Chapter 2. Model the domain objects

The goal of the Red Hat Business Optimizer timetable project is to assign each lesson to a time slot and a room. To do this, add three classes, Timeslot, Lesson, and Room, as shown in the following diagram:

timeTableClassDiagramPure

Timeslot

The Timeslot class represents a time interval when lessons are taught, for example, Monday 10:30 - 11:30 or Tuesday 13:30 - 14:30. In this example, all time slots have the same duration and there are no time slots during lunch or other breaks.

A time slot has no date because a high school schedule just repeats every week. There is no need for continuous planning. A Timeslot is called a problem fact because no Timeslot instances change during solving. Such classes do not require any Red Hat Business Optimizer specific annotations.

Room

The Room class represents a location where lessons are taught, for example, Room A or Room B. In this example, all rooms are without capacity limits and they can accommodate all lessons.

Room instances do not change during solving so Room is also a problem fact.

Lesson

During a lesson, represented by the Lesson class, a teacher teaches a subject to a group of students, for example, Math by A.Turing for 9th grade or Chemistry by M.Curie for 10th grade. If a subject is taught multiple times each week by the same teacher to the same student group, there are multiple Lesson instances that are only distinguishable by id. For example, the 9th grade has six math lessons a week.

During solving, Red Hat Business Optimizer changes the timeslot and room fields of the Lesson class to assign each lesson to a time slot and a room. Because Red Hat Business Optimizer changes these fields, Lesson is a planning entity:

timeTableClassDiagramAnnotated

Most of the fields in the previous diagram contain input data, except for the orange fields. A lesson’s timeslot and room fields are unassigned (null) in the input data and assigned (not null) in the output data. Red Hat Business Optimizer changes these fields during solving. Such fields are called planning variables. In order for Red Hat Business Optimizer to recognize them, both the timeslot and room fields require an @PlanningVariable annotation. Their containing class, Lesson, requires an @PlanningEntity annotation.

Procedure

  1. Create the src/main/java/com/example/domain/Timeslot.java class:

    package com.example.domain;
    
    import java.time.DayOfWeek;
    import java.time.LocalTime;
    
    public class Timeslot {
    
        private DayOfWeek dayOfWeek;
        private LocalTime startTime;
        private LocalTime endTime;
    
        private Timeslot() {
        }
    
        public Timeslot(DayOfWeek dayOfWeek, LocalTime startTime, LocalTime endTime) {
            this.dayOfWeek = dayOfWeek;
            this.startTime = startTime;
            this.endTime = endTime;
        }
    
        @Override
        public String toString() {
            return dayOfWeek + " " + startTime.toString();
        }
    
        // ********************************
        // Getters and setters
        // ********************************
    
        public DayOfWeek getDayOfWeek() {
            return dayOfWeek;
        }
    
        public LocalTime getStartTime() {
            return startTime;
        }
    
        public LocalTime getEndTime() {
            return endTime;
        }
    
    }

    Notice the toString() method keeps the output short so it is easier to read Red Hat Business Optimizer’s DEBUG or TRACE log, as shown later.

  2. Create the src/main/java/com/example/domain/Room.java class:

    package com.example.domain;
    
    public class Room {
    
        private String name;
    
        private Room() {
        }
    
        public Room(String name) {
            this.name = name;
        }
    
        @Override
        public String toString() {
            return name;
        }
    
        // ********************************
        // Getters and setters
        // ********************************
    
        public String getName() {
            return name;
        }
    
    }
  3. Create the src/main/java/com/example/domain/Lesson.java class:

    package com.example.domain;
    
    import org.optaplanner.core.api.domain.entity.PlanningEntity;
    import org.optaplanner.core.api.domain.variable.PlanningVariable;
    
    @PlanningEntity
    public class Lesson {
    
        private Long id;
    
        private String subject;
        private String teacher;
        private String studentGroup;
    
        @PlanningVariable(valueRangeProviderRefs = "timeslotRange")
        private Timeslot timeslot;
    
        @PlanningVariable(valueRangeProviderRefs = "roomRange")
        private Room room;
    
        private Lesson() {
        }
    
        public Lesson(Long id, String subject, String teacher, String studentGroup) {
            this.id = id;
            this.subject = subject;
            this.teacher = teacher;
            this.studentGroup = studentGroup;
        }
    
        @Override
        public String toString() {
            return subject + "(" + id + ")";
        }
    
        // ********************************
        // Getters and setters
        // ********************************
    
        public Long getId() {
            return id;
        }
    
        public String getSubject() {
            return subject;
        }
    
        public String getTeacher() {
            return teacher;
        }
    
        public String getStudentGroup() {
            return studentGroup;
        }
    
        public Timeslot getTimeslot() {
            return timeslot;
        }
    
        public void setTimeslot(Timeslot timeslot) {
            this.timeslot = timeslot;
        }
    
        public Room getRoom() {
            return room;
        }
    
        public void setRoom(Room room) {
            this.room = room;
        }
    
    }

    The Lesson class has an @PlanningEntity annotation, so Red Hat Business Optimizer knows that this class changes during solving because it contains one or more planning variables.

    The timeslot field has an @PlanningVariable annotation, so Red Hat Business Optimizer knows that it can change its value. In order to find potential Timeslot instances to assign to this field, Red Hat Business Optimizer uses the valueRangeProviderRefs property to connect to a value range provider that provides a List<Timeslot> to pick from. See Chapter 4, Gather the domain objects in a planning solution for information about value range providers.

    The room field also has an @PlanningVariable annotation for the same reasons.