Date - Learn Java by Examples

How do I convert between old Date and Calendar object with the new Java 8 Date Time?

By Wayan in Java 8, Java Date Time API Last modified: June 21, 2024 0 Comment

In this example we will learn how to convert the old java.util.Date and java.util.Calendar objects to the new Date Time introduced in Java 8. The first method in the code snippet below dateToNewDate() show conversion of java.util.Date while the calendarToNewDate() show the conversion of java.util.Calendar.

The java.util.Date and java.util.Calendar provide a toInstant() method to convert the objects to the new Date Time API class of the java.time.Instant. To convert the old date into the Java 8 LocalDate, LocalTime and LocalDateTime we first can create an instance of ZonedDateTime using the atZone() method of the Instant class.

ZonedDateTime zonedDateTime = instant.atZone(ZoneId.systemDefault());

From an instance of ZonedDateTime class we can call the toLocalDate(), toLocalTime() and toLocalDateTime() to get instance of LocalDate, LocalTime and LocalDateTime.

To convert back from the new Java 8 date to the old java.util.Date we can use the Date.from() static factory method and passing and instance of java.time.Instant that we can obtain by calling the following code.

Instant instant1 = dateTime.atZone(ZoneId.systemDefault()).toInstant();
Date now1 = Date.from(instant1);

Here are the complete code snippet to convert java.util.Date to the new Java 8 Date Time.

private static void dateToNewDate() {
    Date now = new Date();
    Instant instant = now.toInstant();

    ZonedDateTime zonedDateTime = instant.atZone(ZoneId.systemDefault());

    LocalDate date = zonedDateTime.toLocalDate();
    LocalTime time = zonedDateTime.toLocalTime();
    LocalDateTime dateTime = zonedDateTime.toLocalDateTime();

    Instant instant1 = dateTime.atZone(ZoneId.systemDefault()).toInstant();
    Date now1 = Date.from(instant1);

    System.out.println("java.util.Date          = " + now);
    System.out.println("java.time.LocalDate     = " + date);
    System.out.println("java.time.LocalTime     = " + time);
    System.out.println("java.time.LocalDateTime = " + dateTime);
    System.out.println("java.util.Date          = " + now1);
    System.out.println();
}

The steps for converting from the java.util.Calendar to the new Java 8 date can be seen in the code snippet below. As with java.util.Date the Calendar class provide toInstant() method to convert the calendar to java.time.Instant object.

Using the LocalDateTime.ofInstant() method we can create a LocalDateTime object from the instant object. By having the LocalDateTime object we can then get an instance of LocalDate and LocalTime by calling the toLocalDate() and toLocalTime() method.

Finally, to convert back to java.util.Calendar we can use the GregorianCalendar.from() static factory method which require an instance of ZonedDateTime to be passed as a parameter. To get an instance of ZonedDateTime we can call LocalDateTime.atZone() method. You can see the complete code in the code snippet below.

private static void calendarToNewDate() {
    Calendar now = Calendar.getInstance();

    LocalDateTime dateTime = LocalDateTime.ofInstant(now.toInstant(),
            ZoneId.systemDefault());

    LocalDate date = dateTime.toLocalDate();
    LocalTime time = dateTime.toLocalTime();

    ZonedDateTime zonedDateTime = dateTime.atZone(ZoneId.systemDefault());
    Calendar now1 = GregorianCalendar.from(zonedDateTime);

    System.out.println("java.util.Calendar      = " + now);
    System.out.println("java.time.LocalDateTime = " + dateTime);
    System.out.println("java.time.LocalDate     = " + date);
    System.out.println("java.time.LocalTime     = " + time);
    System.out.println("java.util.Calendar      = " + now1);
}

Below is the main Java class to run the code snippet. You must place the above methods inside this class to run the code snippet.

package org.kodejava.datetime;

import java.time.*;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class LegacyDateCalendarToNewDateExample {
    public static void main(String[] args) {
        dateToNewDate();
        calendarToNewDate();
    }
}

Here are the result of the code snippet above. The first group is conversion the java.util.Date to the new Date Time API. The second group is conversion from the java.util.Calendar to the new Date Time API.

java.util.Date          = Tue Nov 16 08:44:51 CST 2021
java.time.LocalDate     = 2021-11-16
java.time.LocalTime     = 08:44:51.031
java.time.LocalDateTime = 2021-11-16T08:44:51.031
java.util.Date          = Tue Nov 16 08:44:51 CST 2021

java.util.Calendar      = java.util.GregorianCalendar[time=1637023491089,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Shanghai",offset=28800000,dstSavings=0,useDaylight=false,transitions=31,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2021,MONTH=10,WEEK_OF_YEAR=47,WEEK_OF_MONTH=3,DAY_OF_MONTH=16,DAY_OF_YEAR=320,DAY_OF_WEEK=3,DAY_OF_WEEK_IN_MONTH=3,AM_PM=0,HOUR=8,HOUR_OF_DAY=8,MINUTE=44,SECOND=51,MILLISECOND=89,ZONE_OFFSET=28800000,DST_OFFSET=0]
java.time.LocalDateTime = 2021-11-16T08:44:51.089
java.time.LocalDate     = 2021-11-16
java.time.LocalTime     = 08:44:51.089
java.util.Calendar      = java.util.GregorianCalendar[time=1637023491089,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Shanghai",offset=28800000,dstSavings=0,useDaylight=false,transitions=31,lastRule=null],firstDayOfWeek=2,minimalDaysInFirstWeek=4,ERA=1,YEAR=2021,MONTH=10,WEEK_OF_YEAR=46,WEEK_OF_MONTH=3,DAY_OF_MONTH=16,DAY_OF_YEAR=320,DAY_OF_WEEK=3,DAY_OF_WEEK_IN_MONTH=3,AM_PM=0,HOUR=8,HOUR_OF_DAY=8,MINUTE=44,SECOND=51,MILLISECOND=89,ZONE_OFFSET=28800000,DST_OFFSET=0]

How do I checks if two dates are on the same day?

