Java date data type conversion (Date, Calendar, String)

When dealing with date data in Java, I was often confused, so I tried to organize it myself.

Data type for date

Date

Can represent data for a particular date up to milliseconds
However, methods for getting and setting hours, minutes, and seconds are not recommended.
The Calendar class described later can handle hours, minutes, and seconds.

Calendar

Can generate all the time format values needed when formatting dates and times
Can be used when molding any date and time

java.util.Date

java.util is one of the packages that can handle Date type
Inheriting methods for comparing and retrieving date data
ʻEquals`: Compare if two dates are equal
getDay: You can get the data of the day (0 = Sunday, 1 = Monday, ...)
etc

java.sql.Date

java.sql is a package
It will provide an API for accessing and processing data stored in a data source (usually RDB).
When dealing with dates in java.sql.Date, hours, minutes, seconds and milliseconds must be set to 0

Date ⇄ string conversion

Date→String

Specify the format of the date you want to output with SimpleDateFormat
Convert to string with format (Date date) method

Date date = new Date(); //Today's date
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
String strDate = dateFormat.format(date);

System.out.println(date); // => Mon Dec 31 10:33:59 JST 2018
System.out.println(strDate); // => 2018-12-31

If you use the toString () method, it will be a string as it is

Date date = new Date(); //Today's date
String strDate = date.toString();

System.out.println(date); // => Mon Dec 31 10:33:59 JST 2018
System.out.println(strDate); // => Mon Dec 31 10:33:59 JST 2018

String→Date

Conversion using the parse method
It is necessary to decide the format in advance
You need to declare ParseException in a class that uses the parse method

public static void main(String[] args) throws ParseException{
   String strDate = "2018-12-31";
   SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
   Date date = dateFormat.parse(strDate);

   System.out.println(date); // => Mon Dec 31 00:00:00 JST 2018
}

Conversion using the valueOf (String s) method
The return value of the valueOf method will be a java.sql.Date object


String strDate = "2018-12-31";
Date sqlDate = java.sql.Date.valueOf(strDate);

System.out.println(sqlDate); // => 2018-12-31

Date⇄Calendar conversion

Date→Calendar

Create a new instance of the Calendar class
Convert to Calendar class object using setTime (Date date) method

Date date = new Date(); //Today's date
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);

System.out.println(calendar);
// =>java.util.GregorianCalendar[time=1546228443561,areFieldsSet=true, ...]

Calendar→Date

Convert to Date class object with getTime () method

Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.YEAR, 2018); //Set year
calendar.set(Calendar.MONTH, 11); //Set month(* MONTH starts from 0 0 → Jan, 1 → Feb,...become)
calendar.set(Calendar.DAY_OF_MONTH, 31);//Set date
Date date = calendar.getTime();

System.out.println(date); // => Mon Dec 31 13:01:05 JST 2018

Reference site

at the end

I summarized how to handle date data that I have implemented. I would be grateful if you could give me some advice if you say "this is better" or "I often do this"! Wish you a good year!