[Java] Get date information 10 days later using milliseconds in the Date class

Practice handling the Date class

I was studying about Date,

• Understanding epoch milliseconds
• How to display by specifying the format

I feel that there were some parts that I didn't understand very well in these two points, so I will summarize them.

Actually, it was swamped because the fields of the Calendar class were not well understood, and it seems that Java 8 has a new date API and it can be handled a little more intuitively, but it is also a primitive Date class. I felt that it was better to start understanding, so I tried to draw while verifying various things in my own way.

__ Version using Calendar class is also helpful __

Let's display the date information 10 days later using the Date class

TenDaysDate.java

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

public class TenDaysDate {

	public static void main(String[] args) {
		
		//Change the long value of Date type to get the date 10 days later
		Date d = new Date();									//Get the current date and time
		long ts = System.currentTimeMillis(); //Get the current epoch milliseconds
		
		//Compare the return value of getTime method of Date type with the value of currentTimeMillis
		System.out.println("d.getTime:\t\t\t" + d.getTime());				// Date.getTime value
		System.out.println("currentTimeMillis:\t" + ts);						//value of currentTimeMillis
		if(ts == d.getTime()) System.out.println("Together");			//long values match
		
		//Display Date type date information by specifying the format
		SimpleDateFormat f = new SimpleDateFormat("yyyy year MM month dd day"); //Specify display format
		System.out.println("Today's date:\t" + f.format(d));						//Pass Date type argument to SimpleDateFormat type
		
		//Add 10 days worth of milliseconds with the Date type setTime method and change to date information 10 days later.
		d.setTime(d.getTime() + (long)(24 * 60 * 60 * 1000) * 10);	// ()Is a millisecond of the day
		System.out.println("Date 10 days later:\t" + f.format(d));					//Pass to SimpleDateFormat type as an argument of Date type
	}

}

Output result

d.getTime:			1602415769204
currentTimeMillis:	1602415769204
Together
Today's date:October 11, 2020
Date 10 days later:October 21, 2020

get and set

d.getTime(); //Epoch milliseconds are returned
d.setTime(Epoch milliseconds); // Epoch millisecondsを引数に渡して日付情報をセットする

Epoch milliseconds

January 1, 1970 00:00:00 Milliseconds elapsed from UTC Also known as Unix Time One day's worth of milliseconds is 24 * 60 * 60 * 1000. Multiply this by 10 for 10 days worth of milliseconds.

Also, by setting getTime () + 24 * 60 * 60 * 1000 * 10, you can get the date information 10 days after the lapse of milliseconds from the current time.

Do you need to cast to a long type as (long) 24 * 60 * 60 * 1000 in some cases?

It may not be the date you expected depending on the year of the moist

If a leap year comes out 10 years later or something like that, even if the millisecond one year later is 24 * 60 * 60 * 1000 * 365 * 10, it will not be the same month and day 10 years later. ..

When calculating over a long span, you have to add it to the year field etc. It's complicated.