java字符串日期类Date和Calendar相互转化及相关常用方法

String dateString = "2023-07-28";
DateFormat format = new SimpleDateFormat("yyyy-MM-dd");
Date date = format.parse(dateString);

Date date = new Date();
DateFormat format = new SimpleDateFormat("yyyy-MM-dd");
String dateString = format.format(date);

Date date = new Date();
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);

Calendar calendar = Calendar.getInstance();
Date date = calendar.getTime();

public static void main(String[] args) {
    Calendar calendar = Calendar.getInstance();
    System.out.println(calendar.getTime().toString());  // Mon Aug 15 18:19:55 CST 2022

    calendar.add(Calendar.YEAR, 2);                     // Mon Aug 15 18:19:55 CST 2024
    calendar.add(Calendar.MONTH, -6);                   // Thu Feb 15 18:19:55 CST 2024
    calendar.add(Calendar.DAY_OF_MONTH, 14);            // Thu Feb 29 18:19:55 CST 2024

    System.out.println(calendar.getTime().toString());  // Thu Feb 29 18:19:55 CST 2024
}

public class Main {

    /**
     * 对日期加减数天
     *
     * @param yyyyMMdd 形如 "20240228"
     * @param day      形如 2（支持负数）
     * @return 形如 "20240301"
     */
    public static String addDay(String yyyyMMdd, int day) {

        // String to Date
        SimpleDateFormat format = new SimpleDateFormat("yyyyMMdd");
        Date date;
        try {
            date = format.parse(yyyyMMdd);
        } catch (ParseException e) {
            throw new RuntimeException(e);
        }

        // Change Date
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        calendar.add(Calendar.DAY_OF_MONTH, day);
        date = calendar.getTime();

        // Date to String
        yyyyMMdd = format.format(date);

        return yyyyMMdd;
    }

    public static void main(String[] args) {
        System.out.println(addDay("20240228", 2));  // 20240301
    }
}

System.out.println(addDay("20240228", 2333));  // 20300719