Arrays of Arrays in Java

Question

This is a nasty one for me... I'm a PHP guy working in Java on a JSP project. I know how to do what I'm attempting through too much code and a complete lack of finesse.

I'd prefer to do it right. Here is the situation:

I'm writing a small display to show customers what days they can water their lawns based on their watering group (ABCDE) and what time of year it is. Our seasons look like this: Summer (5-1 to 8-31) Spring (3-1 to 4-30) Fall (9-1 to 10-31) Winter (11-1 to 2-28)

An example might be:

If I'm in group A, here would be my allowed times: Winter: Mondays only Spring: Tues, Thurs, Sat Summer: Any Day Fall: Tues, Thurs, Sat

If I was writing this in PHP I would use arrays like this:

//M=Monday,t=Tuesday,T=Thursday.... etc
$schedule["A"]["Winter"]='M';
$schedule["A"]["Spring"]='tTS';
$schedule["A"]["Summer"]='Any';
$schedule["A"]["Fall"]='tTS';
$schedule["B"]["Winter"]='t';

I COULD make the days arrays (array("Tuesday","Thursday","Saturday")) etc, but it is not necessary for what I'm really trying to accomplish.

I will also need to setup arrays to determine what season I'm in:

$seasons["Summer"]["start"]=0501;
$seasons["Summer"]["end"]=0801;

Can anyone suggest a really cool way to do this? I will have today's date and the group letter. I will need to get out of my function a day (M) or a series of days (tTS), (Any).

Answer 1

You could do essentially the same code with Hashtables (or some other Map):

Hashtable<String, Hashtable<String, String>> schedule
    = new Hashtable<String, Hashtable<String, String>>();
schedule.put("A", new Hashtable<String, String>());
schedule.put("B", new Hashtable<String, String>());
schedule.put("C", new Hashtable<String, String>());
schedule.put("D", new Hashtable<String, String>());
schedule.put("E", new Hashtable<String, String>());

schedule.get("A").put("Winter", "M");
schedule.get("A").put("Spring", "tTS");
// Etc...

Not as elegant, but then again, Java isn't a dynamic language, and it doesn't have hashes on the language level.

Note: You might be able to do a better solution, this just popped in my head as I read your question.

Answer 2

Don't try to be as dynamic as PHP is. You could try to first define what you need.

interface Season
{
    public string getDays();
}

interface User
{
    public Season getWinter();
    public Season getSpring();
    public Season getSummer();
    public Season getFall();
}

interface UserMap
{
    public User getUser(string name);
}

And please, read the documentation of Hashtable before using it. This class is synchronized which means that each call is protected against multithreading which really slows the access when you don't need the extra protection. Please use any Map implementation instead like HashMap or TreeMap.

Answer 3

It seems like everyone is trying to find the Java way to do it like you're doing it in PHP, instead of the way it ought to be done in Java. Just consider each piece of your array an object, or, at the very least, the first level of the array as an object and each sub level as variables inside the object. The build a data structure that you populate with said objects and access the objects through the data structure's given accessors.

Something like:

class Schedule
{
  private String group;
  private String season;
  private String rundays;
  public Schedule() { this.group = null; this.season = null; this.rundays= null; }
  public void setGroup(String g) { this.group = g; }
  public String getGroup() { return this.group; }
  ...
}

public ArrayList<Schedule> schedules = new ArrayList<Schedule>();
Schedule s = new Schedule();
s.setGroup(...);
...
schedules.add(s);
...

Of course that probably isn't right either. I'd make each season an object, and maybe each weekday list as an object too. Anyway, its more easily reused, understood, and extensible than a hobbled-together Hashtable that tries to imitate your PHP code. Of course, PHP has objects too, and you should use them in a similar fashion instead of your uber-arrays, wherever possible. I do understand the temptation to cheat, though. PHP makes it so easy, and so fun!

