Is a superclass protected field which is only used in a subclass bad practice?

Question

I have a superclass like this which I expect alot of classes to inherit:

public abstract class Super {
    protected Object myField; //Not used anywhere in this class
    //a load more code here which does useful stuff
}

All these classes will need to use an instance of myField. However the superclass does not. Have I made a bad design decision somewhere?

Answer 1

Not necessarily. If all the subclasses need the same field for the same reason, then that's no different than providing any other common functionality in a base class. as your classes grow you may find that you add common functionality which uses this field (e.g. referencing it in an equals/hashCode method).

Now, if the field is of type Object and each sub-class shoves something completely different into it, i would consider that a code smell.

Answer 2

Well IMHO, a field should not be present in a class if it's not really used by that class. What it seems to me that you really want here is to have a base class that tells its subclasses "you should ALL have some way of keeping state for X but I (the base class) will not modify that X state, in which case you should make an abstract method in order to convey that message, something like this:

public abstract class Super {
    protected abstract Object getMyField();
}