Hello world Java quick start

This guide walks you through the process of creating a simple Java application with Timefold's constraint solving Artificial Intelligence (AI).

1. What you will build

You will build a command-line application that optimizes a school timetable for students and teachers:

...
INFO  Solving ended: time spent (5000), best score (0hard/9soft), ...
INFO
INFO  |            | Room A     | Room B     | Room C     |
INFO  |------------|------------|------------|------------|
INFO  | MON 08:30  | English    | Math       |            |
INFO  |            | I. Jones   | A. Turing  |            |
INFO  |            | 9th grade  | 10th grade |            |
INFO  |------------|------------|------------|------------|
INFO  | MON 09:30  | History    | Physics    |            |
INFO  |            | I. Jones   | M. Curie   |            |
INFO  |            | 9th grade  | 10th grade |            |
INFO  |------------|------------|------------|------------|
INFO  | MON 10:30  | History    | Physics    |            |
INFO  |            | I. Jones   | M. Curie   |            |
INFO  |            | 10th grade | 9th grade  |            |
INFO  |------------|------------|------------|------------|
...
INFO  |------------|------------|------------|------------|

Your application will assign Lesson instances to Timeslot and Room instances automatically by using AI to adhere to hard and soft scheduling constraints, for example:

A room can have at most one lesson at the same time.
A teacher can teach at most one lesson at the same time.
A student can attend at most one lesson at the same time.
A teacher prefers to teach all lessons in the same room.
A teacher prefers to teach sequential lessons and dislikes gaps between lessons.
A student dislikes sequential lessons on the same subject.

Mathematically speaking, school timetabling is an NP-hard problem. This means it is difficult to scale. Simply brute force iterating through all possible combinations takes millions of years for a non-trivial dataset, even on a supercomputer. Fortunately, AI constraint solvers such as Timefold have advanced algorithms that deliver a near-optimal solution in a reasonable amount of time.

2. Solution source code

Follow the instructions in the next sections to create the application step by step (recommended).

Alternatively, review the completed example:

Complete one of the following tasks:
1. Clone the Git repository:
  $ git clone https://github.com/TimefoldAI/timefold-quickstarts
2. Download an archive.
Find the solution in the hello-world directory.
Follow the instructions in the README file to run the application.

3. Prerequisites

To complete this guide, you need:

JDK 11+ with JAVA_HOME configured appropriately
Apache Maven 3.8.1+ or Gradle 4+
An IDE, such as IntelliJ IDEA, VSCode or Eclipse

4. The build file and the dependencies

Create a Maven or Gradle build file and add these dependencies:

timefold-solver-core (compile scope) to solve the school timetable problem.
timefold-solver-test (test scope) to JUnit test the school timetabling constraints.
A logging implementation, such as logback-classic (runtime scope), to see what Timefold is doing.

If you choose Maven, your pom.xml file has the following content:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>org.acme</groupId>
  <artifactId>timefold-solver-hello-world-school-timetabling-quickstart</artifactId>
  <version>1.0-SNAPSHOT</version>

  <properties>
    <maven.compiler.release>11</maven.compiler.release>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>ai.timefold.solver</groupId>
        <artifactId>timefold-solver-bom</artifactId>
        <version>0.8.42</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
      <dependency>
        <groupId>ch.qos.logback</groupId>
        <artifactId>logback-classic</artifactId>
        <version>1.2.3</version>
      </dependency>
    </dependencies>
  </dependencyManagement>

  <dependencies>
    <dependency>
      <groupId>ai.timefold.solver</groupId>
      <artifactId>timefold-solver-core</artifactId>
    </dependency>
    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <scope>runtime</scope>
    </dependency>

    <!-- Testing -->
    <dependency>
      <groupId>ai.timefold.solver</groupId>
      <artifactId>timefold-solver-test</artifactId>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>exec-maven-plugin</artifactId>
        <version>3.0.0</version>
        <configuration>
          <mainClass>org.acme.schooltimetabling.TimeTableApp</mainClass>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

