[JAVA] A memo when you want to clear the time part of the calendar

What is this?

A personal memo when you want to clear the time part of the date and time data acquired by Calendar.

First of all, an example that comes out when you google

calenderObj.clear(Calendar.HOUR_OF_DAY);
calenderObj.clear(Calendar.HOUR);
calenderObj.clear(Calendar.MINUTE);
calenderObj.clear(Calendar.SECOND);
calenderObj.clear(Calendar.MILLISECOND);

What's wrong?

Write a program and check it.

import java.util.Calendar;

public class CalendarClear {

    public static void main(String[] args) {

        Calendar calendar = Calendar.getInstance();
        sysout(calendar);

        calendar.clear(Calendar.HOUR);
        calendar.clear(Calendar.MINUTE);
        calendar.clear(Calendar.SECOND);
        calendar.clear(Calendar.MILLISECOND);
        sysout(calendar);

        calendar.clear(Calendar.HOUR_OF_DAY);
        sysout(calendar);

    }

    private static final void sysout(Calendar calc) {

        System.out.println("/--------------------");

        System.out.println("YEAR:" + calc.get(Calendar.YEAR));
        System.out.println("MONTH:" + calc.get(Calendar.MONTH));
        System.out.println("DATE:" + calc.get(Calendar.DATE));
        System.out.println("HOUR_OF_DAY:" + calc.get(Calendar.HOUR_OF_DAY));
        System.out.println("HOUR:" + calc.get(Calendar.HOUR));
        System.out.println("MINUTE:" + calc.get(Calendar.MINUTE));
        System.out.println("SECOND:" + calc.get(Calendar.SECOND));
        System.out.println("MILLISECOND:" + calc.get(Calendar.MILLISECOND));

    }

}

The result is this.

/--------------------
YEAR:2017
MONTH:4
DATE:22
HOUR_OF_DAY:17
HOUR:5
MINUTE:25
SECOND:35
MILLISECOND:527
/--------------------
YEAR:2017
MONTH:4
DATE:22
HOUR_OF_DAY:17
HOUR:5
MINUTE:0
SECOND:0
MILLISECOND:0
/--------------------
YEAR:2017
MONTH:4
DATE:22
HOUR_OF_DAY:17
HOUR:5
MINUTE:0
SECOND:0
MILLISECOND:0

The values of HOUR_OF_DAY and HOUR are not cleared. This is the correct description.

public static void main(String[] args) {

    Calendar calendar = Calendar.getInstance();
    sysout(calendar);

    calendar.clear(Calendar.MINUTE);
    calendar.clear(Calendar.SECOND);
    calendar.clear(Calendar.MILLISECOND);
    //To clear the time part, you have to put it in set.
    calendar.set(Calendar.HOUR_OF_DAY, 0);
    sysout(calendar);

    calendar.clear();
    sysout(calendar);

}

It is clearly stated in JavaDoc. Calendar (Java Platform SE 7 )

The HOUR_OF_DAY, HOUR, and AM_PM fields are treated separately and the time resolution rule applies. Clearing any of the> fields does not reset the "hours" in this Calendar. Use set (Calendar.HOUR_OF_DAY, 0) to reset the "hour" value.

Processing result.

/--------------------
YEAR:2017
MONTH:4
DATE:22
HOUR_OF_DAY:17
HOUR:5
MINUTE:27
SECOND:12
MILLISECOND:718
/--------------------
YEAR:2017
MONTH:4
DATE:22
HOUR_OF_DAY:0
HOUR:0
MINUTE:0
SECOND:0
MILLISECOND:0

bonus

calendar.clear();

When you do

/--------------------
YEAR:1970
MONTH:0
DATE:1
HOUR_OF_DAY:0
HOUR:0
MINUTE:0
SECOND:0
MILLISECOND:0

All items are cleared.

Calendar (Java Platform SE 7 )

Set all calendar field and time values (offsets in milliseconds from the epoch) for this calendar to undefined.