How do I use TemporalAdjusters dayOfWeekInMonth() method?

By I Wayan Saryada in Java 8, Java Date Time API 0 Comment

dayOfWeekInMonth() is a handy method in java.time.temporal.TemporalAdjusters class that returns an adjuster which changes the date to the n-th day of week in the current month.

Here is an example of how you could use it:

package org.kodejava.datetime;

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.temporal.TemporalAdjusters;

public class DayOfWeekInMonthExample {
    public static void main(String[] args) {
        // Current date
        LocalDate date = LocalDate.now();

        // The 2nd Tuesday in the month of the date.
        LocalDate secondTuesday = date.with(
                TemporalAdjusters.dayOfWeekInMonth(2, DayOfWeek.TUESDAY));

        System.out.println("Current Date: " + date);
        System.out.println("Second Tuesday: " + secondTuesday);
    }
}

Output:

Current Date: 2024-01-18
Second Tuesday: 2024-01-09

This code would display the date of the second Tuesday in the current month.

Here’s what’s going on in this case:

  • TemporalAdjusters.dayOfWeekInMonth(2, DayOfWeek.TUESDAY) specifies that we want the second occurrence of DayOfWeek.TUESDAY in the current month.

  • date.with(TemporalAdjuster) modifies the LocalDate date based on the TemporalAdjuster that is passed in. The original date object is unchanged; a new LocalDate reflecting the adjusted date is returned.

This TemporalAdjusters method is very useful when you have conditional logic based on things like “if it’s the third Monday of the month, then…”

How do I use java.time.Period class?

By I Wayan Saryada in Java 8, Java Date Time API 0 Comment

java.time.Period is a class that represents a quantity or amount of time in terms of years, months, and days. Here’s a brief guide on how to use it:

1. Creating a Period instance

The Period class provides several static methods named of() and between() to create an instance. To create Period of 1 year, 2 months, and 3 days:

Period period = Period.of(1, 2, 3);

Or you can create a Period using LocalDate:

LocalDate start = LocalDate.of(2020, Month.JANUARY, 1);
LocalDate end = LocalDate.of(2023, Month.MARCH, 31);
Period period = Period.between(start, end);

2. Retrieving Years, Months, and Days

Use the getDays(), getMonths(), and getYears() methods to get the number of days, months, and years in the Period.

int days = period.getDays();
int months = period.getMonths();
int years = period.getYears();

3. Adding/subtracting Period to/from a LocalDate

The LocalDate.plus() or LocalDate.minus() methods can be used to add or subtract a Period from a LocalDate.

LocalDate ld = LocalDate.of(2021, Month.JULY, 1);
Period period = Period.of(1, 2, 3);
LocalDate newDate = ld.plus(period);

In this case, newDate will be 1 year, 2 months, and 3 days after July 1, 2021.

4. Mixing units of time

You can use plusDays(), plusMonths(), or plusYears() to add to a Period.

Period period = Period.of(1, 2, 3);
Period newPeriod = period.plusDays(10);

The newPeriod will then be 1 year, 2 months, and 13 days.

How do I use plus and minus method in the Java Date-Time API?

By I Wayan Saryada in Java 8, Java Date Time API 0 Comment

In the Java Date-Time API, the plus and minus methods can be used to calculate and modify dates, times, date/times, and durations.

Each temporal class (LocalDate, LocalTime, LocalDateTime, and Duration) includes these methods.

Here’s a basic example using the LocalDate class:

package org.kodejava.datetime;

import java.time.LocalDate;

public class PlusMinusExample {
    public static void main(String[] args) {
        LocalDate date = LocalDate.now();

        // Calculate the date 5 days into the future
        LocalDate futureDate = date.plusDays(5);
        System.out.println("Date five days in the future: " + futureDate);

        // Calculate the date 5 days in the past.
        LocalDate pastDate = date.minusDays(5);
        System.out.println("Date five days in the past: " + pastDate);
    }
}

Output:

Date five days in the future: 2024-01-22
Date five days in the past: 2024-01-12

You can also use plusWeeks, plusMonths, plusYears, minusWeeks, minusMonths, minusYears methods in a similar manner to add or subtract the respective time period.

Note: All the datetime manipulation methods return a new instance of the date/time object; they do not modify the original object because the classes are immutable.

The plus() and minus() methods in the Java Date-Time API offer finer control over date-time arithmetic by allowing you to add or subtract different types of date-time units such as days, months, or years.

The plus() method is used to add specific time units to a date or time, while the minus() method is used to subtract specific time units.

Here’s an example:

package org.kodejava.datetime;

import java.time.LocalDate;
import java.time.Period;

public class PlusMinusOtherExample {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();

        // 1 year, 2 months, and 3 days.
        Period periodToAdd = Period.of(1, 2, 3);
        LocalDate futureDate = today.plus(periodToAdd);
        System.out.println("Date after adding a period: " + futureDate);

        // 2 years, 4 months, and 6 days.
        Period periodToSubtract = Period.of(2, 4, 6);
        LocalDate pastDate = today.minus(periodToSubtract);
        System.out.println("Date after subtracting a period: " + pastDate);
    }
}

The Period class is part of the Java Date-Time API and is used to represent a quantity of time in terms of years, months, and days.