On the other hand, in Gradle, your build.gradle file has this content:

plugins {
    id "java"
    id "application"
}

def timefoldSolverVersion = "0.8.42"
def logbackVersion = "1.2.9"

group = "org.acme"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
}

dependencies {
    implementation platform("ai.timefold.solver:timefold-solver-bom:${timefoldSolverVersion}")
    implementation "ai.timefold.solver:timefold-solver-core"
    testImplementation "ai.timefold.solver:timefold-solver-test"

    runtimeOnly "ch.qos.logback:logback-classic:${logbackVersion}"
}

java {
    sourceCompatibility = JavaVersion.VERSION_11
    targetCompatibility = JavaVersion.VERSION_11
}

compileJava {
    options.encoding = "UTF-8"
    options.compilerArgs << "-parameters"
}

compileTestJava {
    options.encoding = "UTF-8"
}

application {
    mainClass = "org.acme.schooltimetabling.TimeTableApp"
}

test {
    // Log the test execution results.
    testLogging {
        events "passed", "skipped", "failed"
    }
}

5. Model the domain objects

Your goal is to assign each lesson to a time slot and a room. You will create these classes:

5.1. 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. For simplicity’s sake, 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. So there is no need for continuous planning.

Create the src/main/java/org/acme/schooltimetabling/domain/Timeslot.java class:

package org.acme.schooltimetabling.domain;

import java.time.DayOfWeek;
import java.time.LocalTime;

public class Timeslot {

    private DayOfWeek dayOfWeek;
    private LocalTime startTime;
    private LocalTime endTime;

    public Timeslot() {
    }

    public Timeslot(DayOfWeek dayOfWeek, LocalTime startTime, LocalTime endTime) {
        this.dayOfWeek = dayOfWeek;
        this.startTime = startTime;
        this.endTime = endTime;
    }

    public DayOfWeek getDayOfWeek() {
        return dayOfWeek;
    }

    public LocalTime getStartTime() {
        return startTime;
    }

    public LocalTime getEndTime() {
        return endTime;
    }

    @Override
    public String toString() {
        return dayOfWeek + " " + startTime;
    }

}

Because no Timeslot instances change during solving, a Timeslot is called a problem fact. Such classes do not require any Timefold specific annotations.

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

5.2. Room

The Room class represents a location where lessons are taught, for example, Room A or Room B. For simplicity’s sake, all rooms are without capacity limits and they can accommodate all lessons.

Create the src/main/java/org/acme/schooltimetabling/domain/Room.java class:

package org.acme.schooltimetabling.domain;

public class Room {

    private String name;

    public Room() {
    }

    public Room(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return name;
    }

}

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

5.3. 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 per 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, Timefold changes the timeslot and room fields of the Lesson class, to assign each lesson to a time slot and a room. Because Timefold changes these fields, Lesson is a planning entity:

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. Timefold changes these fields during solving. Such fields are called planning variables. In order for Timefold to recognize them, both the timeslot and room fields require an @PlanningVariable annotation. Their containing class, Lesson, requires an @PlanningEntity annotation.

Create the src/main/java/org/acme/schooltimetabling/domain/Lesson.java class:

package org.acme.schooltimetabling.domain;

import ai.timefold.solver.core.api.domain.entity.PlanningEntity;
import ai.timefold.solver.core.api.domain.lookup.PlanningId;
import ai.timefold.solver.core.api.domain.variable.PlanningVariable;

@PlanningEntity
public class Lesson {

    @PlanningId
    private Long id;

    private String subject;
    private String teacher;
    private String studentGroup;

    @PlanningVariable
    private Timeslot timeslot;
    @PlanningVariable
    private Room room;

    public Lesson() {
    }

    public Lesson(Long id, String subject, String teacher, String studentGroup) {
        this.id = id;
        this.subject = subject;
        this.teacher = teacher;
        this.studentGroup = studentGroup;
    }

    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;
    }

    @Override
    public String toString() {
        return subject + "(" + id + ")";
    }

}

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

The timeslot field has an @PlanningVariable annotation, so Timefold knows that it can change its value. In order to find potential Timeslot instances to assign to this field, Timefold uses the variable type to connect to a value range provider that provides a List<Timeslot> to pick from.

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

Determining the @PlanningVariable fields for an arbitrary constraint solving use case is often challenging the first time. Read the domain modeling guidelines to avoid common pitfalls.

6. Define the constraints and calculate the score

A score represents the quality of a specific solution. The higher the better. Timefold looks for the best solution, which is the solution with the highest score found in the available time. It might be the optimal solution.

Because this use case has hard and soft constraints, use the HardSoftScore class to represent the score:

Hard constraints must not be broken. For example: A room can have at most one lesson at the same time.
Soft constraints should not be broken. For example: A teacher prefers to teach in a single room.

Hard constraints are weighted against other hard constraints. Soft constraints are weighted too, against other soft constraints. Hard constraints always outweigh soft constraints, regardless of their respective weights.

