How do I add or subtract a date?

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

The java.util.Calendar allows us to do a date arithmetic function such as add or subtract a unit of time to the specified date field.

The method that done this process is the Calendar.add(int field, int amount). Where the value of the field can be Calendar.DATE, Calendar.MONTH, Calendar.YEAR. So this mean if you want to subtract in days, months or years use Calendar.DATE, Calendar.MONTH or Calendar.YEAR respectively.

package org.kodejava.util;

import java.util.Calendar;

public class CalendarAddExample {
    public static void main(String[] args) {
        Calendar cal = Calendar.getInstance();

        System.out.println("Today : " + cal.getTime());

        // Subtract 30 days from the calendar
        cal.add(Calendar.DATE, -30);
        System.out.println("30 days ago: " + cal.getTime());

        // Add 10 months to the calendar
        cal.add(Calendar.MONTH, 10);
        System.out.println("10 months later: " + cal.getTime());

        // Subtract 1 year from the calendar
        cal.add(Calendar.YEAR, -1);
        System.out.println("1 year ago: " + cal.getTime());
    }
}

In the code above we want to know what is the date back to 30 days ago. The sample result of the code is shown below:

Today : Wed Sep 15 17:58:49 CST 2021
30 days ago: Mon Aug 16 17:58:49 CST 2021
10 months later: Thu Jun 16 17:58:49 CST 2022
1 year ago: Wed Jun 16 17:58:49 CST 2021

How do I get the current month name?

By I Wayan Saryada in Core API, Util Package 2 Comments

To get the current month name from the system we can use java.util.Calendar class. The Calendar.get(Calendar.MONTH) returns the month value as an integer starting from 0 as the first month and 11 as the last month. This mean January equals to 0, February equals to 1 and December equals to 11.

Let’s see the code below:

package org.kodejava.example.util;

import java.util.Calendar;

public class GetMonthNameExample {
    public static void main(String[] args) {
        String[] monthName = {"January", "February",
                "March", "April", "May", "June", "July",
                "August", "September", "October", "November",
                "December"};

        Calendar cal = Calendar.getInstance();
        String month = monthName[cal.get(Calendar.MONTH)];

        System.out.println("Month name: " + month);
    }
}

On the first line inside the main method we declare an array of string that keep our month names. Next we get the integer value of the current month and at the last step we look for the month name inside our previously defined array.

The example result of this program is:

Month name: September

The better way to get the month names or week names is to use the java.text.DateFormatSymbols class. The example on using this class can be found on the following links: How do I get a list of month names? and How do I get a list of weekday names?.

How do I check if a string is a valid number?

By I Wayan Saryada in Core API, Lang Package 0 Comment

When building a computer program we will use a lot of string to represent our data. The data might not just information about our customer name, email or address, but will also contain numeric data represented as string. So how do we know if this string contains a valid number?

Java provides some wrappers to the primitive data types that can be used to do the checking. These wrappers come with the parseXXX() method such as Integer.parseInt(), Float.parseFloat() and Double.parseDouble() methods.

package org.kodejava.lang;

public class NumericParsingExample {
    public static void main(String[] args) {
        String age = "15";
        String height = "160.5";
        String weight = "55.9";

        try {
            int theAge = Integer.parseInt(age);
            float theHeight = Float.parseFloat(height);
            double theWeight = Double.parseDouble(weight);

            System.out.println("Age    = " + theAge);
            System.out.println("Height = " + theHeight);
            System.out.println("Weight = " + theWeight);
        } catch (NumberFormatException e) {
            e.printStackTrace();
        }
    }
}

In the example code we use Integer.parseInt(), Float.parseFloat(), Double.parseDouble() methods to check the validity of our numeric data. If the string is not a valid number java.lang.NumberFormatException will be thrown.

The result of our example:

Age    = 15
Height = 160.5
Weight = 55.9

How do I get date, month, year values from the current date?

By I Wayan Saryada in Core API, Util Package 0 Comment

What date, month, year, day-of-week, day-of-month, day-of-year is it today? If we want to answer these question we can use java.util.Calendar and java.util.GregorianCalendar which is the implementation of Calendar abstract class.

These classes can help us to get integer value such as date, month, year from a Date object. Let’s see the example code.

package org.kodejava.util;

import java.util.Calendar;

public class CalendarExample {
    public static void main(String[] args) {
        // Get various information from the Date object.
        Calendar cal = Calendar.getInstance();
        int day = cal.get(Calendar.DATE);
        int month = cal.get(Calendar.MONTH) + 1;
        int year = cal.get(Calendar.YEAR);
        int dow = cal.get(Calendar.DAY_OF_WEEK);
        int dom = cal.get(Calendar.DAY_OF_MONTH);
        int doy = cal.get(Calendar.DAY_OF_YEAR);

        System.out.println("Current Date: " + cal.getTime());
        System.out.println("Day         : " + day);
        System.out.println("Month       : " + month);
        System.out.println("Year        : " + year);
        System.out.println("Day of Week : " + dow);
        System.out.println("Day of Month: " + dom);
        System.out.println("Day of Year : " + doy);
    }
}

Here is the result of this example:

Current Date: Wed Sep 15 17:34:44 CST 2021
Day         : 15
Month       : 9
Year        : 2021
Day of Week : 4
Day of Month: 15
Day of Year : 258

You might also want to try to use the Joda Time library for this. Here another example for getting information about date and time using Joda: How do I get date / time fields of date in Joda?.

How do I get the minimum and maximum value of a primitive data types?

By I Wayan Saryada in Core API, Lang Package 0 Comment

To get the minimum or maximum value of a primitive data types such as byte, short, int, long, float and double we can use the wrapper class provided for each of them, the wrapper classes are Byte, Short, Integer, Long, Float and Double which is located in java.lang package.

package org.kodejava.lang;

public class MinMaxExample {
    public static void main(String[] args) {
        System.out.println("Byte.MIN    = " + Byte.MIN_VALUE);
        System.out.println("Byte.MAX    = " + Byte.MAX_VALUE);
        System.out.println("Short.MIN   = " + Short.MIN_VALUE);
        System.out.println("Short.MAX   = " + Short.MAX_VALUE);
        System.out.println("Integer.MIN = " + Integer.MIN_VALUE);
        System.out.println("Integer.MAX = " + Integer.MAX_VALUE);
        System.out.println("Long.MIN    = " + Long.MIN_VALUE);
        System.out.println("Long.MAX    = " + Long.MAX_VALUE);
        System.out.println("Float.MIN   = " + Float.MIN_VALUE);
        System.out.println("Float.MAX   = " + Float.MAX_VALUE);
        System.out.println("Double.MIN  = " + Double.MIN_VALUE);
        System.out.println("Double.MAX  = " + Double.MAX_VALUE);
    }
}

The result of the code above shows the minimum and maximum value for each data types.

Byte.MIN    = -128
Byte.MAX    = 127
Short.MIN   = -32768
Short.MAX   = 32767
Integer.MIN = -2147483648
Integer.MAX = 2147483647
Long.MIN    = -9223372036854775808
Long.MAX    = 9223372036854775807
Float.MIN   = 1.4E-45
Float.MAX   = 3.4028235E38
Double.MIN  = 4.9E-324
Double.MAX  = 1.7976931348623157E308