Remember, you can create a Period using the Period.of(int years, int months, int days) method, where years, months, and days are the specific units of time to be represented.

Here are examples using LocalTime and LocalDateTime:

package org.kodejava.datetime;

import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.temporal.ChronoUnit;

public class PlusMinusTime {
    public static void main(String[] args) {
        // Creating a LocalTime object and adding/subtracting hours, minutes, seconds
        LocalTime time = LocalTime.now();

        LocalTime futureTime = time.plus(2, ChronoUnit.HOURS);
        System.out.println("Time after two hours: " + futureTime);

        LocalTime pastTime = time.minus(30, ChronoUnit.MINUTES);
        System.out.println("Time 30 minutes ago: " + pastTime);

        // Creating a LocalDateTime object and adding/subtracting days, months, years
        LocalDateTime dateTime = LocalDateTime.now();

        LocalDateTime futureDateTime = dateTime.plus(1, ChronoUnit.YEARS);
        System.out.println("Date and Time one year into the future: " + futureDateTime);

        LocalDateTime pastDateTime = dateTime.minus(2, ChronoUnit.MONTHS);
        System.out.println("Date and Time two months ago: " + pastDateTime);

        // You can also use plus or minus Days, Weeks, Months, Years directly
        LocalDateTime exactDateTimeFuture = dateTime.plusDays(1).plusWeeks(1).plusMonths(1).plusYears(1);
        System.out.println("Date and Time after one day, week, month, and year: " + exactDateTimeFuture);
    }
}

Note that when we are adding/subtracting units like hours, minutes, and seconds, we use java.time.temporal.ChronoUnit. When adding/subtracting days, weeks, months, and years, we use directly plusDays or minusDays and so on.

How do I use atDate() method of Java Date-Time API?

By I Wayan Saryada in Java 8, Java Date Time API 0 Comment

The atDate() method is a part of LocalTime class in the Java Date-Time API. This method combines this time with a date to create an instance of LocalDateTime.

Here’s an example:

package org.kodejava.datetime;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;

public class AtDateExample {
    public static void main(String[] args) {
        // Create a LocalTime instance
        LocalTime time = LocalTime.of(14, 20);

        // Create a LocalDate instance
        LocalDate date = LocalDate.of(2023, 1, 23);

        // Using atDate to combine time and date into a LocalDateTime
        LocalDateTime dateTime = time.atDate(date);

        System.out.println(dateTime);
    }
}

Output:

2023-01-23T14:20

In this example, a LocalTime and a LocalDate are combined into a LocalDateTime using the atDate() method. This method is useful when you have a LocalTime instance and want to combine it with a date. It’s in some sense a converse operation to LocalDate‘s atTime().

How do I use atTime() method of Java Date-Time API?

By I Wayan Saryada in Java Date Time API 0 Comment

The atTime() method belongs to the LocalDate class in the Java Date-Time API, not the Date class. This method combines this date with a time to create a LocalDateTime.

Here’s an example:

package org.kodejava.datetime;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;

public class AtTimeExample {
    public static void main(String[] args) {
        // Create a LocalDate instance
        LocalDate date = LocalDate.of(2023, 1, 23);

        // Create a LocalTime instance
        // 24-hour clocks
        LocalTime time = LocalTime.of(13, 45);

        // Use atTime() to combine date and time into a LocalDateTime
        LocalDateTime dateTime = date.atTime(time);

        System.out.println(dateTime);
    }
}

Output:

2023-01-23T13:45

In this example, a LocalDate and a LocalTime are combined into a LocalDateTime using the atTime() method.

The LocalDate class also has an overloaded atTime method that takes the hour and minute directly, instead of a LocalTime instance.

Here’s an example where we set 14 hours and 30 minutes directly.

package org.kodejava.datetime;

import java.time.LocalDate;
import java.time.LocalDateTime;

public class AtTimeOtherExample {
    public static void main(String[] args) {
        // Create a LocalDate instance
        LocalDate date = LocalDate.of(2023, 1, 23);

        // Use atTime() to combine date and hour and minute into a
        // LocalDateTime
        LocalDateTime dateTime = date.atTime(14, 30);

        System.out.println(dateTime);
    }
}

Output:

2023-01-23T14:30

In this example, 14:30 (in 24-hour clock) is directly passed into atTime. There is another version of atTime() that takes hrs, min, and sec. That would look like this:

// include seconds
LocalDateTime dateTime = date.atTime(14, 30, 20);

// include nano seconds
LocalDateTime dateTime = date.atTime(14, 30, 20, 200);

These are all the overloaded versions of atTime() in LocalDate.

Remember, LocalDate, LocalTime, LocalDateTime and others from Java Date-Time API are designed to replace the old Date and Calendar classes from java.util package. They are more consistent, easier to understand and use.