To calculate the score, you could implement an EasyScoreCalculator class:

public class TimeTableEasyScoreCalculator implements EasyScoreCalculator<TimeTable, HardSoftScore> {

    @Override
    public HardSoftScore calculateScore(TimeTable timeTable) {
        List<Lesson> lessonList = timeTable.getLessonList();
        int hardScore = 0;
        for (Lesson a : lessonList) {
            for (Lesson b : lessonList) {
                if (a.getTimeslot() != null && a.getTimeslot().equals(b.getTimeslot())
                        && a.getId() < b.getId()) {
                    // A room can accommodate at most one lesson at the same time.
                    if (a.getRoom() != null && a.getRoom().equals(b.getRoom())) {
                        hardScore--;
                    }
                    // A teacher can teach at most one lesson at the same time.
                    if (a.getTeacher().equals(b.getTeacher())) {
                        hardScore--;
                    }
                    // A student can attend at most one lesson at the same time.
                    if (a.getStudentGroup().equals(b.getStudentGroup())) {
                        hardScore--;
                    }
                }
            }
        }
        int softScore = 0;
        // Soft constraints are only implemented in the timefold-quickstarts code
        return HardSoftScore.of(hardScore, softScore);
    }

}

Unfortunately that does not scale well, because it is non-incremental: every time a lesson is assigned to a different time slot or room, all lessons are re-evaluated to calculate the new score.

Instead, create a src/main/java/org/acme/schooltimetabling/solver/TimeTableConstraintProvider.java class to perform incremental score calculation. It uses Timefold’s ConstraintStream API which is inspired by Java Streams and SQL:

package org.acme.schooltimetabling.solver;

import org.acme.schooltimetabling.domain.Lesson;
import ai.timefold.solver.core.api.score.buildin.hardsoft.HardSoftScore;
import ai.timefold.solver.core.api.score.stream.Constraint;
import ai.timefold.solver.core.api.score.stream.ConstraintFactory;
import ai.timefold.solver.core.api.score.stream.ConstraintProvider;
import ai.timefold.solver.core.api.score.stream.Joiners;

public class TimeTableConstraintProvider implements ConstraintProvider {

    @Override
    public Constraint[] defineConstraints(ConstraintFactory constraintFactory) {
        return new Constraint[] {
                // Hard constraints
                roomConflict(constraintFactory),
                teacherConflict(constraintFactory),
                studentGroupConflict(constraintFactory),
                // Soft constraints are only implemented in the timefold-quickstarts code
        };
    }

    private Constraint roomConflict(ConstraintFactory constraintFactory) {
        // A room can accommodate at most one lesson at the same time.

        // Select a lesson ...
        return constraintFactory
                .forEach(Lesson.class)
                // ... and pair it with another lesson ...
                .join(Lesson.class,
                        // ... in the same timeslot ...
                        Joiners.equal(Lesson::getTimeslot),
                        // ... in the same room ...
                        Joiners.equal(Lesson::getRoom),
                        // ... and the pair is unique (different id, no reverse pairs) ...
                        Joiners.lessThan(Lesson::getId))
                // ... then penalize each pair with a hard weight.
                .penalize(HardSoftScore.ONE_HARD)
                .asConstraint("Room conflict");
    }

    private Constraint teacherConflict(ConstraintFactory constraintFactory) {
        // A teacher can teach at most one lesson at the same time.
        return constraintFactory.forEach(Lesson.class)
                .join(Lesson.class,
                        Joiners.equal(Lesson::getTimeslot),
                        Joiners.equal(Lesson::getTeacher),
                        Joiners.lessThan(Lesson::getId))
                .penalize(HardSoftScore.ONE_HARD)
                .asConstraint("Teacher conflict");
    }

    private Constraint studentGroupConflict(ConstraintFactory constraintFactory) {
        // A student can attend at most one lesson at the same time.
        return constraintFactory.forEach(Lesson.class)
                .join(Lesson.class,
                        Joiners.equal(Lesson::getTimeslot),
                        Joiners.equal(Lesson::getStudentGroup),
                        Joiners.lessThan(Lesson::getId))
                .penalize(HardSoftScore.ONE_HARD)
                .asConstraint("Student group conflict");
    }

}

The ConstraintProvider scales an order of magnitude better than the EasyScoreCalculator: O(n) instead of O(n²).