Answer 4

Here's one way it could look like, you can figure the rest out:

A = new Group();
A.getSeason(Seasons.WINTER).addDay(Days.MONDAY);
A.getSeason(Seasons.SPRING).addDay(Days.TUESDAY).addDay(Days.THURSDAY);
A.getSeason(Seasons.SPRING).addDays(Days.MONDAY, Days.TUESDAY, ...);

schedule = new Schedule();
schedule.addWateringGroup( A );

Answer 5

I'm not a Java programmer, but getting away from Java and just thinking in terms that are more language agnostic - a cleaner way to do it might be to use either constants or enumerated types. This should work in any langauge that supports multi-dimensional arrays.

If using named constants, where, for example:

int A = 0;
int B = 1;
int C = 2;
int D = 3;

int Spring = 0; 
int Summer = 1;
int Winter = 2; 
int Fall = 3;
...

Then the constants serve as more readable array subscripts:

schedule[A][Winter]="M";
schedule[A][Spring]="tTS";
schedule[A][Summer]="Any";
schedule[A][Fall]="tTS";
schedule[B][Winter]="t";

Using enumerated types:

enum groups
{
  A = 0,
  B = 1,
  C = 2,
  D = 3
}

enum seasons
{
  Spring = 0,
  Summer = 1,
  Fall = 2,
  Winter = 3
}
...
schedule[groups.A][seasons.Winter]="M";
schedule[groups.A][seasons.Spring]="tTS";
schedule[groups.A][seasons.Summer]="Any";
schedule[groups.A][seasons.Fall]="tTS";
schedule[groups.B][seasons.Winter]="t";

Answer 6

I'm totally at a loss as to why some of you seem to think that throwing gobs of objects at the code is the way to go. For example, there are exactly four seasons, and they don't do or store anything. How does it simplify anything to make them objects? Wing is quite right that these should probably be constants (or maybe enums).

What Bruce needs, at it's heart, is simply a lookup table. He doesn't need a hierarchy of objects and interfaces; he needs a way to look up a schedule based on a season and a group identifier. Turning things into objects only makes sense if they have responsibilities or state. If they have neither, then they are simply identifiers, and building special objects for them just makes the codebase larger.

You could build, e.g., Group objects that each contain a set of schedule strings (one for each season), but if all the Group object does is provide lookup functionality, then you've reinvented the lookup table in a much less intuitive fashion. If he has to look up the group, and then lookup the schedule, all he has is a two-step lookup table that took longer to code, is more likely to be buggy, and will be harder to maintain.

Answer 7

I'm with those that suggest encapsulating function in objects.

import java.util.Date;
import java.util.Map;
import java.util.Set;

public class Group {

    private String groupName;

    private Map<Season, Set<Day>> schedule;

    public String getGroupName() {
        return groupName;
    }

    public void setGroupName(String groupName) {
        this.groupName = groupName;
    }

    public Map<Season, Set<Day>> getSchedule() {
        return schedule;
    }

    public void setSchedule(Map<Season, Set<Day>> schedule) {
        this.schedule = schedule;
    }

    public String getScheduleFor(Date date) {
        Season now = Season.getSeason(date);
        Set<Day> days = schedule.get(now);
        return Day.getDaysForDisplay(days);
    }

}

EDIT: Also, your date ranges don't take leap years into account:

Our seasons look like this: Summer (5-1 to 8-31) Spring (3-1 to 4-30) Fall (9-1 to 10-31) Winter (11-1 to 2-28)

Answer 8

I agree that you should definitely put this logic behind the clean interface of:

public String lookupDays(String group, String date);

but maybe you should stick the data in a properties file. I'm not against hardcoding this data in your source files but, as you noticed, Java can be pretty wordy when it comes to nested Collections. Your file might looks like:

A.Summer=M
A.Spring=tTS
B.Summer=T

Usually I don't like to move static data like this to an external file because it increases the "distance" between the data and the code that uses it. However, whenever you're dealing with nested Collections, especially maps, things can get real ugly, real fast.

If you don't like this idea, maybe you can do something like this:

public class WaterScheduler
{
  private static final Map<String, String> GROUP2SEASON = new HashMap<String, String>();
  static
  {
    addEntry("A", "Summer", "M");
    addEntry("A", "Spring", "tTS");
    addEntry("B", "Summer", "T");
  }

  private static void addEntry(String group, String season, String value)
  {
    GROUP2SEASON.put(group + "." + season, value);
  }

}

You lose some readability but at least the data is closer to where it's going to be used.

