Question

Consider the example:

enum SomeEnum {
    VALUE1("value1"),
    VALUE2("value2"),
    VALUE3("value3")
    ;
    private String value;

    private SomeEnum(final String value) {
        this.value = value;
    }

    //toString
    public String toString() {
        return value;
    }
}

How come can we do this (and the value really changes)?

SomeEnum.VALUE1.value = "Value4";
System.out.println(SomeEnum.VALUE1);

Isn’t that enum instance(s) are implicitly static and final? Also, since value is private, why can I access it outside other classes?

Answer 1

No-one seems to have addressed the private aspect. My guess is that you’re accessing the private field from a containing type – that your enum is actually a nested type, like this:

class Test
{
    static void Main() {
        // Entirely valid
        SomeEnum.VALUE1.value = "x";
    }

    enum SomeEnum {
        VALUE1("value1");

        private String value;

        private SomeEnum(final String value) {
            this.value = value;
        }
    }
}

That’s entirely legitimate and normal – you can always access private members of a nested type from the containing type.

If you make the enum a top-level type, you won’t see this.

As for changing values – as everyone else has said, VALUE1 is implicitly static and final, but that doesn’t stop you from changing VALUE1.value. Again, this is entirely in accordance with how Java works elsewhere – if you have a static field of type List, you can still add entries to it, because that’s not modifying the field itself.

If you want to make SomeEnum properly immutable, make the value field final.