How do I get operating system temporary directory / folder?

By I Wayan Saryada in Core API, Lang Package 1 Comment

To get the location of temporary directory we can use the System.getProperty(String) method and pass a property key "java.io.tmpdir" as the argument to the method.

package org.kodejava.lang;

public class TempDirExample {
    public static void main(String[] args) {
        // This is the property name for accessing OS temporary directory
        // or folder.
        String property = "java.io.tmpdir";

        // Get the temporary directory and print it.
        String tempDir = System.getProperty(property);
        System.out.println("OS temporary directory is " + tempDir);
    }
}

The output of code snippet above is:

OS temporary directory is C:\Users\wsaryada\AppData\Local\Temp\

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

By I Wayan Saryada 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 I Wayan Saryada 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 I Wayan Saryada 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 I Wayan Saryada 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);
    }
}