Answer 9

A better solution would perhaps be to put all that data into a database, instead of hard-coding it in your sources or using properties files.

Using a database will be much easier to maintain, and there are a variety of free database engines to choose from.

Two of these database engines are implemented entirely in Java and can be embedded in an application just by including a jar file. It's a little heavyweight, sure, but it's a lot more scalable and easier to maintain. Just because there are 20 records today doesn't mean there won't be more later due to changing requirements or feature creep.

If, in a few weeks or months, you decide you want to add, say, time of day watering restrictions, it will be much easier to add that functionality if you're already using a database. Even if that never happens, then you've spent a few hours learning how to embed a database in an application.

Answer 10

Does the "date" have to be a parameter? If you're just showing the current watering schedule the WateringSchedule class itself can figure out what day it is, and therefore what season it is. Then just have a method which returns a map where the Key is the group letter. Something like:

public Map<String,List<String>> getGroupToScheduledDaysMap() {
  // instantiate a date or whatever to decide what Map to return
}

Then in the JSP page

<c:forEach var="day" items="${scheduler.groupToScheduledDaysMap["A"]}">
   ${day}
</c:forEach>

If you need to show the schedules for more than one season, you should have a method in the WateringSchedule class that returns a map where Seasons are the keys, and then Maps of groupToScheduledDays are the values.

Answer 11

There is no pretty solution. Java just doesn't do things like this well. Mike's solution is pretty much the way to do it if you want strings as the indices (keys). Another option if the hash-of-hashes setup is too ugly is to append the strings together (shamelessly stolen from Mike and modified):

Hashtable<String, String> schedule = new Hashtable<String, String>();
schedule.put("A-Winter", "M");
schedule.put("A-Spring", "tTS");

and then lookup:

String val = schedule.get(group + "-" + season);

If you're unhappy with the general ugliness (and I don't blame you), put it all behind a method call:

String whenCanIWater(String group, Date date) { /* ugliness here */ }

Answer 12

tl;dr

Using modern Java language features and classes, define your own enums to represent the seasons and groups.

Schedule.daysForGroupOnDate( 
    Group.D , 
    LocalDate.now()
)  

That method yields a Set of DayOfWeek enum objects (not mere text!), such as DayOfWeek.TUESDAY & DayOfWeek.THURSDAY.

Modern Java

Can anyone suggest a really cool way to do this?

Yes.

Modern Java has built-in classes, collections, and enums to help you with this problem.

The java.time framework built into Java offers the Month enum and DayOfWeek enum.

The EnumSet and EnumMap provide implementations of Set and Map that are optimized for use with enums for fast execution in very little memory.

You can define your own enums to represent your season and your groups (A, B, and so on). The enum facility in Java is far more useful and powerful than seen in other languages. If not familiar, see the Oracle Tutorial.

The simple syntax of defining your own enums actually provides much of the functionality needed to solve this Question, eliminating some complicated coding. New literals syntax for sets and maps in the collections factory methods in Java 9 (JEP 269) makes the code even simpler now.

Here is a complete working app. Note how little code there is for algorithms. Defining your own custom enums does most of the heavy-lifting.

One caveat with this app code: It assumes nothing changes in your business definitions, or at least if there is a change you only care about the current rules, “latest is greatest”. If your rules change over time and you need to represent all the past, present, and future versions, I would build a very different app, probably with a database to store the rules. But this app here solves the Question as asked.

Season

Represent your season as an enum Season. Each season object holds a List of Month enum objects that define the length of that particular season. The new List.of syntax added to Java 9 defines an immutable list in a literals syntax via static factory methods.

package com.basilbourque.watering;

import java.time.LocalDate;
import java.time.Month;
import java.util.EnumSet;
import java.util.List;
import java.util.Set;

public enum Season
{
    SPRING( List.of( Month.MARCH , Month.APRIL ) ),
    SUMMER( List.of( Month.MAY , Month.JUNE, Month.JULY , Month.AUGUST ) ),
    FALL( List.of( Month.SEPTEMBER , Month.OCTOBER ) ),
    WINTER( List.of( Month.NOVEMBER , Month.DECEMBER , Month.JANUARY , Month.FEBRUARY ) );

    private List< Month > months;

