I have an immutable class called Employees like this:
public final class Employees {
private final List<Person> persons;
public Employees() {
persons = new LinkedList<Person>();
}
public List<Person> getPersons() {
return persons;
}
}
How do I keep this class immutable?
I made the field private and final, and I didn't provide a setter method. Is this enough to achieve immutability?
Answer edited to explain not only the case with a mutable version of Person, but also with an immutable version of Person.
Your class is mutable because you can do that:
Employees employees = new Employees();
employees.getPersons().add(new Person());
Note that not passing a list of persons to the constructor if you change your code to create an immutable class you will have a not useful class holding an ever empty list of persons, so I assume that is necessary to pass a List<Person> to the constructor.
Now there are two scenarios, with different implementations:
Personis immutablePersonis mutable
Scenario 1 - Person is immutable
You need only to create an immutable copy of the persons parameter in the constructor.
You need also to make final the class or at least the method getPersons to be sure that nobody provide a mutable overwritten version of the getPersons method.
public final class Employees {
private final List<Persons> persons;
public Employees(List<Person> persons) {
persons = Collections.unmodifiableList(new ArrayList<>(persons));
}
public List<Employees> getPersons() {
return persons;
}
}
Scenario 2 - Person is mutable
You need to create a deep copy of persons in the getPersons method.
You need to create a deep copy of persons on the constructor.
You need also to make final the class or at least the method getPersons to be sure that nobody provide a mutable overwritten version of the getPersons method.
public final class Employees {
private final List<Persons> persons;
public Employees(List<Person> persons) {
persons = new ArrayList<>();
for (Person person : persons) {
persons.add(deepCopy(person)); // If clone is provided
// and creates a deep copy of person
}
}
public List<Employees> getPersons() {
List<Person> temp = new ArrayList<>();
for (Person person : persons) {
temp.add(deepCopy(person)); // If clone is provided
// and creates a deep copy of person
}
return temp;
}
public Person deepCopy(Person person) {
Person copy = new Person();
// Provide a deep copy of person
...
return copy;
}
}
This part of the answer is to show why a not deep copy of the personsParameter passed to the constructor can create mutable versions of Employees:
List<Person> personsParameter = new ArrayList<>();
Person person = new Person();
person.setName("Pippo");
personsParameter.add(person);
Employees employees = new Employees(personsParameter);
// Prints Pippo
System.out.println(employees.getPersons().get(0).getName());
employees.getPersons().get(0).setName("newName");
// Prints again Pippo
System.out.println(employees.getPersons().get(0).getName());
// But modifiyng something reachable from the parameter
// used in the constructor
person.setName("Pluto");
// Now it prints Pluto, so employees has changed
System.out.println(employees.getPersons().get(0).getName());