7. Gather the domain objects in a planning solution

A TimeTable wraps all Timeslot, Room, and Lesson instances of a single dataset. Furthermore, because it contains all lessons, each with a specific planning variable state, it is a planning solution and it has a score:

If lessons are still unassigned, then it is an uninitialized solution, for example, a solution with the score -4init/0hard/0soft.
If it breaks hard constraints, then it is an infeasible solution, for example, a solution with the score -2hard/-3soft.
If it adheres to all hard constraints, then it is a feasible solution, for example, a solution with the score 0hard/-7soft.

Create the src/main/java/org/acme/schooltimetabling/domain/TimeTable.java class:

package org.acme.schooltimetabling.domain;

import java.util.List;

import ai.timefold.solver.core.api.domain.solution.PlanningEntityCollectionProperty;
import ai.timefold.solver.core.api.domain.solution.PlanningScore;
import ai.timefold.solver.core.api.domain.solution.PlanningSolution;
import ai.timefold.solver.core.api.domain.solution.ProblemFactCollectionProperty;
import ai.timefold.solver.core.api.domain.valuerange.ValueRangeProvider;
import ai.timefold.solver.core.api.score.buildin.hardsoft.HardSoftScore;

@PlanningSolution
public class TimeTable {

    @ValueRangeProvider
    @ProblemFactCollectionProperty
    private List<Timeslot> timeslotList;
    @ValueRangeProvider
    @ProblemFactCollectionProperty
    private List<Room> roomList;
    @PlanningEntityCollectionProperty
    private List<Lesson> lessonList;

    @PlanningScore
    private HardSoftScore score;

    public TimeTable() {
    }

    public TimeTable(List<Timeslot> timeslotList, List<Room> roomList, List<Lesson> lessonList) {
        this.timeslotList = timeslotList;
        this.roomList = roomList;
        this.lessonList = lessonList;
    }

    public List<Timeslot> getTimeslotList() {
        return timeslotList;
    }

    public List<Room> getRoomList() {
        return roomList;
    }

    public List<Lesson> getLessonList() {
        return lessonList;
    }

    public HardSoftScore getScore() {
        return score;
    }

}

The TimeTable class has an @PlanningSolution annotation, so Timefold knows that this class contains all of the input and output data.

Specifically, this class is the input of the problem:

A timeslotList field with all time slots
- This is a list of problem facts, because they do not change during solving.
A roomList field with all rooms
- This is a list of problem facts, because they do not change during solving.
A lessonList field with all lessons
- This is a list of planning entities, because they change during solving.
- Of each Lesson:
  - The values of the timeslot and room fields are typically still null, so unassigned. They are planning variables.
  - The other fields, such as subject, teacher and studentGroup, are filled in. These fields are problem properties.

However, this class is also the output of the solution:

A lessonList field for which each Lesson instance has non-null timeslot and room fields after solving
A score field that represents the quality of the output solution, for example, 0hard/-5soft

7.1. The value range providers

The timeslotList field is a value range provider. It holds the Timeslot instances which Timefold can pick from to assign to the timeslot field of Lesson instances. The timeslotList field has an @ValueRangeProvider annotation to connect the @PlanningVariable with the @ValueRangeProvider, by matching the type of the planning variable with the type returned by the value range provider.

Following the same logic, the roomList field also has an @ValueRangeProvider annotation.

7.2. The problem fact and planning entity properties

Furthermore, Timefold needs to know which Lesson instances it can change as well as how to retrieve the Timeslot and Room instances used for score calculation by your TimeTableConstraintProvider.

The timeslotList and roomList fields have an @ProblemFactCollectionProperty annotation, so your TimeTableConstraintProvider can select from those instances.

The lessonList has an @PlanningEntityCollectionProperty annotation, so Timefold can change them during solving and your TimeTableConstraintProvider can select from those too.

8. Create the application

Now you are ready to put everything together and create a Java application. The main() method performs the following tasks:

Creates the SolverFactory to build a Solver per dataset.
Loads a dataset.
Solves it with Solver.solve().
Visualizes the solution for that dataset.

Typically, an application has a single SolverFactory to build a new Solver instance for each problem dataset to solve. A SolverFactory is thread-safe, but a Solver is not. In this case, there is only one dataset, so only one Solver instance.