    // Constructor
    Season ( List < Month > monthsArg )
    {
        this.months = monthsArg;
    }

    public List < Month > getMonths ( )
    {
        return this.months;
    }

    // For any given month, determine the season.
    static public Season ofLocalMonth ( Month monthArg )
    {
        Season s = null;
        for ( Season season : EnumSet.allOf( Season.class ) )
        {
            if ( season.getMonths().contains( monthArg ) )
            {
                s = season;
                break; // Bail out of this FOR loop.
            }
        }
        return s;
    }

    // For any given date, determine the season.
    static public Season ofLocalDate ( LocalDate localDateArg )
    {
        Month month = localDateArg.getMonth();
        Season s = Season.ofLocalMonth( month );
        return s;
    }

    // Run `main` for demo/testing.
    public static void main ( String[] args )
    {
        // Dump all these enum objects to console.
        for ( Season season : EnumSet.allOf( Season.class ) )
        {
            System.out.println( "Season: " + season.toString() + " = " + season.getMonths() );
        }
    }
}

Group

Represent each grouping of customers’ lawns/yards, (A, B, C, D, E) as an enum named Group. Each of these enum objects holds a Map, mapping a Season enum object to a Set of DayOfWeek enum objects. For example, Group.A in Season.SPRING allows watering on two days, DayOfWeek.TUESDAY & DayOfWeek.THURSDAY.

package com.basilbourque.watering;

import java.time.DayOfWeek;
import java.util.EnumMap;
import java.util.EnumSet;
import java.util.Map;
import java.util.Set;

public enum Group
{
    A(
            Map.of(
                    Season.SPRING , EnumSet.of( DayOfWeek.TUESDAY , DayOfWeek.THURSDAY ) ,
                    Season.SUMMER , EnumSet.allOf( DayOfWeek.class ) ,
                    Season.FALL , EnumSet.of( DayOfWeek.TUESDAY , DayOfWeek.THURSDAY ) ,
                    Season.WINTER , EnumSet.of( DayOfWeek.TUESDAY )
            )
    ),
    B(
            Map.of(
                    Season.SPRING , EnumSet.of( DayOfWeek.FRIDAY ) ,
                    Season.SUMMER , EnumSet.allOf( DayOfWeek.class ) ,
                    Season.FALL , EnumSet.of( DayOfWeek.TUESDAY , DayOfWeek.FRIDAY ) ,
                    Season.WINTER , EnumSet.of( DayOfWeek.FRIDAY )
            )
    ),
    C(
            Map.of(
                    Season.SPRING , EnumSet.of( DayOfWeek.MONDAY ) ,
                    Season.SUMMER , EnumSet.allOf( DayOfWeek.class ) ,
                    Season.FALL , EnumSet.of( DayOfWeek.MONDAY , DayOfWeek.TUESDAY ) ,
                    Season.WINTER , EnumSet.of( DayOfWeek.MONDAY )
            )
    ),
    D(
            Map.of(
                    Season.SPRING , EnumSet.of( DayOfWeek.WEDNESDAY , DayOfWeek.FRIDAY ) ,
                    Season.SUMMER , EnumSet.allOf( DayOfWeek.class ) ,
                    Season.FALL , EnumSet.of( DayOfWeek.FRIDAY ) ,
                    Season.WINTER , EnumSet.of( DayOfWeek.WEDNESDAY )
            )
    ),
    E(
            Map.of(
                    Season.SPRING , EnumSet.of( DayOfWeek.TUESDAY ) ,
                    Season.SUMMER , EnumSet.allOf( DayOfWeek.class ) ,
                    Season.FALL , EnumSet.of( DayOfWeek.TUESDAY , DayOfWeek.WEDNESDAY ) ,
                    Season.WINTER , EnumSet.of( DayOfWeek.WEDNESDAY )
            )
    );

    private Map < Season, Set < DayOfWeek > > map;

    // Constructor
    Group ( Map < Season, Set < DayOfWeek > > mapArg )
    {
        this.map = mapArg;
    }

    // Getter
    private Map < Season, Set < DayOfWeek > > getMapOfSeasonToDaysOfWeek() {
        return this.map ;
    }

