I. Introduction of Calendar class

In Java, time classes mainly include Date and Calendar. Java officially recommends using Calendar to replace Date. There can be free conversion between Calendar and Date, and the link of conversion is time. The java.util.Calendar class is an abstract class that provides methods for converting between a specific time and a set of Calendar fields such as YEAR, MONTH, DAY_OF_MONTH, HOUR, and so on, as well as methods for manipulating Calendar fields, such as getting next week’s date. A moment can be represented by a millisecond value, which is the offset from the epoch (00:00:00.000 GMT, January 1, 1970, Gregorian calendar).

Constants in the Calendar class

Common methods of Calendar class

3.1 Constructors

Because the Decorator of the Calendar class is protected, this object cannot be created directly; it needs to be created using a static method that returns a subclass object.

// Get a calendar with the default time zone and locale
public static synchronized Calendar getInstance(a)
// Get a calendar with the given time zone and default locale
public static synchronized Calendar getInstance(TimeZone zone)
// Get a calendar with the default time zone and given locale
public static synchronized Calendar getInstance(Locale aLocale)
// Get a calendar with the given time zone and locale
public static synchronized Calendar getInstance(TimeZone zone, Locale aLocale)
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3.2 set values

Get the value 3.3

3.4 compare the value

• Compares the time values represented by two Calendar objects, and returns a value of 0 if the time represented by the parameter is equal to the time represented by this Calendar;
• Returns a value less than 0 if the Calendar time is before the time represented by the parameter;
• If the time of this Calendar is after the time indicated by the parameter, a value greater than 0 is returned.

3.5 calculated value

Four, the instance,

• Basic example

  public class WorkCalendarHelper {
  
      private final static String[] WEEKS = {"Monday"."Tuesday"."Wednesday"."Thursday"."Friday"."Saturday"."Sunday"};
  
      public static void main(String[] args) {
          // Get an instance of Calendar using Calendar getInstance()
          Calendar calendar = Calendar.getInstance();
          // Initialize the Calendar object, but it is not necessary unless you need to reset the time
          calendar.setTime(new Date());
          // Get the time value
          System.out.println(calendar.getTime());
          // Displays the year
          System.out.println("The year is:" + calendar.get(Calendar.YEAR));
  
          // Displays the month (Calendar month starts from 0, that is, the 12 months of the year are represented by 0 to 11.)
          System.out.println("Month is:" + (calendar.get(Calendar.MONTH) + 1));
          / /,
          System.out.println("Date:" + calendar.get(Calendar.DATE));
          / / week
          System.out.println("For the week:" + WEEKS[calendar.get(Calendar.DAY_OF_WEEK) - 1]);
          //
          System.out.println("Date:" + calendar.get(Calendar.DAY_OF_WEEK_IN_MONTH));
          // the NTH day of a month
          System.out.println("The first of the month" + calendar.get(Calendar.DAY_OF_MONTH) + "Day");
          // The NTH day of the year
          System.out.println("Number one of the year" + calendar.get(Calendar.DAY_OF_YEAR) + "Day");
  
          // get morning and afternoon
          System.out.println(calendar.get(Calendar.AM_PM) == 0 ? "Morning" : "Afternoon");
          / /
          System.out.println("When:" + calendar.get(Calendar.HOUR));
          // The default value is 24 hours
          System.out.println("When:" + calendar.get(Calendar.HOUR_OF_DAY));
          / / points
          System.out.println("Minutes are:" + calendar.get(Calendar.MINUTE));
          / / SEC.
          System.out.println("Seconds are:" + calendar.get(Calendar.SECOND));
          / / ms
          System.out.println("Milliseconds are:" + calendar.get(Calendar.MILLISECOND));
          System.out.println("Week of month:" + calendar.get(Calendar.WEEK_OF_MONTH));
          System.out.println("Weeks of the year:" + calendar.get(Calendar.WEEK_OF_YEAR));
          System.out.println("The calendar type is:" + calendar.get(Calendar.ERA));
          System.out.println("Zone for."+ calendar.get(Calendar.ZONE_OFFSET)); }}Copy the code

• Gets all dates for a year

  public static List<Map<String,Object>> initCalendar(int year) {
      List<Map<String,Object>> resultList = new ArrayList<>();
      Calendar calendar = Calendar.getInstance();
      int monthSum = 12;
      for (int i = 0; i < monthSum; i++) {
          calendar.set(year, i, 1);
          // Obtain the minimum number of days in a month
          int monthMin = calendar.getActualMinimum(Calendar.DAY_OF_MONTH);
          // Obtain the maximum number of days in a month
          int monthMax = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
  