In this example, you will learn how to find out if two defined date objects are on the same day. It means that we are only interested in the date information and ignoring the time information of these date objects. We will be using an API provided by the Apache Commons Lang library. So here is the code snippet:

package org.kodejava.commons.lang;

import org.apache.commons.lang3.time.DateUtils;

import java.util.Calendar;
import java.util.Date;

public class CheckSameDay {
    public static void main(String[] args) {
        Date date1 = new Date();
        Date date2 = new Date();

        // Checks to see if the dates is on the same day.
        if (DateUtils.isSameDay(date1, date2)) {
            System.out.printf("%1$te/%1$tm/%1$tY and %2$te/%2$tm/%2$tY " +
                    "is on the same day.%n", date1, date2);
        }

        Calendar cal1 = Calendar.getInstance();
        Calendar cal2 = Calendar.getInstance();

        // Checks to see if the calendars is on the same day.
        if (DateUtils.isSameDay(cal1, cal2)) {
            System.out.printf("%1$te/%1$tm/%1$tY and %2$te/%2$tm/%2$tY " +
                    "is on the same day.%n", cal1, cal2);
        }

        cal2.add(Calendar.DAY_OF_MONTH, 10);
        if (!DateUtils.isSameDay(cal1, cal2)) {
            System.out.printf("%1$te/%1$tm/%1$tY and %2$te/%2$tm/%2$tY " +
                    "is not on the same day.", cal1, cal2);
        }
    }
}

The example results produced by this snippet are:

31/10/2021 and 31/10/2021 is on the same day.
31/10/2021 and 31/10/2021 is on the same day.
31/10/2021 and 10/11/2021 is not on the same day.

Maven Dependencies

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.14.0</version>
</dependency>

How do I convert string to Date in GMT timezone?

By Wayan in Core API, Text Package Last modified: June 2, 2023 0 Comment

The following code snippet convert a string representation of a date into a java.util.Date object and the timezone is set to GMT. To parse the string so that the result is in GMT you must set the TimeZone of the DateFormat object into GMT.

package org.kodejava.joda;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;

public class WithTimezoneStringToDate {
    public static void main(String[] args) {
        // Create a DateFormat and set the timezone to GMT.
        DateFormat df = new SimpleDateFormat("E, dd MMM yyyy HH:mm:ss z");
        df.setTimeZone(TimeZone.getTimeZone("GMT"));

        try {
            // Convert string into Date
            Date today = df.parse("Fri, 29 Oct 2021 00:00:00 GMT+08:00");
            System.out.println("Today = " + df.format(today));
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

The code snippet above print the following output:

Today = Thu, 28 Oct 2021 16:00:00 GMT

How do I get pattern string of a SimpleDateFormat?

By Wayan in Core API, Text Package Last modified: June 1, 2023 0 Comment

To format a java.util.Date object we use the SimpleDateFormat class. To get back the string pattern that were used to format the date we can use the toPattern() method of this class.

package org.kodejava.text;

import java.text.SimpleDateFormat;
import java.util.Date;

public class SimpleDateFormatToPattern {
    public static void main(String[] args) {
        SimpleDateFormat format = new SimpleDateFormat("EEEE, dd/MM/yyyy");

        // Gets a pattern string describing this date format used by the
        // SimpleDateFormat object.
        String pattern = format.toPattern();

        System.out.println("Pattern = " + pattern);
        System.out.println("Date    = " + format.format(new Date()));
    }
}

The result of the program will be as follow:

Pattern = EEEE, dd/MM/yyyy
Date    = Tuesday, 26/10/2021

How do I create JSpinner component with date value?

By Wayan in Swing Last modified: May 24, 2023 0 Comment

The SpinnerDateModel allow us to display and select date information from the JSpinner component. By default, the initial value of the model will be set to the current date. To change it we can call the setValue method of the JSpinner object.

package org.kodejava.swing;

import javax.swing.*;
import java.awt.*;
import java.util.GregorianCalendar;
import java.util.Calendar;

public class JSpinnerDate extends JFrame {
    public JSpinnerDate() {
        initializeUI();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(
                () -> new JSpinnerDate().setVisible(true));
    }

    private void initializeUI() {
        setSize(500, 500);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setLayout(new BorderLayout());

        // Create a SpinnerDateModel with current date as the initial value.
        SpinnerDateModel model = new SpinnerDateModel();

        // Set the spinner value to October 9, 2021.
        JSpinner spinner = new JSpinner(model);
        Calendar calendar = new GregorianCalendar(2021, Calendar.OCTOBER, 9);
        spinner.setValue(calendar.getTime());

        getContentPane().add(spinner, BorderLayout.NORTH);
    }
}