    // Retrieve the DayOfWeek set for this particular Group.
    public Set<DayOfWeek> daysForSeason (Season season ) {
        Set<DayOfWeek> days =   this.map.get( season ) ; // Retrieve the value (set of days) for this key (a season) for this particular grouping of lawns/yards.
        return days;
    }



    // Run `main` for demo/testing.
    public static void main ( String[] args )
    {
        // Dump all these enum objects to console.
        for ( Group group : EnumSet.allOf( Group.class ) )
        {
            System.out.println( "Group: " + group.toString() + " = " + group.getMapOfSeasonToDaysOfWeek() );
        }
    }

}

Schedule

Pull it all together in this Schedule class.

This class makes use of the two enums defined above to get useful work done. The only method implemented so far tells you which days of the week are allowed for a particular group on a particular date. The method determines which Season applies for that date.

Run the main method here to dump the contents of our two enums, and report on the days of the week allowing for watering in each group for a particular hard-coded date.

package com.basilbourque.watering;

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.ZoneId;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoField;
import java.time.temporal.IsoFields;
import java.util.EnumSet;
import java.util.Set;

public class Schedule
{
    static private DateTimeFormatter isoWeekFormatter = DateTimeFormatter.ofPattern( "uuuu-'W'ww" ) ;

    static public Set < DayOfWeek > daysForGroupOnDate ( Group group , LocalDate localDate )
    {
        Season season = Season.ofLocalDate( localDate );
        Set < DayOfWeek > days = group.daysForSeason( season );
        return days;
    }

    // Run `main` for demo/testing.
    public static void main ( String[] args )
    {
        Season.main( null  );
        Group.main( null  );
        // Dump all these enum objects to console.
        for ( Group group : EnumSet.allOf( Group.class ) )
        {
            LocalDate localDate = LocalDate.now( ZoneId.of( "Africa/Tunis" ) );
            Set < DayOfWeek > days = Schedule.daysForGroupOnDate( group , localDate );
            String week = localDate.format( Schedule.isoWeekFormatter  ) ; // Standard ISO 8601 week, where week number one has the first Thursday of the calendar year, and week starts on Monday, so year is either 52 or 53 weeks long.
            String message = "Group " + group + " – Watering days on " + localDate + " week # " + week + " is: " + days;
            System.out.println( message );
        }
    }
}

Console

When running Schedule.main, we see this dumped to the console.

Season: SPRING = [MARCH, APRIL]

Season: SUMMER = [MAY, JUNE, JULY, AUGUST]

Season: FALL = [SEPTEMBER, OCTOBER]

Season: WINTER = [NOVEMBER, DECEMBER, JANUARY, FEBRUARY]

Group: A = {SPRING=[TUESDAY, THURSDAY], FALL=[TUESDAY, THURSDAY], SUMMER=[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY], WINTER=[TUESDAY]}

Group: B = {SPRING=[FRIDAY], FALL=[TUESDAY, FRIDAY], SUMMER=[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY], WINTER=[FRIDAY]}

Group: C = {SPRING=[MONDAY], FALL=[MONDAY, TUESDAY], SUMMER=[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY], WINTER=[MONDAY]}

Group: D = {SPRING=[WEDNESDAY, FRIDAY], FALL=[FRIDAY], SUMMER=[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY], WINTER=[WEDNESDAY]}

Group: E = {SPRING=[TUESDAY], FALL=[TUESDAY, WEDNESDAY], SUMMER=[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY], WINTER=[WEDNESDAY]}

Group A – Watering days on 2018-01-30 week # 2018-W05 is: [TUESDAY]

Group B – Watering days on 2018-01-30 week # 2018-W05 is: [FRIDAY]

Group C – Watering days on 2018-01-30 week # 2018-W05 is: [MONDAY]

Group D – Watering days on 2018-01-30 week # 2018-W05 is: [WEDNESDAY]

Group E – Watering days on 2018-01-30 week # 2018-W05 is: [WEDNESDAY]

ISO 8601 week

You may find it helpful to learn about the ISO 8601 standard for a definition of week. The standard gives a specific meaning to “week” and defines a textual format for representing a particular week or a particular day within that week.

