Extending non-instantiable class in java

Question

Hi, I read "Effective Java" by Joshua Bloch and removed the 'Constant Interface anti-pattern' from our application. The trick is to use Non-instantiable util class whose constructor is private, and define all the constants as 'public static final'

I have to extend the this constant util class. I can only do this when I change the constructor to protected.

Could somebody suggest a better way.

public class Constants {
    private Constants () {} // Prevent instantiation
    public static final String MyString = "MyString";
}

public class MyConstants extends Constants {
    private MyConstants () {} // Compiler error : Implicit super constructor Constants() is not visible.
    public static final String MySecondString = "MySecondString";
}

Answer 1

I don't see anything wrong with changing the constructor to protected.

I also don't see anything wrong with using interfaces to hold constants.

Answer 2

Is there a problem with using the protected keyword ? It looks like a pragmatic solution to me.

I have to say (in addition) that I don't have a particular problem with using an interface to hold constants. Use static import to make them available in your client classes. I wouldn't advocate implementing it, though.

Answer 3

You are not typically supposed to extend these constant classes. Could you provide us with a more concrete example of what you're trying to do?

Typically you'd want to group constants together when they are related, e.g., maths constants or configuration parameter names for a particular functional component.

If the constants are really related, is there anyway you could just add them to the original class? Alternatively, is there any reason you can't create a separate class for your constants?

Answer 4

If the classes are in the same package and you want to forbid inheritance for non-package classes, consider using default access:

public class Constants {
    Constants () {} // Prevent instantiation
    public static final String MyString = "MyString";
}