How do I compare two dates?

By Wayan in Core API, Util Package 0 Comment

In this example you’ll see the utilization of SimpleDateFormat class for comparing two dates for equality. We convert the date object into string using the DateFormat instance and then compare it using String.equals() method.

package org.kodejava.util;

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

public class ComparingDate {
    public static void main(String[] args) {
        // Create a SimpleDateFormat instance with dd/MM/yyyy format
        DateFormat df = new SimpleDateFormat("dd/MM/yyyy");

        // Get the current date
        Date today = new Date();

        // Create an instance of calendar that represents a new year date
        Calendar calendar = Calendar.getInstance();
        calendar.set(Calendar.DATE, 1);
        calendar.set(Calendar.MONTH, Calendar.JANUARY);
        calendar.set(Calendar.YEAR, 2021);

        String newYear = df.format(calendar.getTime());
        String now = df.format(today);

        // Using the string equals method we can compare the date.
        if (now.equals(newYear)) {
            System.out.println("Happy New Year!");
        } else {
            System.out.println("Have a nice day!");
        }
    }
}

How do I convert milliseconds value to date?

By Wayan in Core API, Util Package 0 Comment 
package org.kodejava.util;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;

public class MillisecondsToDate {
    public static void main(String[] args) {
        // Create a DateFormatter object for displaying date information.
        DateFormat formatter = new SimpleDateFormat("dd/MM/yyyy hh:mm:ss.SSS");

        // Get date and time information in milliseconds
        long now = System.currentTimeMillis();

        // Create a calendar object that will convert the date and time value
        // in milliseconds to date. We use the setTimeInMillis() method of the
        // Calendar object.
        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(now);

        System.out.println(now + " = " + formatter.format(calendar.getTime()));
    }
}

The output of the code snippet above is:

1632868662762 = 29/09/2021 06:37:42.762

How do I convert time between timezone?

By Wayan in Core API, Util Package 0 Comment 
package org.kodejava.util;

import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.TimeZone;

public class TimeZoneExample {
    public static void main(String[] args) {
        // Create a calendar object and set it time based on the local
        // time zone
        Calendar localTime = Calendar.getInstance();
        localTime.set(Calendar.HOUR, 17);
        localTime.set(Calendar.MINUTE, 15);
        localTime.set(Calendar.SECOND, 20);

        int hour = localTime.get(Calendar.HOUR);
        int minute = localTime.get(Calendar.MINUTE);
        int second = localTime.get(Calendar.SECOND);

        // Print the local time
        System.out.printf("Local time  : %02d:%02d:%02d\n", hour, minute, second);

        // Create a calendar object for representing a Germany time zone. Then we
        // wet the time of the calendar with the value of the local time
        Calendar germanyTime = new GregorianCalendar(TimeZone.getTimeZone("Germany"));
        germanyTime.setTimeInMillis(localTime.getTimeInMillis());
        hour = germanyTime.get(Calendar.HOUR);
        minute = germanyTime.get(Calendar.MINUTE);
        second = germanyTime.get(Calendar.SECOND);

        // Print the local time in Germany time zone
        System.out.printf("Germany time: %02d:%02d:%02dn", hour, minute, second);
    }
}

How do I get the last day of a month?

By Wayan in Core API, Util Package 2 Comments 
package org.kodejava.util;

import java.text.DateFormatSymbols;
import java.util.Calendar;

public class LastDayOfMonth {
    public static void main(String[] args) {
        // Get a calendar instance
        Calendar calendar = Calendar.getInstance();

        // Get the last date of the current month. To get the last date for a
        // specific month you can set the calendar month using calendar object
        // calendar.set(Calendar.MONTH, theMonth) method.
        int lastDate = calendar.getActualMaximum(Calendar.DATE);

        // Set the calendar date to the last date of the month so then we can
        // get the last day of the month
        calendar.set(Calendar.DATE, lastDate);
        int lastDay = calendar.get(Calendar.DAY_OF_WEEK);

        // Print the current date and the last date of the month
        System.out.println("Last Date: " + calendar.getTime());

        // The lastDay will be in a value from 1 to 7 where 1 = Sunday and 7 =
        // Saturday. The first day of the week is based on the locale.
        System.out.println("Last Day : " + lastDay);

        // Get weekday name
        DateFormatSymbols dfs = new DateFormatSymbols();
        System.out.println("Last Day : " + dfs.getWeekdays()[lastDay]);
    }
}

Here is the output of the code snippet above:

Last Date: Thu Sep 30 06:44:05 CST 2021
Last Day : 5
Last Day : Thursday

How do I get the last date of a month?