For working with such weeks within Java, consider adding the ThreeTen-Extra library to your project to make use of the YearWeek class.

LocalDate

The LocalDate class represents a date-only value without time-of-day and without time zone.

A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec.

If no time zone is specified, the JVM implicitly applies its current default time zone. That default may change at any moment, so your results may vary. Better to specify your desired/expected time zone explicitly as an argument.

Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).

ZoneId z = ZoneId.of( "America/Montreal" ) ;  
LocalDate today = LocalDate.now( z ) ;

If you want to use the JVM’s current default time zone, ask for it and pass as an argument. If omitted, the JVM’s current default is applied implicitly. Better to be explicit.

ZoneId z = ZoneId.systemDefault() ;  // Get JVM’s current default time zone.

Or specify a date. You may set the month by a number, with sane numbering 1-12 for January-December.

LocalDate ld = LocalDate.of( 1986 , 2 , 23 ) ;  // Years use sane direct numbering (1986 means year 1986). Months use sane numbering, 1-12 for January-December.

Or, better, use the Month enum objects pre-defined, one for each month of the year. Tip: Use these Month objects throughout your codebase rather than a mere integer number to make your code more self-documenting, ensure valid values, and provide type-safety.

LocalDate ld = LocalDate.of( 1986 , Month.FEBRUARY , 23 ) ;

Immutable collections

The lists, sets, and maps seen above should ideally be immutable collections, as changing the membership of those collections is likely to be confounding and erroneous.

The new Java 9 syntax List.of and Map.of are already promised to be immutable. However, in our case the Map should ideally be an EnumMap for efficiency of performance and memory. The current implementation of Map.of and Set.of apparently do not detect the use of enums as members and automatically optimize with an internal use of EnumMap and EnumSet. There is a OpenJDK issue opened to consider such issues: consider enhancements to EnumMap and EnumSet.

One way to obtain an immutable EnumSet and immutable EnumMap is through the Google Guava library:

• Sets.immutableEnumSet( … )
• Maps.immutableEnumMap( … )

The results of each use an underlying EnumSet/EnumMap. Operations such as get and put throw an exception. So you get the enum-related optimizations along with immutability.

Here are the Season and Group classes seen above modified to use the Google Guava 23.6 library.

Season with immutability

package com.basilbourque.watering;

import java.time.LocalDate;
import java.time.Month;
import java.util.EnumSet;
import java.util.List;

public enum Season
{
    SPRING( List.of( Month.MARCH , Month.APRIL ) ),  // `List.of` provides literals-style syntax, and returns an immutable `List`. New in Java 9.
    SUMMER( List.of( Month.MAY , Month.JUNE, Month.JULY , Month.AUGUST ) ),
    FALL( List.of( Month.SEPTEMBER , Month.OCTOBER ) ),
    WINTER( List.of( Month.NOVEMBER , Month.DECEMBER , Month.JANUARY , Month.FEBRUARY ) );

    private List< Month > months;

    // Constructor
    Season ( List < Month > monthsArg )
    {
        this.months = monthsArg;
    }

    public List < Month > getMonths ( )
    {
        return this.months;
    }

    // For any given month, determine the season.
    static public Season ofLocalMonth ( Month monthArg )
    {
        Season s = null;
        for ( Season season : EnumSet.allOf( Season.class ) )
        {
            if ( season.getMonths().contains( monthArg ) )
            {
                s = season;
                break; // Bail out of this FOR loop.
            }
        }
        return s;
    }

    // For any given date, determine the season.
    static public Season ofLocalDate ( LocalDate localDateArg )
    {
        Month month = localDateArg.getMonth();
        Season s = Season.ofLocalMonth( month );
        return s;
    }

    // Run `main` for demo/testing.
    public static void main ( String[] args )
    {
        // Dump all these enum objects to console.
        for ( Season season : EnumSet.allOf( Season.class ) )
        {
            System.out.println( "Season: " + season.toString() + " = " + season.getMonths() );
        }
    }
}

