Question

I’ve heard that implementing an interface of constants is an anti-pattern ’cause such interfaces provide implementation details to the classes (which is not a purpose of an interface). But what about having an inner interface of constants in a class? Is this a bad practice?

EDIT:
I mean I don’t want to implement it, just to use it’s constants.

Answer 1

An interface’s sole purpose is to be implemented. So, using an interface to group constants is still bad practice, even if you don’t implement it. Just use a class to put the constants:

public final class SomeClass {
    private SomeClass() {
    }

    public static final int FOO = 6;
    public static final String BAR = "bar";
}

If these connstants are tightly coupled with a given class, put them directly in this class. If they’re common to several classes, put them in an external class as shown above.