来自 DAY、MONTH、YEAR 的日期对象

Question

I am trying to create a method which converts DAY, MONTH, YEAR to Date object in Java

I have

String DAY = "31"
String YEAR = "2012"
String MONTH = "11"

I need a configurable Format as output.

String format = "MM/dd/yyy";

I will always get DAY string from "01" to "31" only
I will always get YEAR string from "1000" to "9999" only
I will always get MONTH string from "01" to "12" only (Never get Jan, Feb etc)

I understand that SimpleDateFormat(format) can work up to some extend.

new SimpleDateFormat("MM/dd/yyyy") will parse "02/01/2010" as Feb 01 2010 but
new SimpleDateFormat("mm/dd/yyyy") will parse "02/01/2010" as Jan 01 2010

Is it possible to write a generic method in java which converts given Strings (DAY, MONTH, YEAR) to Date object based on a configurable pattern?? and which can throw exception is I supply a wrong combination of DAY, MONTH, YEAR (like DAY = "31", MONTH = "02", YEAR = "2010")

Something like :

Date dateParser(String format){
  
  String DAY = "01";
  String YEAR = "1922";
  String MONTH = "02";
  
  
  return date;
}

Answer 1

There are basically two ways:

1. Parse each string to a number and put the numbers together to a date.
2. Put the strings together to one string and parse it into a date.

Some middle forms are thinkable, but I recommend you take a pure stance and use one of the ways only. The answer by Arvind Kumar Avinash shows option 1. My taste is rather for option 2., so let me demonstrate.

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

    String format = "MM/dd/yyy";
    
    String day = "31";
    String year = "2012";
    String month = "11";
    
    DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern(format, Locale.US);
    
    String isoDateString = year + '-' + month + '-' + day;
    LocalDate date = LocalDate.parse(isoDateString);
    
    String formattedDate = date.format(dateFormatter);
    System.out.println(formattedDate);

Since there are only 30 days in November (month 11), this code very sensibly throws an exception:

Exception in thread "main" java.time.format.DateTimeParseException: Text '2012-11-31' could not be parsed: Invalid date 'NOVEMBER 31'

That is, we have got input validation for free. For a valid date the code will parse the date and format as requested.

Link

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

Answer 2

I recommend you switch from the outdated and error-prone java.util date-time API and SimpleDateFormat to the modern java.time date-time API and the corresponding formatting API (package, java.time.format ). Learn more about the modern date-time API from Trail: Date Time .

Note that mm is used for minute ; not for month . For month , you use MM . Check this for more information.

Using the modern date-time API:

import java.time.LocalDate;

public class Main {
    public static void main(String[] args) {
        // Test
        System.out.println(getLocalDate("2010", "02", "28"));
        System.out.println(getLocalDate("2010", "02", "31"));
    }

    static LocalDate getLocalDate(String year, String month, String dayOfMonth) {
        return LocalDate.of(Integer.parseInt(year), Integer.parseInt(month), Integer.parseInt(dayOfMonth));
    }
}

Output:

2010-02-28
Exception in thread "main" java.time.DateTimeException: Invalid date 'FEBRUARY 31'
    at java.base/java.time.LocalDate.create(LocalDate.java:459)
    at java.base/java.time.LocalDate.of(LocalDate.java:271)
    at Main.getLocalDate(Main.java:11)
    at Main.main(Main.java:7)

Note that a date-time object is supposed to store the information about date, time, time-zone etc. but not about the formatting. When you print an object of a date-time type, its is supposed to print what its toString method returns. If you need to print the date-time in a different format, you need a formatting class (eg DateTimeFormatter ) which can return you a string in your desired pattern eg

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

public class Main {
    public static void main(String[] args) {
        // Test
        System.out.println(getFormattedLocalDate("2010/02/28", "yyyy/MM/dd", "EEEE dd MMMM yyyy"));
        System.out.println(getFormattedLocalDate("28/02/2010", "dd/MM/yyyy", "yyyy MMM EEE dd"));
    }

    static String getFormattedLocalDate(String date, String inputPattern, String outputPattern) {
        return LocalDate.parse(date, DateTimeFormatter.ofPattern(inputPattern))
                .format(DateTimeFormatter.ofPattern(outputPattern));
    }
}

Output:

Sunday 28 February 2010
2010 Feb Sun 28