          Map<String,Object> workCalendar;
          String timeCode;
          for (int day = monthMin; day <= monthMax; day++) {
              workCalendar = new HashMap<String,Object>(4);
              calendar.set(year, i, day);
              if ((calendar.get(7) = =1) || (calendar.get(7) = =7)) {
                  timeCode = "sys_weekend";
              } else {
                  timeCode = "sys_workday";
              }
              // Work time code
              workCalendar.put("timeCode",timeCode);
              / / year
              workCalendar.put("year",year);
              / / in
              workCalendar.put("month",(i+1));
              / / day
              workCalendar.put("day",day); resultList.add(workCalendar); }}return resultList;
  }
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• Gets all the dates of the current month

  /** * gets all the dates of the current month */
  public static List<Map<String, Object>> currentMonthDay() {
      List<Map<String, Object>> resultList = new ArrayList<>();
      // Create a calendar
      Calendar ca = Calendar.getInstance();
      // Set the time
      ca.setTime(new Date());
      // Adjust the time to the last day of the current calendar month
      ca.set(ca.get(Calendar.YEAR), ca.get(Calendar.MONTH) + 1.0);
      // Get the actual total number of days in the current calendar month
      int dayNumOfMonth = ca.get(Calendar.DAY_OF_MONTH);
      // Point the calendar time to the first of the current calendar month
      ca.set(Calendar.DAY_OF_MONTH, 1);
      Map<String, Object> workCalendar;
      String timeCode;
      for (int i = 0; i < dayNumOfMonth; i++, ca.add(Calendar.DATE, 1)) {
          workCalendar = new HashMap<String, Object>(4);
          // Set to 0 hours, 0 minutes, 0 seconds, 0 milliseconds
          ca.set(Calendar.HOUR_OF_DAY, 0);
          ca.set(Calendar.MINUTE, 0);
          ca.set(Calendar.SECOND, 0);
          ca.set(Calendar.MILLISECOND, 0);
          if ((ca.get(7) = =1) || (ca.get(7) = =7)) {
              timeCode = "sys_weekend";
          } else {
              timeCode = "sys_workday";
          }
          // Work time code
          workCalendar.put("timeCode", timeCode);
          / / year
          workCalendar.put("year", ca.get(Calendar.YEAR));
          / / in
          workCalendar.put("month", (ca.get(Calendar.MONTH) + 1));
          / / day
          workCalendar.put("day", ca.get(Calendar.DAY_OF_MONTH));
          resultList.add(workCalendar);
      }
      return resultList;
  }
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• Gets all the dates of the current week

  /** * get all the dates of the week, sorted by time */
  public static List<Map<String, Object>> currentMeekDay() {
      List<Map<String, Object>> resultList = new ArrayList<>();
      // Create a calendar
      Calendar ca = Calendar.getInstance();
      // Set the time
      ca.setTime(new Date());
      // Set Monday to the first day of the week
      ca.setFirstDayOfWeek(Calendar.MONDAY);
      // Point the Calendar date object to the Monday of the week
      ca.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
      Map<String, Object> workCalendar;
      String timeCode;
      for (int i = 0; i < 7; i++, ca.add(Calendar.DATE, 1)) {
          workCalendar = new HashMap<String, Object>(4);
          // Set the calendar time to 0 hours 0 minutes 0 seconds
          ca.set(Calendar.HOUR_OF_DAY, 0);
          ca.set(Calendar.MINUTE, 0);
          ca.set(Calendar.SECOND, 0);
          ca.set(Calendar.MILLISECOND, 0);
          // Add time to list
          if ((ca.get(7) = =1) || (ca.get(7) = =7)) {
              timeCode = "sys_weekend";
          } else {
              timeCode = "sys_workday";
          }
          // Work time code
          workCalendar.put("timeCode", timeCode);
          / / year
          workCalendar.put("year", ca.get(Calendar.YEAR));
          / / in
          workCalendar.put("month", (ca.get(Calendar.MONTH) + 1));
          / / day
          workCalendar.put("day", ca.get(Calendar.DAY_OF_MONTH));
          resultList.add(workCalendar);
      }
      return resultList;
  }
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