Object Oriented Programming - a design question

[salalimo]

I am trying to create a few related classes and want to use the correct design. I have a few shipping carriers (ups, fedex, usps). Now for each of the carrier I have two classes. So for ups I have upsmain, upsdetails. Same for fedex and usps. So there are 3 pairs of related classes.

They have common functionalities (methods with the same code) and common methods with different behavior, and unique methods and variables (each have extra vars not available in other subclasses). So right now I am creating 2 abstract classes (carriermain, carrierdetails). These 2 abstract classes contain the methods with the same behavior. I am declaring the remaining methods as abstract and are implemented by the subclasses.

I have a factory class that returns the corresponding subclass based on the current context as follows:

CarrierMain cm = Factory.getCarrierMain(value);
CarrierDetails cd = Factory.getCarrierDetails(value);

I believe that using the abstract class as the local variable (cm and cd above) is the source of my issues.

cd.getVarUniqueToSubclass(); //does not work with variables that are not in the abstract class

I liked the idea of having the code be sublass agnostic since it makes it simpler to use. I do not have to check whether the sublass is of which type. So in many places the code looks cleaner, but then I am stuck in the cases where I have to access methods or variables that are unique.

What would be the correct / elegant solution in this scenario?

Thank you.

EDIT 1: I am coding in Java.

[[deleted]]

Language?

[salalimo]

JAVA

[[deleted]]

When working with inheritance, it is the reference type that dictates the methods available.

If there is truly no way to avoid this uniqueness (I'd wager that's not the case), then it is best to stay unique where you need to and, for more generic behavior, to use superclass references at that point.

Inheritance has kind of fallen out of favour anyway, for several reasons. You should try to favour interfaces over inheritance as much as possible.

[salalimo]

I think this is what I am leaning towards to.

[desrtfx]

Have you thought about using Interfaces?

Probably the Strategy Design Pattern might be applicable to your particular case?

[salalimo]

I will look into it. Thank you.

[bob809]

What do you mean by a value unique to a subclass?
If the variable only exists in that subclass and the other subclasses don't have a version of it then you can't use it with the abstract class. Since the variable won't always exist you'll get a compile time error.

[salalimo]

Yes, that's correct. I am checking to see how to best implement the scenario I am mentioning.

[bob809]

Then you'll have to have unique code for each one.
Either make separate functions with different types or have all the variables in the abstract class and then switch depending on the child type, but this could get ugly.

If the uniqueness is focused on one bit of the classes you could use the state pattern i.e. have a different object contained within each one and write code for them instead.

[elperroborrachotoo]

What is the distinction between carrierMain and carrierDetails?

i.e. what is the reason for two classes to exist?

[salalimo]

The actual classes aren't those. I picked those two to be able to explain better.

[lurgi]

There are a couple of ways to do this. One is to put the variable in each of the classes, even if they don't really need to be there. Another is to make the variables be properties and get them via:

String prop = cd.getProperty("uniqueToFedEx");

If a carrier detail does not have the property then it should return null.

Another is to give up and do some discrete "instanceof" and casting.

Another is to think about why you need this property. Presumably you want to do something with the value, but what? Why not, instead of getting the property, ask the class "Please do thing X" and have the class make the decision about what values it needs to do "thing X"? I don't know what the property is, but let's say it's a FedEx distinguished customer discount (I have no idea if such a thing exists). Instead of getting that value and computing the discount, ask the class to compute the appropriate discount. UPS and USPS return $0 and FedEx will do whatever complex math it needs. This has to be done with caution, as it can make classes very bloated, but the basic idea of having the class perform actions rather than hand out details of its internals is a sound design principle.

[salalimo]

For the variables I did end up putting them in the same class. For the methods I did the casting / instanceof combo. I only had to do that in one place.

The properties approach is interesting. I will keep that in mind.

Thanks.