Query Superclass

Throughout the object-oriented software development process, you're likely to find new and better ways to query object properties. These properties aren't necessarily the defined attributes for the object's class. The methods used to query these attributes are often obvious enough. What about derived properties? These are computed properties and they sometimes go unnoticed until some iteration further down the line.

Is the best approach to adding these newly discovered query methods to add them to the class directly? Probably not because you are adding new responsibilities directly to the class. There are exceptions to not not doing this. For instance, there might be some aspect of the objects that you may want to query that are painfully obvious and were missed for some reason. Also, if the class is somewhat specialized already, the newly discovered query methods may only apply to instances of this class.

When more general query methods are discovered, we can use the inheritance mechanism to extend the functionality of our class. This can go in either direction. You can define a specialized version of your class that is a child of that class. This class would then define the new query methods. The problem with this approach is that rather than being more generalized, you are increasing the specificity level. This is fine if the newly discovered query method are specific to your class. More often than not, however, derived properties that are computed can often be applied to a group of disparate classes. If this is the case, spending the time to develop a highly-reusable query super class may be worthwhile.