The pit of Calendar in Java

package com.dev.tool.log.service;

import org.junit.Test;

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

/**
 * Created by kenny.dong on 2018/4/2.
 */
public class CalendarTest {

    /**
     * Calendar.getTime method is to return the milliseconds since 1970-01-01 00:00:00 UTC (Epoch Time), and then convert it to Date type.
     * Now a case is:
     * The current time is 20180329, and now there is a year with year=2018, month=2, and I want the format to be yyyyMM format
     * If you give the year and month directly to Calendar, the problem comes out
     * @throws InterruptedException
     */
    @Test
    public void calendarGetTime() throws InterruptedException, ParseException {
        int year = 2018;
        int month = 2;
        SimpleDateFormat format = new SimpleDateFormat("yyyyMMdd");
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(format.parse("20180331"));
        calendar.set(Calendar.YEAR,year);
        calendar.set(Calendar.MONTH,month-1);
        String DateStr = format.format(calendar.getTime());
        System.out.println(DateStr);
        /**
         * The output result is 20180301. The output result we expect should be 20180201. Why is there an extra month?
         * This is because the set method of Calendar only changes the value of the month. But Calendar is ultimately calculated based on milliseconds
         * The time it represents, then his DAY days have not changed, because 201802 just doesn't have the 29th, then Calendar is equal to February 28th
         * Then add 1 day, then it is 20180301! If the current time is 20180331, then the output is 20180303!
         *So to use Calendar, you can set calendar.set(Calendar.DAY_OF_MONTH, 1); to be safer at the beginning of the month.
         */
    }

    @Test
    public void calendarGetTimeOk() throws InterruptedException, ParseException {
        int year = 2018;
        int month = 2;
        SimpleDateFormat format = new SimpleDateFormat("yyyyMMdd");
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(format.parse("20180331"));
        calendar.set(Calendar.YEAR,year);
        calendar.set(Calendar.MONTH,month-1);
        calendar.set(Calendar.DAY_OF_MONTH,1);//Set as the beginning of the month
        String DateStr = format.format(calendar.getTime());
        System.out.println(DateStr);
    }
}