Extending an already written class using OOPS

Question

I am trying to learn Design practices and OOPs. I am using parking lot problem as sample to start.

I have a GeneralParkingLot interface and a Vehicle interface. GeneralParkingLot has only one function returnParkingLotSize, Vehicle interface has multiple Vehicle attributes.

I have created a class DowntownParkingLot which extends GeneralParkingLot and has other attributes like listOfCars, availableSlots etc. and a Car Class which extends Vehicle class.

I have a HandlerClass which handles incoming command and inside that class I have decalared a DownTownParkingLot object and multiple functions to handle commands, so in each function I just pass the object of DowntownParkingLot and operate on it.

I have created different services like CreateParkingLotObject, ParkACar, FreeASlot etc. which are called by the command handler.

I have also created unit Tests to test my application.

My problem is if I want to extend my current parking lot to have additional functionality like lets say multiple floor attribute or If I want to now handle multiple parking lots instead of one, so what would be the best way to extend my GeneralParkingLot class or DowntownParkingLot class. I have also read about adapter and decorator patterns, but I think those are useful when I am already following a particular design pattern from start, In my case I didnt follow any particular pattern, so what would be the best way to extend my code. I am asking this because sometimes we encounter a class which is not made according to any design pattern and is being used in multiple places (like a lot of API's etc), so what is the best way to extend such a code. Is refactoring from start the only option? or creating new classes which inherit from old classes? What would be the best way? Also I would like to use the already created unit tests as much as possible and not rewrite same test cases again.

Answer 1

In your case, your top most class is an interface so you cannot add attributes directly.

One way to solve your problem is create a top most implementation class which implement the GeneralParkingLot interface:

public abstract CommonParkingLot implements GeneralParkingLot {
     // add your missing attributes here
     protected int floorNo;
}

(If you just want to add attributes without provide some default implementation for GeneralParkingLot then you can omit the implements part)

And then have your implementation class extends this CommonParkingLot:

public class DowntownParkingLot extends CommonParkingLot implement GeneralParkingLot {
     // Now all missing attributes are accessible here
}

For missing functionalities, the default function interface should come handy. (the keyword is language specified), you can add as more function as you need to the GeneralParkingLot interface with a default body for it.

One more point, your current implementation kinda restricted to a DowntownParkingLot because you don't add common functionality to the interface. I would suggest you to refactor the HandlerClass class a bit:

public interface GeneralParkingLot {
    int returnParkingLotSize();
    ParkingLotObject createParkingLotObject();
    boolean parkACar();
    int freeASlot();
    default public void parkMultipleCar() {
         throws new UnsupportedOperationException();
    }
}

Now in your HandlerClass should operate on the GeneralParkingLot instead of DowntownParkingLot.

By doing this, you can add more GeneralParkingLot without changing code.