Group with immutability

package com.basilbourque.watering;

import com.google.common.collect.Maps;
import com.google.common.collect.Sets;

import java.time.DayOfWeek;
import java.util.EnumSet;
import java.util.Map;
import java.util.Set;

public enum Group
{
    A(
            Maps.immutableEnumMap(
                    Map.of(  // `Map.of` provides literals-style syntax, and returns an immutable `Map`. New in Java 9.
                            Season.SPRING , Sets.immutableEnumSet( DayOfWeek.TUESDAY , DayOfWeek.THURSDAY ) ,
                            Season.SUMMER , Sets.immutableEnumSet( EnumSet.allOf( DayOfWeek.class ) ) ,
                            Season.FALL , Sets.immutableEnumSet( DayOfWeek.TUESDAY , DayOfWeek.THURSDAY ) ,
                            Season.WINTER , Sets.immutableEnumSet( DayOfWeek.TUESDAY )
                    )
            )
    ),

    B(
            Maps.immutableEnumMap(
                    Map.of(
                            Season.SPRING , Sets.immutableEnumSet( DayOfWeek.FRIDAY ) ,
                            Season.SUMMER , Sets.immutableEnumSet( EnumSet.allOf( DayOfWeek.class ) ) ,
                            Season.FALL , Sets.immutableEnumSet( DayOfWeek.TUESDAY , DayOfWeek.FRIDAY ) ,
                            Season.WINTER , Sets.immutableEnumSet( DayOfWeek.FRIDAY )
                    )
            )
    ),

    C(
            Maps.immutableEnumMap(
                    Map.of(
                            Season.SPRING , Sets.immutableEnumSet( DayOfWeek.MONDAY ) ,
                            Season.SUMMER , Sets.immutableEnumSet( EnumSet.allOf( DayOfWeek.class ) ) ,
                            Season.FALL , Sets.immutableEnumSet( DayOfWeek.MONDAY , DayOfWeek.TUESDAY ) ,
                            Season.WINTER , Sets.immutableEnumSet( DayOfWeek.MONDAY )
                    )
            )
    ),

    D(
            Maps.immutableEnumMap(
                    Map.of(
                            Season.SPRING , Sets.immutableEnumSet( DayOfWeek.WEDNESDAY , DayOfWeek.FRIDAY ) ,
                            Season.SUMMER , Sets.immutableEnumSet( EnumSet.allOf( DayOfWeek.class ) ) ,
                            Season.FALL , Sets.immutableEnumSet( DayOfWeek.FRIDAY ) ,
                            Season.WINTER , Sets.immutableEnumSet( DayOfWeek.WEDNESDAY )
                    )
            )
    ),

    E(
            Maps.immutableEnumMap(
                    Map.of(
                            Season.SPRING , Sets.immutableEnumSet( DayOfWeek.TUESDAY ) ,
                            Season.SUMMER , Sets.immutableEnumSet( EnumSet.allOf( DayOfWeek.class ) ) ,
                            Season.FALL , Sets.immutableEnumSet( EnumSet.of( DayOfWeek.TUESDAY , DayOfWeek.WEDNESDAY ) ) ,
                            Season.WINTER , Sets.immutableEnumSet( DayOfWeek.WEDNESDAY )
                    )
            )
    );

    private Map < Season, Set < DayOfWeek > > map;

    // Constructor
    Group ( Map < Season, Set < DayOfWeek > > mapArg )
    {
        this.map = mapArg;
    }

    // Getter
    private Map < Season, Set < DayOfWeek > > getMapOfSeasonToDaysOfWeek ( )
    {
        return this.map;
    }

    // Retrieve the DayOfWeek set for this particular Group.
    public Set < DayOfWeek > daysForSeason ( Season season )
    {
        Set < DayOfWeek > days = this.map.get( season ); // Retrieve the value (set of days) for this key (a season) for this particular grouping of lawns/yards.
        return days;
    }

    // Run `main` for demo/testing.
    public static void main ( String[] args )
    {
        // Dump all these enum objects to console.
        for ( Group group : EnumSet.allOf( Group.class ) )
        {
            System.out.println( "Group: " + group.toString() + " = " + group.getMapOfSeasonToDaysOfWeek() );
        }
    }

}

---

About java.time

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

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

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android, the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use 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.