By Wayan in Core API, Util Package 0 Comment

You want to know what is the last date for a current month. The code below show you how to get it.

package org.kodejava.util;

import java.util.Calendar;

public class LastDateOfMonth {
    public static void main(String[] args) {
        // Get a calendar instance
        Calendar calendar = Calendar.getInstance();

        // Get the last date of the current month. To get the last date for 
        // a specific month you can set the calendar month using calendar 
        // object calendar.set(Calendar.MONTH, theMonth) method.
        int lastDate = calendar.getActualMaximum(Calendar.DATE);

        // Print the current date and the last date of the month
        System.out.println("Date     : " + calendar.getTime());
        System.out.println("Last Date: " + lastDate);
    }
}

Here is the result of the code snippet:

Date     : Fri Sep 24 06:42:18 CST 2021
Last Date: 30

How do I convert day-of-the-year to date?

By Wayan in Core API, Util Package 4 Comments

In the following example we want to get the date of the specified day-of-the-year. We can define a calendar for a specific day of the year by setting the java.util.Calendar object DAY_OF_YEAR field using the set() method. The method take the field to be set and a value.

package org.kodejava.util;

import java.util.Calendar;

public class DayOfYearToDate {
    public static void main(String[] args) {
        // In the example we want to get the date value of the specified
        // day of the year. Using the calendar object we can define our
        // calendar for a specific day of the year.
        int dayOfYear = 112;
        Calendar calendar = Calendar.getInstance();
        calendar.set(Calendar.DAY_OF_YEAR, dayOfYear);
        System.out.println("Day " + dayOfYear + " of the current year = "
                + calendar.getTime());

        // If you want to get the date for a specific day of year and for
        // a specific year, you can also pass the year information to the
        // calendar object.
        int year = 2020;
        calendar.set(Calendar.YEAR, year);
        calendar.set(Calendar.DAY_OF_YEAR, dayOfYear);
        System.out.println("Day " + dayOfYear + " in year " + year
                + " = " + calendar.getTime());
    }
}

And here is an example result of the code above:

Day 112 of the current year = Thu Apr 22 06:34:14 CST 2021
Day 112 in year 2020 = Tue Apr 21 06:34:14 CST 2020

How do I convert day-of-year to day-of-month?

By Wayan in Core API, Util Package 0 Comment 
package org.kodejava.util;

import java.util.Calendar;

public class DayYearToDayMonth {
    public static void main(String[] args) {
        // Create an instance of calendar for the year 2017 and set the
        // day to the 180 day of the year.
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, 2021);
        cal.set(Calendar.DAY_OF_YEAR, 180);

        // Print the date of the calendar.
        System.out.println("Calendar date is: " + cal.getTime());

        // To know what day in month of the calendar we can obtain the
        // value by calling Calendar's instance get() method and pass
        // the Calendar.DAY_OF_MONTH field.
        int dayOfMonth = cal.get(Calendar.DAY_OF_MONTH);

        // Print which month day is it in number.
        System.out.println("Calendar day of month: " + dayOfMonth);

        // To know what day in week of the calendar we can obtain the
        // value by calling Calendar's instance get() method and pass
        // the Calendar.DAY_OF_WEEK field.
        int dayOfWeek = cal.get(Calendar.DAY_OF_WEEK);

        // Print which week day is it in number.
        System.out.println("Calendar day of week: " + dayOfWeek);
    }
}

The result of our above example is.

Calendar date is: Tue Jun 29 08:14:06 CST 2021
Calendar day of month: 29
Calendar day of week: 3

How do I check if a year is a leap year?

