How do I get day of week?

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

How do I get number of days in a month?

By I Wayan Saryada in Core API, Util Package 3 Comments

Let say you want to know the number of days in a month, or we can say it as the last date of a month. The example below shows you how to obtain the number of days or the date.

package org.kodejava.util;

import java.util.Calendar;

public class MonthDaysExample {
    public static void main(String[] args) {
        // First get an instance of calendar object.
        Calendar calendar = Calendar.getInstance();

        // We'll set the date of the calendar to the following
        // date. We can use constant variable in the calendar
        // for months value (JANUARY - DECEMBER). Be informed that
        // month in Java started from 0 instead of 1.
        int year = 2021;
        int month = Calendar.FEBRUARY;
        int date = 1;
        // We have a new date of 2021-02-01
        calendar.set(year, month, date);

        // Here we get the maximum days for the date specified
        // in the calendar. In this case we want to get the number
        // of days for february 2021
        int maxDay = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
        System.out.println("Max Day: " + maxDay);

        // Here we want to see what is the days for february on
        // a leap year.
        calendar.set(2020, Calendar.FEBRUARY, 1);
        maxDay = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
        System.out.println("Max Day: " + maxDay);
    }
}

The result of the code snippet above:

Max Day: 28
Max Day: 29

In JDK 8 you can use the new Date Time API to get the number of days in a month. Here an example that show you how to do it: How do I get the length of month represented by a date object?.

How do I create random number?

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

The java.lang.Math.random() method returns random number between 0.0 and 1.0 including 0.0 but not including 1.0. By multiplying Math.random() result with a number, for example 10 will give us a range of random number between 0.0 and 10.0.

To get a random number between two numbers (n and m) we can use the formula of: n + (Math.random() * (m - n)). Where n is the lowest number (inclusive) and m is the highest number (exclusive).

package org.kodejava.lang;

public class RandomNumberExample {
    public static void main(String[] args) {
        // The Math.random() returns a random number between 0.0 and 1.0 
        // including 0.0 but not including 1.0.
        double number = Math.random();
        System.out.println("Generated number: " + number);

        // By multiplying Math.random() result with a number will give
        // us a range of random number between, for instance 0.0 to 10.0 as
        // shown in the example below.
        number = Math.random() * 10;
        System.out.println("Generated number: " + number);

        // To get a random number between n and m we can use the formula:
        // n + (Math.random() * (m - n)). The example below creates random
        // number between 100.0 and 200.0.
        int n = 100;
        int m = 200;
        number = n + (Math.random() * (m - n));
        System.out.println("Generated number: " + number);

        // Creates an integer random number
        int random = 100 + (int) (Math.random() * 100);
        System.out.println("Generated number: " + random);
    }
}

Here is an example result of our program.

Generated number: 0.024974902600698234
Generated number: 2.271051510086771
Generated number: 147.542543014888
Generated number: 112