Question

I’m trying to write a driving simulation program and I need some help on the design. I have a interface called RoadObjects which for now contains vehicles and mammals. I need to implement different kind of cars such as trucks, sedans, semis. they have some methods in common and some unique. Later on, I will add other kinds of vehicles and mammals. I don’t want to use inheritance and I am trying to do this according to design principles and patterns such as open-closed principle and factory. Now my question is, should I design it like this:

RoadObject interface

Vehicle extends RoadObject

Mammal extends RoadObject

SedanImpl implements Vehicle

TruckImpl implements Vehicle

People implements Mammal

…

…

or should I just not have vehicle and people interface and do this?

RoadObject interface

SedanImpl implements RoadObject

TruckImpl implements RoadObject

People implements RoadObject

New question:
I want to use a factory pattern, to chose which kind of vehicles for example. So I make a VechicleImpl and VechicleImplFactory so it might look like this:

RoadObject interface

Vehicle extends RoadObject

VehicleImpl implements Vehicle

Mammal extends RoadObject

SedanImpl implements Vehicle

TruckImpl implements Vehicle

People implements Mammal

Is this kind of bad design? Because VehicleImpl will have a lot of duplicate methods of SedanImpl,TruckImpl etc. Or if I wanted to have an factory that createVehicle, where should this be?

Answer 1

Both approaches are valid, whether you need an explicit abstraction for vehicles depends on your requirements. SedanImpl, TruckImpl etc. may share some features common to vehicles but not to humen/animals:

public interface Vehcile extends RoadObject
    getTires()
    getEngine()

As you tagged this in Java common abstract classes may be useful as well:

public abstract class AbstractVehicle implements Vehicle {
    private Integer tires = 4;

    public Integer getTires() { return this.tires; }

    /* ... */
}

So that TruckImpl can inherit from AbstractVehicle:

public class TruckImpl extends AbstractVehicle {
    /* ... */
}