解析方法未正确处理格式

Question

My code is supposed to print out the difference between the entered date and now. Then the output will be formatted according to the ISO LocalDate format.

Parse method not handling formatting correctly.

import java.time.Period;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;
import java.io.*;
import java.time.*;

//Main class
public class TimeBetween{
public static void main(String[] args){
    String dateFormat = "MMMM d, yyyy";
    LocalDate aDate = null;
    boolean validStr = false;

 //parse method not handling formatting correctly.
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern(dateFormat);
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    while(validStr==false){
        System.out.print("Enter a date: ");
//Try and catch 
        try {
            String dateEntered = br.readLine();
            aDate = LocalDate.parse(dateEntered,formatter);
            validStr = true;
        }catch(IOException | DateTimeParseException ex){
            validStr = false;
        }

    }

    System.out.println("Date entered was: "+aDate);
    LocalDate now = LocalDate.now(); 

    Period between;
    if(aDate.isBefore(now)){
        between = Period.between(aDate,now);
    }else{
        between = Period.between(now,aDate);
    }

    int years = between.getYears();
    int months = between.getMonths();
    int days =  between.getDays();

//Printing out year month and day differences
    System.out.println("There are"    +years+"years"+months+"months"+days+"days");
    }
}

Answer 1

Enter month in the case used in your locale

As your program stands in the question the user is required to enter the month in the expected case, for example uppercase F and lowercase ebruary for February. (Edit:) Pattern letter uppercase M is used for month while lowercase m means minutes. Therefore the M s in the format pattern string have to be in uppercase even though you are not supposed to type the month that way, and it also isn't formatted in uppercase if you format a date back into a string.

Tip: When parsing using a formatter won't work, try formatting a value with the same formatter to see what it expects your input string to look like. For example:

    DateTimeFormatter formatter
            = DateTimeFormatter.ofPattern(dateFormat, Locale.forLanguageTag("en-IE"));
    System.out.println(LocalDate.now(ZoneId.of("Europe/Dublin")).format(formatter));

Example output:

October 17, 2018

Here you can see that the month is not in uppercase.

Tip 2: give explicit locale for your formatter so you know it will also work as expected on computers with a different default locale and if some day you happen to play with the locale setting of your own computer.

To accept month in uppercase

I don't think it was what you wanted, but for the sake of completeness: To allow the month to be entered in all uppercase letters use a DateTimeFormatterBuilder and its parseCaseInsensitive method like this:

    DateTimeFormatter formatter = new DateTimeFormatterBuilder()
            .parseCaseInsensitive()
            .appendPattern(dateFormat)
            .toFormatter(Locale.forLanguageTag("en-IE"));

With this change here's a sample session:

Enter a date: FEBRUARY 27, 1999
Date entered was: 1999-02-27
There are19years7months19days

The date is output in ISO 8601 format for a calendar date. And I believe the calculation of the period is correct.

Link

DateTimeFormatter documentation where you can see the uppercase and lowercase pattern letters and study the difference between m and M , d and D and y and Y .