeFormatter = DateTimeFormatter
    .ofPattern(dateFormat)
    .withResolverStyle(ResolverStyle.STRICT);
    try {
        LocalDate date = LocalDate.parse(strDate, dateTimeFormatter);
        return true;
    } catch (DateTimeParseException e) {
       return false;
    } 
}

Which will result in:

29/02/2016 eh uma data valida? true  //2016 é bissexto, data válida  
29/02/2017 eh uma data valida? false   //inválida  
31/06/2017 eh uma data valida? true    //data válida  
31/04/2017 eh uma data valida? false   //abril só tem 30 dias, data inválida 

See a test on Ideone

This response was based on Soen. In this another question there is an explanation of why the use of u(new year representation) instead of y(year representation of an era) for formatting within the new API.

Answer 2

Using the Java 8 API, you can test with LocalDate and the parse method as follows:

public class DateValidator {

   public boolean isValid(String date) {
      try {
         DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd/MM/yyyy");
         LocalDate d = LocalDate.parse(date, formatter);    
         return true;
      } catch (DateTimeParseException e) {
        return false;
      }   
   }
}

Run the class created via Ideone.