Create the src/main/java/org/acme/schooltimetabling/TimeTableApp.java class:

package org.acme.schooltimetabling;

import java.time.DayOfWeek;
import java.time.Duration;
import java.time.LocalTime;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

import org.acme.schooltimetabling.domain.Lesson;
import org.acme.schooltimetabling.domain.Room;
import org.acme.schooltimetabling.domain.TimeTable;
import org.acme.schooltimetabling.domain.Timeslot;
import org.acme.schooltimetabling.solver.TimeTableConstraintProvider;
import ai.timefold.solver.core.api.solver.Solver;
import ai.timefold.solver.core.api.solver.SolverFactory;
import ai.timefold.solver.core.config.solver.SolverConfig;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class TimeTableApp {

    private static final Logger LOGGER = LoggerFactory.getLogger(TimeTableApp.class);

    public static void main(String[] args) {
        SolverFactory<TimeTable> solverFactory = SolverFactory.create(new SolverConfig()
                .withSolutionClass(TimeTable.class)
                .withEntityClasses(Lesson.class)
                .withConstraintProviderClass(TimeTableConstraintProvider.class)
                // The solver runs only for 5 seconds on this small dataset.
                // It's recommended to run for at least 5 minutes ("5m") otherwise.
                .withTerminationSpentLimit(Duration.ofSeconds(5)));

        // Load the problem
        TimeTable problem = generateDemoData();

        // Solve the problem
        Solver<TimeTable> solver = solverFactory.buildSolver();
        TimeTable solution = solver.solve(problem);

        // Visualize the solution
        printTimetable(solution);
    }

    public static TimeTable generateDemoData() {
        List<Timeslot> timeslotList = new ArrayList<>(10);
        timeslotList.add(new Timeslot(DayOfWeek.MONDAY, LocalTime.of(8, 30), LocalTime.of(9, 30)));
        timeslotList.add(new Timeslot(DayOfWeek.MONDAY, LocalTime.of(9, 30), LocalTime.of(10, 30)));
        timeslotList.add(new Timeslot(DayOfWeek.MONDAY, LocalTime.of(10, 30), LocalTime.of(11, 30)));
        timeslotList.add(new Timeslot(DayOfWeek.MONDAY, LocalTime.of(13, 30), LocalTime.of(14, 30)));
        timeslotList.add(new Timeslot(DayOfWeek.MONDAY, LocalTime.of(14, 30), LocalTime.of(15, 30)));

        timeslotList.add(new Timeslot(DayOfWeek.TUESDAY, LocalTime.of(8, 30), LocalTime.of(9, 30)));
        timeslotList.add(new Timeslot(DayOfWeek.TUESDAY, LocalTime.of(9, 30), LocalTime.of(10, 30)));
        timeslotList.add(new Timeslot(DayOfWeek.TUESDAY, LocalTime.of(10, 30), LocalTime.of(11, 30)));
        timeslotList.add(new Timeslot(DayOfWeek.TUESDAY, LocalTime.of(13, 30), LocalTime.of(14, 30)));
        timeslotList.add(new Timeslot(DayOfWeek.TUESDAY, LocalTime.of(14, 30), LocalTime.of(15, 30)));

        List<Room> roomList = new ArrayList<>(3);
        roomList.add(new Room("Room A"));
        roomList.add(new Room("Room B"));
        roomList.add(new Room("Room C"));

        List<Lesson> lessonList = new ArrayList<>();
        long id = 0;
        lessonList.add(new Lesson(id++, "Math", "A. Turing", "9th grade"));
        lessonList.add(new Lesson(id++, "Math", "A. Turing", "9th grade"));
        lessonList.add(new Lesson(id++, "Physics", "M. Curie", "9th grade"));
        lessonList.add(new Lesson(id++, "Chemistry", "M. Curie", "9th grade"));
        lessonList.add(new Lesson(id++, "Biology", "C. Darwin", "9th grade"));
        lessonList.add(new Lesson(id++, "History", "I. Jones", "9th grade"));
        lessonList.add(new Lesson(id++, "English", "I. Jones", "9th grade"));
        lessonList.add(new Lesson(id++, "English", "I. Jones", "9th grade"));
        lessonList.add(new Lesson(id++, "Spanish", "P. Cruz", "9th grade"));
        lessonList.add(new Lesson(id++, "Spanish", "P. Cruz", "9th grade"));

