Use localdate class to complete calendar design

Preface

Before we finish this calendar design , Need to know Java Predefined classes in  LocalDate  Some uses of

grammar

LocalDate.now() // 2022-07-01

A new object will be constructed , Represents the date when the object was constructed .

LocalDate.of(1999, 1, 1)

Can provide years 、 Month and day to construct an object corresponding to a specific date ：

Of course , Usually we want to save the constructed object in an object variable ：

LocalDate newYearsEve = LocalDate.of(1999, 1, 1);

When there is one  LocalDate  object , It can be done in different ways  getYear 、 getMonthValue  and  getDayOfMonth  Get years 、 Month and day ：

int year = newYearsEve.getYear(); // 1999
int month = newYearsEve.getMonthValue(); // 1
int day = newYeaersEve.getDayOfMonth(); // 1

The above method seems meaningless , Because these are the values used when constructing objects . however , Sometimes there may be a calculated date , Then you want to call these methods to learn more about it . for example , plusDays  Method will get a new  LocalDate , If the object to which this method is applied is called the current object , The new date object is a new date from the specified number of days of the current object ：

LocalDate aThousandDaysLater = newYearsEve.plusDays(1000);
year = aThousandDaysLater.getYear();  // 2002
month = aThousandDaysLater.getMonthValue();  // 09
day = aThousandDaysLater.getDayOfMonth();  // 26

aThousandDaysLater  Is added to the original date 1000 God , At this time, the above method is effective

Calendar practice

demand ： Use  LocalDate  Class shows the calendar of the current month , The format is as follows ：

Mon Tue Wed Thu Fri Sat Sun
                  1*  2   3 
  4   5   6   7   8   9  10 
 11  12  13  14  15  16  17 
 18  19  20  21  22  23  24 
 25  26  27  28  29  30  31

The current date uses  *  Number mark . You can see , This program needs to know how to calculate the days of a month and the corresponding day of the week for a given date .

Step breakdown

① First construct an object , And initialize with the current date

LocalDate date = LocalDate.now();`

② Get the current month and date

int month = date.getMonthValue();
int today = date.getDayOfMonth();

③ take date Set to the first day of the month , And get the day of the week

date = date.minusDays(today - 1);  //  Set to the first day of the month 
DayOfWeek weekday = date.getDayOfWeek();
int value = weekday.getValue();  // 1 = Monday    7 = Sunday

Variable  weekday  Set to  DayOfWeek  Object of type . We call the  getValue  Method to get a value of the day of the week . We will get an integer . Return on Monday 1, Return on Tuesday 2, By analogy , Return on Sunday 7.

④ Because the first line of the calendar is indented , This makes the first day of the month point to the corresponding day of the week . The following code will print the header and the indent of the first line

System.out.println("Mon Tue Wed Thu Fri Sat Sun");
for (int i = 1; i < value; i++)
    System.out.print("    ");

⑤ Print the body of the calendar , Into a cycle , among date Traverse every day of the month .

Each iteration , Print date value . If date Is the current date , This date uses a  *  Mark . Next , hold date Advance to the next day . If you arrive on the first day of the new week , Then line feed printing ：

while (date.getMonthValue() == month) {
    System.out.printf("%3d", date.getDayOfMonth());
    if (date.getDayOfMonth() == today)
        System.out.print("*");
    else
        System.out.print(" ");
    date = date.plusDays(1);
    if (date.getDayOfWeek().getValue() == 1)
        System.out.println();
}

⑥ When will it end ？ We don't know how many days this month , yes 28、29、30 still 31. actually , as long as date We will continue to iterate in that month

Complete code

import java.time.DayOfWeek;
import java.time.LocalDate;

/**
 * @author JKC
 * @Description:
 * @date 2022/7/1 10:53
 */
public class  Making a calendar  {
    public static void main(String[] args) {
        //  Create a date object , And initialize 
        LocalDate date = LocalDate.now();
        System.out.println(date);
        //  Get the current month and date 
        int month = date.getMonthValue();
        int today = date.getDayOfMonth();

        //  take date Set to the first day of the month , And get the day of the week 
        date = date.minusDays(today - 1);
        //  Set to DayOfWeek Object of type . Calling this object getValue Method to get a value of the day of the week 
        DayOfWeek weekday = date.getDayOfWeek();
        int value = weekday.getValue();  // 1 = Monday    7 = Sunday

        System.out.println("Mon Tue Wed Thu Fri Sat Sun");
        for (int i = 1; i < value; i++)
            System.out.print("    ");

        while (date.getMonthValue() == month) {
            System.out.printf("%3d", date.getDayOfMonth());
            if (date.getDayOfMonth() == today)
                System.out.print("*");
            else
                System.out.print(" ");
            date = date.plusDays(1);
            if (date.getDayOfWeek().getValue() == 1)
                System.out.println();
        }
        if (date.getDayOfWeek().getValue() != 1)
            System.out.println();
    }
}

Conclusion

The focus of the above example program is to show how to use the interface of a class to complete quite complex tasks , Without knowing the implementation details

LocalDate API

static LocalDate now()
 Construct an object representing the current date 

static LocalDate of(int year, int month, int day)
 Construct an object representing a given date 

int getYear()
int getMonthValue()
int getDayOfMonth()
 Get the year of the current date 、 Month and day .

DayOfWeek getDayOfWeek
 Get what day of the week the current date is , As DayOfWeek An instance of class returns . call getValue To get 1~7 A number between , This is the day of the week ,1 For Monday ,7 Means Sunday 

LocalDate plusDays(int n)
LocalDate minusDays(int n)
 Generated after or before the current date n The date of day