After i added throws ParseException, the parse excception still occured, why?

Question

The question is to calculate the days between two dates. Here's my code

public static void main(String[] args) throws ParseException  {
    // TODO Auto-generated method stub
    SimpleDateFormat sdf=new SimpleDateFormat("yyyy-mm-dd)");
    Scanner input=new Scanner(System.in);
    String date1=input.nextLine();
    String date2=input.nextLine();
    Date d1 =sdf.parse(date1);
    Date d2= sdf.parse(date2);
    if(d1.getTime()>d2.getTime()) {
        System.out.println(d2.getTime()-d1.getTime()/(24*60*60*1000));
    }
    else
        System.out.println(d1.getTime()-d2.getTime()/(24*60*60*1000));

and output is

2010/05/29
2010/01/01
Exception in thread "main" java.text.ParseException: Unparseable date: "2010/05/29"
        at java.base/java.text.DateFormat.parse(DateFormat.java:396)
        at HW10_09156136_1092.HW10_Ex01.main(HW10_Ex01.java:15)

Answer 1

You have used m [Minute in hour] at the place of M [Month in year].

Your input has / as the separator whereas you have specified - for it in the parser. The input should match the pattern specified with the parser.

Apart from this, the java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone.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^*.

Demo using java.time, the modern Date-Time API:

import java.time.LocalDate;
import java.time.temporal.ChronoUnit;

public class Main {
    public static void main(String[] args) {
        String strDateStart = "2021-06-09";
        String strDateEnd = "2021-06-12";
        LocalDate dateStart = LocalDate.parse(strDateStart);
        LocalDate dateEnd = LocalDate.parse(strDateEnd);
        long days = ChronoUnit.DAYS.between(dateStart, dateEnd);
        System.out.println(days);
    }
}

Output:

3

ONLINE DEMO

The modern Date-Time API is based on ISO 8601 and does not require using a DateTimeFormatter object explicitly as long as the Date-Time string conforms to the ISO 8601 standards.

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

Using the legacy API:

Avoid performing calculations yourself if there already exists an API for the same e.g. the following code uses Math#abs and TimeUnit#convert to avoid error-prone if-else and calculations.

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;
import java.util.concurrent.TimeUnit;

public class Main {
    public static void main(String[] args) throws ParseException {
        String strDateStart = "2021-06-09";
        String strDateEnd = "2021-06-12";
        
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd", Locale.ENGLISH);
        Date dateStart = sdf.parse(strDateStart);
        Date dateEnd = sdf.parse(strDateEnd);
        
        long millisInOneDay = TimeUnit.MILLISECONDS.convert(1, TimeUnit.DAYS);
        long days = Math.abs((dateEnd.getTime() - dateStart.getTime()) / millisInOneDay);
        
        System.out.println(days);
    }
}

ONLINE DEMO

After i added throws ParseException, the parse excception still occured, why?

Adding the throws clause to a method forces the caller of the method to either handle the exception using try-catch or rethrow it. It is not a way to prevent the exception from being thrown. Learn more about it from this excellent tutorial from Oracle.

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^{* 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.}