        lessonList.add(new Lesson(id++, "Math", "A. Turing", "10th grade"));
        lessonList.add(new Lesson(id++, "Math", "A. Turing", "10th grade"));
        lessonList.add(new Lesson(id++, "Math", "A. Turing", "10th grade"));
        lessonList.add(new Lesson(id++, "Physics", "M. Curie", "10th grade"));
        lessonList.add(new Lesson(id++, "Chemistry", "M. Curie", "10th grade"));
        lessonList.add(new Lesson(id++, "French", "M. Curie", "10th grade"));
        lessonList.add(new Lesson(id++, "Geography", "C. Darwin", "10th grade"));
        lessonList.add(new Lesson(id++, "History", "I. Jones", "10th grade"));
        lessonList.add(new Lesson(id++, "English", "P. Cruz", "10th grade"));
        lessonList.add(new Lesson(id++, "Spanish", "P. Cruz", "10th grade"));

        return new TimeTable(timeslotList, roomList, lessonList);
    }

    private static void printTimetable(TimeTable timeTable) {
        LOGGER.info("");
        List<Room> roomList = timeTable.getRoomList();
        List<Lesson> lessonList = timeTable.getLessonList();
        Map<Timeslot, Map<Room, List<Lesson>>> lessonMap = lessonList.stream()
                .filter(lesson -> lesson.getTimeslot() != null && lesson.getRoom() != null)
                .collect(Collectors.groupingBy(Lesson::getTimeslot, Collectors.groupingBy(Lesson::getRoom)));
        LOGGER.info("|            | " + roomList.stream()
                .map(room -> String.format("%-10s", room.getName())).collect(Collectors.joining(" | ")) + " |");
        LOGGER.info("|" + "------------|".repeat(roomList.size() + 1));
        for (Timeslot timeslot : timeTable.getTimeslotList()) {
            List<List<Lesson>> cellList = roomList.stream()
                    .map(room -> {
                        Map<Room, List<Lesson>> byRoomMap = lessonMap.get(timeslot);
                        if (byRoomMap == null) {
                            return Collections.<Lesson>emptyList();
                        }
                        List<Lesson> cellLessonList = byRoomMap.get(room);
                        if (cellLessonList == null) {
                            return Collections.<Lesson>emptyList();
                        }
                        return cellLessonList;
                    })
                    .collect(Collectors.toList());

            LOGGER.info("| " + String.format("%-10s",
                    timeslot.getDayOfWeek().toString().substring(0, 3) + " " + timeslot.getStartTime()) + " | "
                    + cellList.stream().map(cellLessonList -> String.format("%-10s",
                            cellLessonList.stream().map(Lesson::getSubject).collect(Collectors.joining(", "))))
                            .collect(Collectors.joining(" | "))
                    + " |");
            LOGGER.info("|            | "
                    + cellList.stream().map(cellLessonList -> String.format("%-10s",
                            cellLessonList.stream().map(Lesson::getTeacher).collect(Collectors.joining(", "))))
                            .collect(Collectors.joining(" | "))
                    + " |");
            LOGGER.info("|            | "
                    + cellList.stream().map(cellLessonList -> String.format("%-10s",
                            cellLessonList.stream().map(Lesson::getStudentGroup).collect(Collectors.joining(", "))))
                            .collect(Collectors.joining(" | "))
                    + " |");
            LOGGER.info("|" + "------------|".repeat(roomList.size() + 1));
        }
        List<Lesson> unassignedLessons = lessonList.stream()
                .filter(lesson -> lesson.getTimeslot() == null || lesson.getRoom() == null)
                .collect(Collectors.toList());
        if (!unassignedLessons.isEmpty()) {
            LOGGER.info("");
            LOGGER.info("Unassigned lessons");
            for (Lesson lesson : unassignedLessons) {
                LOGGER.info("  " + lesson.getSubject() + " - " + lesson.getTeacher() + " - " + lesson.getStudentGroup());
            }
        }
    }

}

The main() method first creates the SolverFactory:

SolverFactory<TimeTable> solverFactory = SolverFactory.create(new SolverConfig()
        .withSolutionClass(TimeTable.class)
        .withEntityClasses(Lesson.class)
        .withConstraintProviderClass(TimeTableConstraintProvider.class)
        // The solver runs only for 5 seconds on this small dataset.
        // It's recommended to run for at least 5 minutes ("5m") otherwise.
        .withTerminationSpentLimit(Duration.ofSeconds(5)));

This registers the @PlanningSolution class, the @PlanningEntity classes, and the ConstraintProvider class, all of which you created earlier.

Without a termination setting or a terminationEarly() event, the solver runs forever. To avoid that, the solver limits the solving time to five seconds.

