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);
    }
}

How do I get day of week?

By Wayan in Core API, Util Package 0 Comment

In this example we want to create a calendar or date from a year and day of the year. Next we will find out what day-of-week that calendar or date represent. In the code snippet below we are trying to find the 180 day of the year 2021.

Let’s see the example below.

package org.kodejava.util;

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

public class DayOfYearToDayOfWeekExample {
    public static void main(String[] args) {
        // Create a calendar with year and day of year.
        Calendar calendar = Calendar.getInstance();
        calendar.set(Calendar.YEAR, 2021);
        calendar.set(Calendar.DAY_OF_YEAR, 180);

        // See the full information of the calendar object.
        System.out.println(calendar.getTime().toString());

        // Get the weekday and print it
        int weekday = calendar.get(Calendar.DAY_OF_WEEK);
        System.out.println("Weekday: " + weekday);

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

Below is the result of our program.

Tue Jun 29 06:04:21 CST 2021
Weekday: 3
Weekday: Tuesday

How do I convert Array to Collection?

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

import java.util.Arrays;
import java.util.List;

public class ArrayToListExample {
    public static void main(String[] args) {
        // Creates an array of object, in this case we create an
        // Integer array.
        Integer[] numbers = {1, 1, 2, 3, 5, 8, 13, 21, 34, 55};

        // Convert the created array above to collection, in this
        // example we convert it to a List.
        List<Integer> list = Arrays.asList(numbers);

        // We've got a list of our array here and iterate it.
        for (Integer number : list) {
            System.out.print(number + ", ");
        }
    }
}

How do I convert collection to ArrayList?

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

import java.util.ArrayList;
import java.util.LinkedList;

public class CollectionToArrayList {
    public static void main(String[] args) {
        // We create LinkedList collection type at put some values
        // in it. Here we put A, B, C and D letter into it.
        LinkedList<String> linkedList = new LinkedList<>();
        linkedList.push("A");
        linkedList.push("B");
        linkedList.push("C");
        linkedList.push("D");

        // Let say you want to convert it to other type of collection,
        // for instance here we convert it into ArrayList. To do it
        // we can pass the collection created above as a parameter to
        // ArrayList constructor.
        ArrayList<String> arrayList = new ArrayList<>(linkedList);

        // Now we have converted the collection into ArrayList and
        // printed what is inside.
        for (String s : arrayList) {
            System.out.println("s = " + s);
        }
    }
}