[Java] Get the date 10 days later with the Calendar class

TenDaysCalendar.java

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

public class TenDaysCalendar {

	public static void main(String[] args) {
		//Get current date information
		Date now = new Date();
		//Set to Calendar
		Calendar c = Calendar.getInstance(); //Calendar does not use new
		c.setTime(now); //Date type as an argument
		
		//Try to display the date
		System.out.println("***********Current date and time***********");
		System.out.println("Calendar.DATE: " + Calendar.DATE); // 5
		System.out.println("c.get(Calendar.DATE): " + c.get(Calendar.DATE)); //Today's date
		
		//Output by specifying the display format
		SimpleDateFormat f = new SimpleDateFormat("yyyy year MM month dd day"); //Specify display format
		
		//I want to add 10 to the date and get the date 10 days later
		c.add(Calendar.DATE, 10);
		System.out.println();
		System.out.println("***********10 days later***********");
		System.out.println("Date 10 days later: " + f.format(c.getTime()));
		System.out.println("Calendar.DATE: " + Calendar.DATE); // 5
		System.out.println("c.get(Calendar.DATE): " + c.get(Calendar.DATE));
		
		//Set the date 10 days later and try to get it
		System.out.println();
		System.out.println("***********After another 10 days***********");
		System.out.println("c.set(Calendar.DATE, (Calendar.DATE + 10))Try to date as 10 days later");
		c.set(Calendar.DATE, (Calendar.DATE + 10));
		System.out.println("Date 10 days later: " + f.format(c.getTime()));
		System.out.println("Calendar.DATE is a constant of 5, so adding 10 gives 15 days.");
		
		System.out.println();
		System.out.println("c.set(Calendar.DATE, (c.get(Calendar.DATE) + 10))Try to date as 10 days later");
		c.set(Calendar.DATE, (c.get(5) + 10));
		System.out.println("Date 10 days later: " + f.format(c.getTime()));
		System.out.println("Since the DATE was set on the 15th above, add 10 to get the 25th.");
	}
}

Execution result

***********Current date and time***********
Calendar.DATE: 5
c.get(Calendar.DATE): 11

***********10 days later***********
Date 10 days later:October 21, 2020
Calendar.DATE: 5
c.get(Calendar.DATE): 21

***********After another 10 days***********
c.set(Calendar.DATE, (Calendar.DATE + 10))Try to date as 10 days later
Date 10 days later:October 15, 2020
Calendar.DATE is a constant of 5, so adding 10 gives 15 days.

c.set(Calendar.DATE, (c.get(Calendar.DATE) + 10))Try to date as 10 days later
Date 10 days later:October 13, 2020
Since the DATE was set on the 15th above, add 10 to get the 25th.

Calendar.DATE is a final constant and cannot be changed

Why can I do a set because it's final?

I couldn't understand this at all. I mean, I still don't understand it well. As a result of various verifications for the time being, there is a constant field value Calendar.DATE seems to contain a constant 5.

You can get the date by doing get

c.get(Calendar.DATE);

Then you can get the date properly. What do you mean? It was like that.

For the time being, Calendar.DATE is a constant 5, so

c.get(5);

I was able to get the date as.

Is the date information stored elsewhere instead of DATE? !!

I did some research, but I didn't understand, but when I passed 5 as an argument to get, The constant value of DATE has arrived! I have to return the date! So, I wonder if it's okay to have the image of fetching the date in another place.

Like doing on the basis

int name = "tanaka";
getName(){
  return name;
}

Is it different from the image of?

I will update the article as soon as I understand it, but for the time being, the above understanding was clear and there was no problem in handling it.