After five seconds, the main() method loads the problem, solves it, and prints the solution:

        // Load the problem
        TimeTable problem = generateDemoData();

        // Solve the problem
        Solver<TimeTable> solver = solverFactory.buildSolver();
        TimeTable solution = solver.solve(problem);

        // Visualize the solution
        printTimetable(solution);

The solve() method doesn’t return instantly. It runs for five seconds before returning the best solution.

Timefold returns the best solution found in the available termination time. Due to the nature of NP-hard problems, the best solution might not be optimal, especially for larger datasets. Increase the termination time to potentially find a better solution.

The generateDemoData() method generates the school timetable problem to solve.

The printTimetable() method pretty prints the timetable to the console, so it’s easy to determine visually whether or not it’s a good schedule.

8.1. Configure logging

To see any output in the console, logging must be configured properly.

Create the src/main/resource/logback.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>

  <appender name="consoleAppender" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%-12.12t] %-5p %m%n</pattern>
    </encoder>
  </appender>

  <logger name="ai.timefold.solver" level="info"/>

  <root level="info">
    <appender-ref ref="consoleAppender" />
  </root>

</configuration>

9. Run the application

9.1. Run the application in IDE

Run that TimeTableApp class as the main class of a normal Java application:

...
INFO  |            | Room A     | Room B     | Room C     |
INFO  |------------|------------|------------|------------|
INFO  | MON 08:30  | English    | Math       |            |
INFO  |            | I. Jones   | A. Turing  |            |
INFO  |            | 9th grade  | 10th grade |            |
INFO  |------------|------------|------------|------------|
INFO  | MON 09:30  | History    | Physics    |            |
INFO  |            | I. Jones   | M. Curie   |            |
INFO  |            | 9th grade  | 10th grade |            |
...

Verify the console output. Does it conform to all hard constraints? What happens if you comment out the roomConflict constraint in TimeTableConstraintProvider?

The info log shows what Timefold did in those five seconds:

... Solving started: time spent (33), best score (-8init/0hard/0soft), environment mode (REPRODUCIBLE), random (JDK with seed 0).
... Construction Heuristic phase (0) ended: time spent (73), best score (0hard/0soft), score calculation speed (459/sec), step total (4).
... Local Search phase (1) ended: time spent (5000), best score (0hard/0soft), score calculation speed (28949/sec), step total (28398).
... Solving ended: time spent (5000), best score (0hard/0soft), score calculation speed (28524/sec), phase total (2), environment mode (REPRODUCIBLE).

9.2. Test the application

A good application includes test coverage.

9.2.1. Test the constraints

To test each constraint in isolation, use a ConstraintVerifier in unit tests. This tests each constraint’s corner cases in isolation from the other tests, which lowers maintenance when adding a new constraint with proper test coverage.

Create the src/test/java/org/acme/schooltimetabling/solver/TimeTableConstraintProviderTest.java class:

package org.acme.schooltimetabling.solver;

import java.time.DayOfWeek;
import java.time.LocalTime;

import org.acme.schooltimetabling.domain.Lesson;
import org.acme.schooltimetabling.domain.Room;
import org.acme.schooltimetabling.domain.TimeTable;
import org.acme.schooltimetabling.domain.Timeslot;
import org.junit.jupiter.api.Test;
import ai.timefold.solver.test.api.score.stream.ConstraintVerifier;

class TimeTableConstraintProviderTest {

    private static final Room ROOM1 = new Room("Room1");
    private static final Timeslot TIMESLOT1 = new Timeslot(DayOfWeek.MONDAY, LocalTime.NOON);
    private static final Timeslot TIMESLOT2 = new Timeslot(DayOfWeek.TUESDAY, LocalTime.NOON);

    ConstraintVerifier<TimeTableConstraintProvider, TimeTable> constraintVerifier = ConstraintVerifier.build(
            new TimeTableConstraintProvider(), TimeTable.class, Lesson.class);

    @Test
    void roomConflict() {
        Lesson firstLesson = new Lesson(1, "Subject1", "Teacher1", "Group1", TIMESLOT1, ROOM1);
        Lesson conflictingLesson = new Lesson(2, "Subject2", "Teacher2", "Group2", TIMESLOT1, ROOM1);
        Lesson nonConflictingLesson = new Lesson(3, "Subject3", "Teacher3", "Group3", TIMESLOT2, ROOM1);
        constraintVerifier.verifyThat(TimeTableConstraintProvider::roomConflict)
                .given(firstLesson, conflictingLesson, nonConflictingLesson)
                .penalizesBy(1);
    }

}

This test verifies that the constraint TimeTableConstraintProvider::roomConflict penalizes with a match weight of 1 when given three lessons in the same room, where two lessons have the same timeslot. Therefore, a constraint weight of 10hard would reduce the score by -10hard.

