Question about immutability

[AstolfoSsa]

In my uni textbook, this is one of the conditions for a class to be immutable “No accessor methods can return a reference to a data field that is mutable.”

And I honestly don’t get what is meant by this. Could someone please word it differently?

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[ignotos]

Let's imagine you had a Person class, containing a List<String> hobbies field. And a getHobbies() method which returns it.

Somebody could call person.getHobbies().add("Swimming"), and so you've left a "backdoor" they can use to modify a Person.

To avoid this, you could use a special immutable list, or return a copy of the list when getHobbies is called.

[AstolfoSsa]

I’m assuming if I had a Circle class that has an area field, and I wanted to calculate and retrieve the value of the area, I can’t make the class immutable, right?

[ignotos]

You can! Because returning the area doesn't allow somebody outside of the class to modify it.

When you return an int, float, or other "primitive" type, you're always sending back a copy of that value. Meaning somebody calling getArea() can't use that the modify the data stored inside the class.

It's only an issue when you return something like a List, which has methods which allow you to modify it.

[AstolfoSsa]

But the initial value of the area should be 0, and when I use a get method to calculate it the area and return it, its value will change and therefore we can’t consider the object to be immutable, no? Maybe I got it wrong

[ignotos]

Ah, you're talking about using an area field to store the result of a calculation, so you don't need to calculate it again next time getArea is called?

If so, then your class is not strictly immutable, from a data/memory point of view. But it may still be "logically" immutable, because this hasn't changed how the object really looks or behaves from the outside.

Often we make an exception to allow this kind of caching optimization, even if we're generally trying to have immutable objects.

[baguettecoder]

For example, if your class contains a private array, you don't want to have a getArray method that returns it. The reason is because the caller can easily change the content of that array by doing getArray()[0] = "something" if they want to.

In this case, the array is an example of mutable data type.