Does one need defensive copies for Java enum types?

Question

One should return so-called "defensive copies" of private reference types. That's to avoid returning the reference of a private field.

I would like to know if that's necessary for private enum types. I read somewhere that enums are immutable reference types, so the answer should be 'no'. Is that right?

Answer 1

Enums aren't inherently immutable - but you wouldn't be able to create a defensive copy anyway, as there's only a fixed set of instances available - you'd have to return a reference to one of the existing instances, rather than creating a new instance.

Enums generally should be immutable anyway, but to counteract the claims that they inherently are immutable:

enum BadEnum {
    INSTANCE;

    private int foo;

    private int getFoo() {
        return foo;
    }

    public int setFoo(int foo) {
        this.foo = foo;
    }
}

class Test {
    public static void main(String[] args) {
        BadEnum.INSTANCE.setFoo(10);
        System.out.println(BadEnum.INSTANCE.getFoo()); // Prints 10
    }
}

So in short:

• Make your enums immutable. I can't remember ever even wanting to create a mutable enum.
• You can't and shouldn't try to make defensive copies.