By Wayan in Core API, Util Package 0 Comment

The following example using the GregorianCalendar.isLeapYear() method to check if the specified year is a leap year.

package org.kodejava.util;

import java.util.GregorianCalendar;

public class LeapYearExample {
    public static void main(String[] args) {
        // Here we show how to know if a specified year is a leap year or 
        // not. The GregorianCalendar object provide a convenient method 
        // to do this. The method is GregorianCalendar.isLeapYear().

        // First, let's obtain an instance of GregorianCalendar.
        GregorianCalendar cal = new GregorianCalendar();

        // The isLeapYear(int year) method will return true for leap 
        // year and otherwise return false. In this example the message 
        // will be printed as 2020 is a leap year.
        if (cal.isLeapYear(2020)) {
            System.out.println("The year 2020 is a leap year!");
        }
    }
}

The result of our code is:

The year 2020 is a leap year!

Another code for checking leap year can be seen in the following example How do I know if a given year is a leap year?.

How do I calculate difference in days between two dates?

By Wayan in Core API, Util Package 3 Comments

In the following example we are going to calculate the differences between two dates. First, we will need to convert the date into the corresponding value in milliseconds and then do the math, so we can get the difference in days, hours, minutes, etc.

For example to get the difference in day we will need to divide the difference in milliseconds with (24 * 60 * 60 * 1000).

package org.kodejava.util;

import java.util.Calendar;

public class DateDifferenceExample {
    public static void main(String[] args) {
        // Creates two calendars instances
        Calendar cal1 = Calendar.getInstance();
        Calendar cal2 = Calendar.getInstance();

        // Set the date for both of the calendar instance
        cal1.set(2020, Calendar.DECEMBER, 30);
        cal2.set(2021, Calendar.SEPTEMBER, 21);

        // Get the represented date in milliseconds
        long millis1 = cal1.getTimeInMillis();
        long millis2 = cal2.getTimeInMillis();

        // Calculate difference in milliseconds
        long diff = millis2 - millis1;

        // Calculate difference in seconds
        long diffSeconds = diff / 1000;

        // Calculate difference in minutes
        long diffMinutes = diff / (60 * 1000);

        // Calculate difference in hours
        long diffHours = diff / (60 * 60 * 1000);

        // Calculate difference in days
        long diffDays = diff / (24 * 60 * 60 * 1000);

        System.out.println("In milliseconds: " + diff + " milliseconds.");
        System.out.println("In seconds: " + diffSeconds + " seconds.");
        System.out.println("In minutes: " + diffMinutes + " minutes.");
        System.out.println("In hours: " + diffHours + " hours.");
        System.out.println("In days: " + diffDays + " days.");
    }
}

Here is the result:

In milliseconds: 22896000016 milliseconds.
In seconds: 22896000 seconds.
In minutes: 381600 minutes.
In hours: 6360 hours.
In days: 265 days.

The better way to get number of days between two dates is to use the Joda Time API. In the following example you can see how to calculate date difference using Joda Time: How do I get number of days between two dates in Joda Time?.

How do I convert Date to String?

By Wayan in Core API, Text Package 1 Comment 
package org.kodejava.text;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.Calendar;
import java.util.Date;

public class DateToString {
    public static void main(String[] args) {
        // Create an instance of SimpleDateFormat used for formatting
        // the string representation of date (day/month/year)
        String pattern = "dd/MM/yyyy";
        DateFormat df = new SimpleDateFormat(pattern);

        // Get the date today using Calendar object.
        Date today = Calendar.getInstance().getTime();

        // Using DateFormat format method we can create a string
        // representation of a date with the defined format.
        String reportDate = df.format(today);

        // Print what date is today!
        System.out.println("Report Date: " + reportDate);

        // Using Java 8.
        // Creates a DateTimeFormatter using the ofPattern() method. Get
        // the current date by calling the .now() method of LocalDate.
        // To convert to string use the format() method of the LocalDate
        // and pass the formatter object as argument.
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern(pattern);
        LocalDate now = LocalDate.now();
        reportDate = now.format(formatter);
        System.out.println("Report Date: " + reportDate);
    }
}