Why is this class not immutable?

Question

I had a Java interview today and I was asked to create an immutable class named Person and I was given a skeleton with some parameters: age, name, etc. I created the below class:

final class Person {
  private final int age;
  private final String name;
  private final List<String> petNames;


  public Person(int a, String n, List<String> p) {
    this.age = a;
    this.name = n;
    this.petNames = p;
  }

  int getAget() {
    return this.age;
  }

  String getName() {
    return this.name;
  }

  List<String> getPetnames() {
    return this.petNames;
  }

}

I was told that it is not complete because by performing the below sequence of code, you prove that it is not immutable:

int x = 3;
String name = "Alex";
List<String> ls = new ArrayList<>();
Person p1 = new Person(x, name, ls);

The hint I was given is that it is related to the list and that I should change sth in the constructor. But I really do not know why and I do not really understand what is wrong.

Can anyone explain me what should be added and why ? Maybe I did not understand completely immutability.

Answer 1

I assume they asked you to keep an immutable copy of the given pet collection.

this.petNames = List.copyOf(p);

Otherwise, you would still be able to modify the pets of a person.

p1.getPetnames().add("newPet");

You shouldn't be bothered with what will happen to ls, but you should take care of what you can do with p1.

1. You make a copy to stop worrying whether the input is going to be changed sometime later.
2. You make the collection immutable, which, along with the 2 final plain fields, makes the class immutable.

Answer 2

To make list immutable, I believe you should use

       this.petnames=  ImmutableList.copyOf(p);

or

     this.petnames  = Collections.unmodifiableList(new ArrayList<>(p));

In java 9, you can use copyOf() method to create immutable list

 this.petnames = List.copyOf(p)

If you return this then its not possible to add or remove elements to it. However, they can be get elements out of it.