2

Based on the answers from this question, given that I have a class with fields initialized on the declaration, what should I do when I extend that class and need the subclass to have a different default value?

Should I assign the new default value in the constructor of the subclass and keep the assignment in the declaration for the superclass?:

public class Bird {
    private boolean flight = true;
}

public class Penguin extends Bird {
    public Penguin() {
        flight = false;
    }
}

Or should I refactor so that both classes initialize fields in the constructor?

From the answers in this question it looks like there are no significant technical differences, so this is a question about semantics.

EJS
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    Currently that code wouldn't even compile, as `Penguin` doesn't have access to `Bird`. Personally I'd provide a protected constructor in `Bird` that lets the subclass provide a value for `flight` - that way you can keep the field private and make it final too. – Jon Skeet Oct 10 '17 at 08:02
  • flight is private in child class. – Optional Oct 10 '17 at 08:03
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    Despite the visibility issue, it would see this as opinion based. Personally, I prefere to define general / default values in the parent `Bird`. But all child class would overwrite them with their own needed values. So the values in parent are just to be there as default or fallback, if I or someone else forgets to set a value in the child. – Korashen Oct 10 '17 at 08:08

2 Answers2

2

First the differences between the two cases.

For an immutable property (the most restrictive - nice - case):

public class Bird {
    public final boolean flight;

    public Bird() {
        this(true);
    }

    protected Bird(boolean flight) {
        this.flight = flight();
    }
}

public class Penguin extends Bird {
    public Penguin() {
        super(false);
    }
}

For a more liberal, dynamic usage:

public class Bird {
    protected boolean flight = true;
    public boolean hasFligth() {
        return flight;
    }
}

public class Penguin extends Bird {
    public Penguin() {
        flight = false;
    }
}

However flight might be considered not a property on every individual Bird / Penguin object: it is a property of the class (Penguin, Ostrich, ...). A kind of strategy pattern, or:

Map<Class<Bird>, Boolean>
Set<Class<Bird>> // better

Situated at the other side, in a class Flight.

Joop Eggen
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1

If you design your superclass for extension and want to allow certain field to be initializable to another value, you should probably expose this via constructor parameter:

public class Bird {
    private final boolean flight = true;
    public Bird() {
        this(true);
    }
    public Bird(boolean flight) {
        this.fligjt = flight;
    }
    public boolean isFlight() {
        return this.flight;
    }
}

public class Penguin extends Bird {
    public Penguin() {
        super(false);
    }
}
lexicore
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