Notice how ConstraintVerifier ignores the constraint weight during testing - even if those constraint weights are hard coded in the ConstraintProvider - because constraints weights change regularly before going into production. This way, constraint weight tweaking does not break the unit tests.

For more, see Testing Constraint Streams.

9.3. Logging

When adding constraints in your ConstraintProvider, keep an eye on the score calculation speed in the info log, after solving for the same amount of time, to assess the performance impact:

... Solving ended: ..., score calculation speed (29455/sec), ...

To understand how Timefold is solving your problem internally, change the logging in the logback.xml file:

  <logger name="ai.timefold.solver" level="debug"/>

Use debug logging to show every step:

... Solving started: time spent (67), best score (-20init/0hard/0soft), environment mode (REPRODUCIBLE), random (JDK with seed 0).
...     CH step (0), time spent (128), score (-18init/0hard/0soft), selected move count (15), picked move ([Math(101) {null -> Room A}, Math(101) {null -> MONDAY 08:30}]).
...     CH step (1), time spent (145), score (-16init/0hard/0soft), selected move count (15), picked move ([Physics(102) {null -> Room A}, Physics(102) {null -> MONDAY 09:30}]).
...

Use trace logging to show every step and every move per step.

9.4. Make a standalone application

In order to run the application outside an IDE easily, you will need to make some changes to the configuration of your build tool.

9.4.1. Executable JAR in Maven

In Maven, add the following to your pom.xml:

  ...
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-assembly-plugin</artifactId>
        <version>${version.assembly.plugin}</version>
        <configuration>
          <finalName>hello-world-run</finalName>
          <appendAssemblyId>false</appendAssemblyId>
          <descriptors>
            <descriptor>src/assembly/jar-with-dependencies-and-services.xml</descriptor>
          </descriptors>
          <archive>
            <manifestEntries>
              <Main-Class>org.acme.schooltimetabling.TimeTableApp</Main-Class>
              <Multi-Release>true</Multi-Release>
            </manifestEntries>
          </archive>
        </configuration>
        <executions>
          <execution>
            <id>make-assembly</id>
            <phase>package</phase>
            <goals>
              <goal>single</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
      ...
    </plugins>
    ...
  </build>
  ...

Also, create a new file in src/assembly directory called jar-with-dependencies-and-services.xml with the following contents:

  <assembly xmlns="http://maven.apache.org/ASSEMBLY/2.1.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://maven.apache.org/ASSEMBLY/2.1.0 http://maven.apache.org/xsd/assembly-2.1.0.xsd">
    <id>jar-with-dependencies-and-services</id>
    <formats>
      <format>jar</format>
    </formats>
    <containerDescriptorHandlers>
      <containerDescriptorHandler>
        <handlerName>metaInf-services</handlerName>
      </containerDescriptorHandler>
    </containerDescriptorHandlers>
    <includeBaseDirectory>false</includeBaseDirectory>
    <dependencySets>
      <dependencySet>
        <outputDirectory>/</outputDirectory>
        <useProjectArtifact>true</useProjectArtifact>
        <unpack>true</unpack>
        <scope>runtime</scope>
      </dependencySet>
    </dependencySets>
  </assembly>

This enables the Maven Assembly Plugin and tells it to do the following:

Take all dependencies of your project and put their classes and resources into a new JAR.
- If any of the dependencies use Java SPI, it properly bundles all the service descriptors.
- If any of the dependencies are multi-release JARs, it takes that into account.
Set that JAR’s main class to be org.acme.schooltimetabling.TimeTableApp.
Make that JAR available as hello-world-run.jar in your project’s build directory, most likely target/.

This executable JAR can be run like any other JAR:

$ mvn clean install
...
$ java -jar target/hello-world-run.jar

9.4.2. Executable application in Gradle

In Gradle, add the following to your build.gradle:

application {
    mainClass = "org.acme.schooltimetabling.TimeTableApp"
}

After building the project, you can find an archive with a runnable application inside the build/distributions/ directory.

10. Summary

Congratulations! You have just developed a Java application with Timefold!

If you ran into any issues, take a look at the quickstart source code.

Read the next guide to build a pretty web application for school timetabling with a REST service and database integration, by leveraging Quarkus.