Implementation question regarding base classes and derived classes

Question

I have a question regarding the best way to implement this. I'm going to describe my current implementation and how I seem to have painted myself into a corner:

I have an abstract class called Package:

public abstract class Package {
    protected String description;
    protected String packagingCode;
    protected Dimension dimensions;
    protected Weight weight;

    protected Package() {
        this.description = null;
        this.packagingCode = null;
        this.dimensions = null;
        this.weight = null;
    }

    protected Package(String description, String packagingCode, Dimension dimensions, Weight weight) throws ShippingException {
        this.description = description;
        this.packagingCode = packagingCode;
        this.dimensions = dimensions;
        this.weight = weight;

        String exceptionMessage = "";

        if(!meetsWeightRequirements()) {
            exceptionMessage = "This package's weight exceeds limits. ";
        }

        if(!meetsDimensionalRequirements()) {
            exceptionMessage += "This package's dimensions exceed limits.";
        }

        if(!StringUtils.isEmpty(exceptionMessage)) {
            throw new ShippingException(exceptionMessage);
        }
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getPackagingCode() {
        return packagingCode;
    }

    public void setPackagingCode(String packagingCode) {
        this.packagingCode = packagingCode;
    }

    public Dimension getPackageDimensions() {
        return dimensions;
    }

    public void setPackageDimensions(Dimension dimensions) throws ShippingException {
        this.dimensions = dimensions;

        if(!meetsDimensionalRequirements()) {
            this.dimensions = null;
            throw new ShippingException("This package's dimensions exceed limits.");
        }
    }

    public Weight getPackageWeight() {
        return weight;
    }

    public void setPackageWeight(Weight weight) throws ShippingException {
        this.weight = weight;

        if(!meetsWeightRequirements()) {
            this.weight = null;
            throw new ShippingException("This package's weight exceeds limits.");
        }
    }


    public abstract boolean meetsWeightRequirements();

    public abstract boolean meetsDimensionalRequirements();
}

Then I have classes that extend this abstract class like so:

public class WeightBasedPackage extends Package {

    public boolean meetsWeightRequirements() {
        Weight weight = this.getPackageWeight();
        boolean meetsRequirements = false;

        if(weight != null) {
            meetsRequirements = (weight.getWeight() > 0);
        }

        return meetsRequirements;
    }

    public boolean meetsDimensionalRequirements() {
        return true;
    }
}

I have another object (ShipRequest) that maintains a List of Packages (List<Package>). I also have a services (eg WeightBasedPackageShipService) that uses this object and can access this list of packages. This implementation has worked fine because the services don't really care what type of package it is. The only difference between the packages is the way they implement the abstract methods.

Now here is where the problem comes in. I created a new class:

public class OrderQuantityPackage extends Package {

    int quantity;

    public OrderQuantityPackage() {
        super();
    }

    public void setQuantity(int quantity) {
        this.quantity = quantity;
    }

    public int getQuantity() {
        return this.quantity;
    }

    public boolean meetsWeightRequirements() {
        return true;
    }

    public boolean meetsDimensionalRequirements() {
        return true;
    }
}

Which has a quantity field. I need to access this field in the service (OrderQuantityPackageShipService). However, since it is of type Package I have to cast it (it seems kinda kludgey).

My question is, how do I implement this in a better fashion (so I don't have to cast) and also ensure type-safety (So that if you are using OrderQuantityPackageShipService, the package must be of type OrderQuantityPackage). I thought about using Generics, but it seems a little to kludgey for what I am trying to do (ShipRequest has a bunch of other attributes and it seemed strange to genericize it based on the type of package).

Thanks.

Answer 1

public abstract class Package {
    protected String description;  // These shouldn't be private fields instead of protected?
    protected String packagingCode; // Nah, I don't think so, otherwise how could I store a value into the Quantity field? =P 
    protected Dimension dimensions;  
    protected Weight weight;  
    protected int quantity;

    // Constructors, getters and setters...

    public virtual int getQuantity {
        throw new NotImplementedException();
    }

    public virtual int setQuantity(int quantity) {
        throw new NotImplementedException();
    }
}

public final class OrderQuantityPackage extends Package {
    public override int getQuantity {
        return super.quantity;
    }

    public override void setQuantity(int quantity) {
        super.quantity = quantity;
    }
}

I'm not completely sure about the syntax though, and neither about the NotImplementedException, but I hope you get the idea. So, any Package derived class that needs or require a quantity may do so by overriding the getter and setter of the Quantity property.

No exception should be thrown as of where the Quantity won't be required, it shouldn't get called, so no exception shall be thrown. Furthermore, it testifies that your model only does what it is required when times come.

In addition to it, OrderQuantityShipService shouldn't require a Weight property within the OrderQuantityPackage, and as written by Vivin, one could access the weight anyway.

Otherwise, a simple cast within your service should do it. It is no dirty way to go to use casting. For instance, one must cast the sender object within an event handler to the proper control type he wishes to check for name, state or other property values! The most general class is then passed on to the event, and one must cast... And this, that is not me who said to opt this way, these are software engineers!...

EDIT Vivin, how do one cast from a data type to another in JAVA, is it as in C/C++/C# ?

CastedType variable = (CastedType)TypeCast; 

Answer 2

Maybe you should create an abstract service and extend it for the different kinds of packages to handle. You could have the handling method be abstract and have each kind of service know what to do with the corresponding package. If you're not to mix types of packages then this might work.

Answer 3

Short Answer: Dependency Inversion

You have a OrderQuantityPackageShipService class that requires certain features from the objects that it processes. So OrderQuantityPackageShipService should be the one specifying those requirements. Typically this is done with an interface. If it is very specific to the service, create the interface nested. ie:

class OrderQuantityPackageShipService {
    //...
    interface QuantityPackage {
        int getQuantity();
        // ...
    }
}

if it can be used in a consistent manner by other services, define it outside of the OrderQuantityPackageShipService class.

Then have certain packages implement that interface...

Answer 4

One thing I can think of is why would you need to access the quantity attribute in the class OrderQuantityPackageShipService ? As I look at it you have a getter and setter for each attribute of the class Package. Are these getters and setters really needed ? Having getters/setters for all those attributes doesn't go well with encapsulation.

Can you think of providing public methods in Package class that operate at a higher level and don't expose the internal attributes ? Wouldn't that help ?