Create a custom collection of days-of-the-week in

Question

I want a custom calendar like this:

enum TradingDays {Monday, Tuesday, Wednesday, Thursday, Friday};

Then I need to iterate over it and check if a particular enum element is the day of week TODAY. The problem is that the JAVA calendar does not match to days of week from my calendar. So:

Calendar now = Calendar.getInstance();

TradingDays.Monday is not equal to any of now.get(Calendar.DAY_OF_WEEK);

So how do I assign Monday, Tuesday etc from my calendar TradingDays the same type (in this case an integer value) from the JAVA calendar?

P.S. I need to have that calendar TradingDays like that because it is then shown to the user so he/she chooses on which days to trade.

Answer 1

Create a constructor inside your enum which takes an int value:

public class CalendarMain {

enum TradingDays {
Monday(2), Tuesday(3), Wednesday(4), Thursday(5), Friday(6);

private int value;

private TradingDays(int value) {
    this.value = value;
}
};

public static void main(String[] args) {

Calendar now = Calendar.getInstance();

if (now.get(Calendar.DAY_OF_WEEK) == TradingDays.Tuesday.value) {
    // Chose Wednesday as the day to trade
    System.out.println(now.get(Calendar.DAY_OF_WEEK));
    System.out.println(TradingDays.Tuesday.value);
}
}

}

Answer 2

tl;dr

Set<DayOfWeek> tradingDays = EnumSet.range( DayOfWeek.MONDAY , DayOfWeek.FRIDAY ) ; // Mon, Tue, Wed, Thu, & Fri.
Boolean todayIsTradingDay = tradingDays.contains( LocalDate.now( ZoneId.of( "America/Montreal" ) ).getDayOfWeek() ) ;

Details

This functionality is built into Java.

java.time.DayOfWeek enum

Java includes the java.time.DayOfWeek enum.

Pass around objects of this enum rather than mere integers 1-7 to make your code more self-documenting, provide type-safety, and ensure a range of valid values.

invoices.printReportForDayOfWeek( DayOfWeek.MONDAY );

Numbering is 1-7 for Monday to Sunday, per ISO 8601 standard.

Get a localized name of the day by calling getDisplayName.

String output = DayOfWeek.MONDAY.getDisplayName( TextStyle.FULL , Locale.CANADA_FRENCH ) ;  // Or Locale.US, etc.

Collection of day-of-week objects

To track multiple days of the week, use an EnumSet (implementation of Set) or EnumMap (implementation of Map).

Set<DayOfWeek> weekend = EnumSet.of( DayOfWeek.SATURDAY , DayOfWeek.SUNDAY ) ;
…
Boolean todayIsWeekend = weekend.contains( LocalDate.now( ZoneId.of( "America/Montreal" ) ).getDayOfWeek() ) ;

Or in the case of this Question specifically, a collection of the weekdays. Perhaps define as a static final constant if the definition does not change during execution of the app.

static final Set<DayOfWeek> tradingDays = EnumSet.of( DayOfWeek.MONDAY , DayOfWeek.TUESDAY , DayOfWeek.WEDNESDAY , DayOfWeek.THURSDAY , DayOfWeek.FRIDAY ) ;

Or make that even shorter by defining an EnumSet as a range of enum objects defined in a sequential order. Specify MONDAY & FRIDAY and let EnumSet fill in the values in between. Use EnumSet.range.

static final Set<DayOfWeek> tradingDays = EnumSet.range( DayOfWeek.MONDAY , DayOfWeek.FRIDAY ) ; // Mon, Tue, Wed, Thu, & Fri.

Then test for today. Note that a time zone is crucial in determining the current date. For any given moment the date varies around the globe by zone.

ZoneId zoneId = ZoneId.of( "America/Montreal" ) ;
LocalDate today = LocalDate.now( zoneId ) ;
DayOfWeek todayDow = today.getDayOfWeek() ;
Boolean todayIsTradingDay = tradingDays.contains( todayDow ).getDayOfWeek() ) ;

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the old troublesome date-time classes such as java.util.Date, .Calendar, & java.text.SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to java.time.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations.

Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport and further adapted to Android in ThreeTenABP.

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Answer 3

You can try using a constructor inside your enum, like in this example:

    public enum Currency {
    PENNY(1),
    NICKLE(5),
    DIME(10),
    QUARTER(25);

     private final int value;

     private Currency(int value) {
         this.value=value;
     }
}

While iterating you can use coin.value like this:

for(Currency coin: Currency.values()){

        System.out.println(coin+" "+coin.value);
        if(coin.value==1){
            System.out.println("THIS is the PENNY");
        }

    }

Which output is:

PENNY 1

THIS is the PENNY

NICKLE 5

DIME 10

QUARTER 25

Answer 4

With enums you can do a lot of things that probably are familiar to you:

File TradingDays.java:

public enum TradingDays {
    Monday(Calendar.MONDAY),
    Tuesday(Calendar.TUESDAY),
    Wednesday(Calendar.WEDNESDAY),
    Thursday(Calendar.THURSDAY),
    Friday(Calendar.FRIDAY);

    private int calendarValue;

    TradingDays(int calendarValue) {
        this.calendarValue = calendarValue;
    }

    public static TradingDays today() {
        return fromCalendarValue(Calendar.getInstance().get(Calendar.DAY_OF_WEEK));
    }

    public static TradingDays fromCalendarValue(int calendarValue) {
        for(TradingDays td : TradingDays.values()) {
            if(td.calendarValue == calendarValue) {
                return td;
            }
        }
        throw new IllegalArgumentException(calendarValue + " is not a valid TradingDays calendarValue");
        // or simply return null
    }

};