June 2003 Technical Tip Processing Dates in Java

Java provides plenty of functionality for processing dates: you just have to know where to look! This month we will look at a simple application which accepts a date from the user, verifies that the user did, in fact, give us a valid date, and then shows that date in a consistent format. First, let's take a quick look at the classes we will be using in this application (comments from the API are shown in italics):

  • java.util.Date - The class Date represents a specific instant in time, with millisecond precision. As of JDK 1.1, the Calendar class should be used to convert between dates and time fields and the DateFormat class should be used to format and parse date strings. The corresponding methods in Date are deprecated. Our objective is to create an instance of a Date.
  • java.text.SimpleDateFormat - A concrete class for formatting and parsing dates in a locale-sensitive manner. This is a subclass of the DateFormat class. We will use SimpleDateFormat for formatting and parsing dates (parsing is converting from a String to a Date.)
  • java.text.ParseException - Signals that an error has been reached unexpectedly while parsing. Thrown when an invalid date is detected. For more information on Exceptions, see our March 2003 article.

Here's our source code, DateDemo.java, with plenty of embedded comments:

// Demo date validation and formatting.
// Written by Bill Qualls.
// (c) 2003 by Caliber Data Training 773.794.1222

import java.io.*;                   // for input from keyboard
import java.util.Date;              // our objective is a Date
import java.text.SimpleDateFormat;  // for parsing and formatting
import java.text.ParseException;    // thrown on invalid date

public class DateDemo
    public static void main(String[] args)
        throws IOException
        // take input from keyboard in this example
        BufferedReader keyboard = new BufferedReader(
            new InputStreamReader(System.in));

        // indicate expected format of dates. Note capital MM.
        SimpleDateFormat df = new SimpleDateFormat("MM/dd/yyyy");

        // new Date() gets today's date. Show in the desired format.
        System.out.println("Today is " + df.format(new Date()) );

        // not lenient, otherwise 02/29/2003 becomes 03/01/2003!

        // get a date from the user, show them the expected format
        System.out.print("Enter a date (" +
            df.toPattern().toLowerCase() + "): ");
        String s = keyboard.readLine();

        // see if it's a valid date; if so then show it
            Date d = df.parse(s);
            System.out.println("Good date! " + df.format(d));
        catch (ParseException pe)
            System.out.println("Invalid date.");
Download source code here.

Sample output follows:

C:\Java\MyJava>java DateDemo
Today is 04/28/2003
Enter a date (mm/dd/yyyy): 02/29/1995
Invalid date.

C:\Java\MyJava>java DateDemo
Today is 04/28/2003
Enter a date (mm/dd/yyyy): 02/29/1996
Good date! 02/29/1996

C:\Java\MyJava>java DateDemo
Today is 04/28/2003
Enter a date (mm/dd/yyyy): 3/1/1996
Good date! 03/01/1996

Caliber Data Training is pleased to offer training in Java programming. We have recently rewritten our Visualage Java course to use WebSphere Studio Application Developer. We have also created a new Intermediate Java Programming course. We hope you will consider us when deciding upon a training provider!

Go to the articles index. Written by Bill Qualls. Copyright 2003 by Caliber Data Training 800.938.1222