How to make a program that reads a date and calculate the number of days left until the end of year?

Question

i have a really simple problem, i don't know how to deduct the user's date by 01/01 / (the user year) +1. Im really stuck at this point.

public static void main(String[] args)
{
    String date;
    Scanner teclado = new Scanner (System.in);
    System.out.println("Dame una fecha formato dd/mm/yyyy");
    date=teclado.next();
    Date mydate =FinalAnio.ParseFecha(date);    
    System.out.println(mydate);
    
    
}

 public static Date ParseFecha(String fecha)
    {
        SimpleDateFormat formato = new SimpleDateFormat("dd/mm/yyyy");
        Date fechaDate = null;
        try 
        {
            fechaDate = formato.parse(fecha);
        } 
        catch (ParseException ex) 
        {
            System.out.println(ex);
        }
        return fechaDate;
    }

Answer 1

1. The date-time API of java.util and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern date-time API.

   • For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7.
   • If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.

2. Do not use mm for the month as it is used for the minute. For the month, the correct symbol is MM. Check DateTimeFormatter to learn more about various symbols used for parsing/formatting string/date-time.

3. Learn about the calculations of the period and duration from Period and Duration tutorial from Oracle. It would also be worth going through this Wikipedia page on Durations.

Demo:

import java.time.LocalDate;
import java.time.Period;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoUnit;
import java.util.Locale;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter date in the format dd/MM/yyyy: ");
        String strDate = scanner.next();
        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("dd/MM/uuuu", Locale.ENGLISH);
        LocalDate userDate = LocalDate.parse(strDate, dtf);

        // The date representing 01/01/(the user year)+1
        LocalDate targetDate = userDate.withDayOfMonth(1).withMonth(1).plusYears(1);

        System.out.println("User's date: " + strDate);
        System.out.println("Target date: " + targetDate.format(dtf));

        Period period = Period.between(userDate, targetDate);
        System.out.printf("Difference: %d days %d months %d years%n", period.getDays(), period.getMonths(),
                period.getYears());

        System.out.println("The difference in terms of days: " + ChronoUnit.DAYS.between(userDate, targetDate));
    }
}

A sample run:

Enter date in the format dd/MM/yyyy: 20/10/2015
User's date: 20/10/2015
Target date: 01/01/2016
Difference: 12 days 2 months 0 years
The difference in terms of days: 73

Learn about the modern date-time API from Trail: Date Time.

Answer 2

java.time

I recommend that you use java.time, the modern Java date and time API, for your date work.

    DateTimeFormatter formatador = DateTimeFormatter.ofPattern("dd/MM/uuuu");
    String entradaUsuario = "02/12/2020";
    LocalDate fecha = LocalDate.parse(entradaUsuario, formatador);
    LocalDate finDeAño = fecha.with(MonthDay.of(Month.DECEMBER, 31));
    long diasRestantes = ChronoUnit.DAYS.between(fecha, finDeAño);
    System.out.println(diasRestantes);

Output is:

29

In the format pattern string upper case MM is for month of year (lower case mm would be minute of hour, so not useful here). uuuu is for year (yyyy would work too).

fecha.with(MonthDay.of(Month.DECEMBER, 31)) adjusts the date to December 31 in the same year.

Link

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