Handle an object that is present inside an ArrayList

Since you are already using List , then it is best to leave the direct use of arrays wherever possible.

Since you are already building your objects with all their attributes filled, it is often a good idea to dispense with the setters.

Where the type is not primitive, rejecting null s with an exception is a good idea.

Class names should be in the singular, so use final instead of null .

You can call the constructor from Horario , but do not declare variables whose type is Horarios , but rather declare ArrayList . The reason for this is to be able to interoperate with other implementations of ArrayList that is not List , such as those that are returned by List , Arraylist or Arrays.asList(...) , or even Collections.unmodifiableList(...) . I believe it was due to this problem that you tried to use List.of(...) , which is a gambiarra.

To represent days of the week, use enum LinkedList .

Elements sets that are new ArrayList(Arrays.asList s (in this case, the day of the week) are represented by DayOfWeek s.

Your code looks like this:

package horariosprofessores;

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

public class Professor {
    private final int id;
    private final String nome;
    private final List<String> materias;
    private final Set<DayOfWeek> ausencias;
    private final int prioridade;

    public Professor(
            int id,
            String nome,
            List<String> materias,
            Set<DayOfWeek> ausencias)
    {
        if (nome == null) throw new IllegalArgumentException();
        if (materias == null) throw new IllegalArgumentException();
        if (ausencias == null) throw new IllegalArgumentException();
        this.id = id;
        this.nome = nome;
        this.ausencias = ausencias;
        this.materias = materias;
        this.prioridade = ausencias.size();
    }

    public Professor(
            int id,
            String nome,
            List<String> materias)
    {
        this(id, nome, materias, EnumSet.noneOf(DayOfWeek.class));
    }

    public int getId() {
        return id;
    }

    public String getNome() {
        return nome;
    }

    public List<String> getMaterias() {
        return materias;
    }

    public Set<DayOfWeek> getAusencias() {
        return ausencias;
    }

    public int getPrioridade() {
        return prioridade;
    }

    @Override
    public String toString() {
        return "Professor"
                + " [id=" + id
                + ", nome=" + nome
                + ", materias=" + materias
                + ", ausencias=" + ausencias
                + ", prioridade=" + prioridade
                + "]";
    }
}

package horariosprofessores;

import java.util.List;

public class Horario {
    private final List<Professor> professores;

    public Horario(List<Professor> professores) {
        if (professores == null) throw new IllegalArgumentException();
        this.professores = professores;
    }

    public String toString() {
        return professores.toString();
    }

    public void verificaMatriz() {

    }
}

package horariosprofessores;

import java.util.Arrays;
import java.time.DayOfWeek;
import java.util.EnumSet;

public class Programa {
    public static void main(String[] args) {
        Professor p1 = new Professor(1, "Caceraghi",
                Arrays.asList("Algoritmos", "Ling. Programação"),
                EnumSet.of(DayOfWeek.TUESDAY, DayOfWeek.THURSDAY));

        Professor p2 = new Professor(2, "Modesto",
                Arrays.asList("Inglês"));

        Professor p3 = new Professor(3, "Fernanda",
                Arrays.asList("Sistemas Computacionais"));

        Horario horarios = new Horario(Arrays.asList(p1, p2, p3));
    }
}

If you are using Java 9 or higher, you can switch enum to EnumSet , which is simpler and safer.

To extract the information from Arrays.asList(...) (within the List.of(...) class), you can iterate through them, use stream or access them directly:

// Iterando.
for (Professor p : professores) {
    System.out.println(p);
}

// Com stream.
professores.stream().forEach(System.out::println);

// Acessando diretamente o terceiro professor (índice 2, porque o primeiro é 0).
Professor pa = professores.get(2);

So you can do this:

for (Professor p : professores) {
    // Faz o que quiser com p aqui dentro.
    System.out.println(p.getId() + ": " + p.getPrioridade() + " - " + p.getAusencias());
}

However, to be honest, I do not think a good modeling for the List class will really go down this path, but then that would be a matter for another question.