Imagine this scenario:
A Customer object that is provided by a third-party system which have multiple properties related to phone numbers.
public class Customer {
private String phoneAreaCode;
private String phoneNumber;
}
Another third-party system which receive his own Customer object which have a Phone object inside it.
public class XCustomer {
private Phone phone;
}
I would like to know if there is a OO design pattern to convert the Customer to XCustomer. Should one of the Customers know about the other Customer object and convert the phone properties into a Phone object? Should I create utility classes to convert one into another? Or there is a better solution?
Since both Customer and CustomerX classes come from third party so you can't modify Customer and CustomerX class.
I prefer a simple utility to do the conversion.
You can create Converter interface/implementation. This will be useful in DI environment or you want to have multi implementations
public interface CustomerConverter {
CustomerX convert(Customer customer);
}
public class CustomerConverterImpl implements CustomerConverter {
#Override
public CustomerX convert(Customer customer) {
// TODO convert
}
}
Related
Why does object's type refer to its interface? Why the term type is used here? In terms of C++ I am not able to understand it.
Gamma, Erich. Design Patterns: Elements of Reusable Object-Oriented
Software (Addison-Wesley Professional Computing Series) (Kindle
Locations 593-596). Pearson Education. Kindle Edition.
An object’s class defines how the object is implemented. The class
defines the object’s internal state and the implementation of its
operations. In contrast, an object’s type only refers to its
interface—the set of requests to which it can respond. An object can
have many types, and objects of different classes can have the same
type.
An oversimplification...
Interface - a list of things that a class have and the things that it can do... a list of things that answer the "Whats"
Implementation - answers the question on "How" the "Whats" are accomplished.
Example:
An interface IPackageMover that does 2 things and 2 classes (types) that actually implements the interface (and also do other things aside from the interface requires)
// the interface
public interface IPackageMover
{
string GetName();
void public void MoveTo(Package package, string newAddress);
}
// the "type" that has the implementation
public class JoeThePackageMover : IPackageMover
{
public string GetName()
{
return "Joe";
}
public void MoveTo(Package package, string newAddress)
{
PickUp(package);
Drive(newAddress);
DropOff(package);
}
public void PickUp(Package package)
{
// do stuff
}
public void Drive(string newAddress)
{
// do stuff
}
public void DropOff(Package package)
{
// do stuff
}
}
// another "type" with the same interface
public class PassTheBuckPackageMover : IPackageMover
{
public string GetName()
{
return "What do you want it to be?";
}
public void MoveTo(Package package, string newAddress)
{
var joe = new JoeThePackageMover();
joe.MoveTo(package, newAddress);
}
public void Chill()
{
//do stuff
}
}
Why does object's type refer to its interface? Why the term type is used here? In terms of C++ I am not able to understand it.
Objects in OOP are not very different from the real world. For example :
A Car IS-A Vehicle. By this definition, a Car has the ability to transport people/cargo from one place to another.
A Car is also a Car. By this definition, it has the ability to be driven using a steering wheel.
In the above example, a Car IS-A Car and a Car is also a Vehicle because it can be driven using a steering wheel to move cargo/people from one place to another. In other words, the type of an object in the real world is defined by the things you can do with it (vis-à-vis it's interface.)
If we use the above analogy in programming, Car is a subclass of Vehicle and code that has a Car object can use all functions from Vehicle as well as Car. This would mean that a Car IS-A Vehicle and a Car. In conclusion, the type of object is defined by its interface, i.e the set of operations it supports.
A Model object which has some private internal state. A component of this state is exposed to the clients. But one of the clients wants a different component of the internal state to be exposed. How should this be dealt with?An example
public GarageModel {
private Vehicle car;
private Vehicle truck;
public Vehicle getMostInterestingVehicle() {
//exposes car as the most interesting vehicle but
//one of the client wants this to return a truck
return car;
}
}
It is hard to say given your sample, you could apply a strategy pattern on your GarageModel class for each different client and override that single method to cater to each of their needs. This only works if you can provide different garage models to your clients though.
Polymorphism is always the answer as a teacher of mine used to say
An example would be
public TruckGarageModel: GarageModel {
public override Vehicle getMostInterestingVehicle(){
return truck;
}
}
public CarGarageModel: GarageModel {
public override Vehicle getMostInterestingVehicle(){
return car;
}
}
Then you would pass the appropriate decorated version of your GarageModel to each different client
You could provide method with parameters which will define criteria by which your client sees most interesting vehicle.
public Vehicle getMostInterestingVehicleByCriteria(VehicleCriteria vehicleCriteria){
// logic which selects correct vehicle in simple way it will be just
if(vehicleCriteria.getMostInterestingVehicleType().equals(VehicleType.TRUCK){
//do your logic
}
// In case of multiple objects you should refactor it with simple inheritance/polymorphism or maybe use some structural pattern
}
public class VehicleCriteria{
VehicleType mostInterestingVehicle; // enum with desired vehicle type
public VehicleCriteria(VehicleType mostInterestingVehicle){
this.mostInterestingVehicle = mostInterestingVehicle;
}
}
If the client knows what type it wants, then let the client say so with a generic argument (C# assumed):
public T getMostInterestingVehicle<T>() where T : Vehicle { }
You can then use a dictionary of 'things' (factories maybe?) that will get a vehicle, keyed by the type they return. This could be a static collection created on construction or resolved by IoC:
private Dictionary<T, Vehicle> _things;
You can then use this to do the work:
public T getMostInterestingVehicle<T>() where T : Vehicle
{
FactoryThing thing;
if (_things.TryGetValue(T, out thing))
{
return thing.GetVehicle();
}
}
Apologies if it's not C# you're working with and if the syntax/usage is incorrect, but I think you'll get my point...
You might have to consider quite a few things before making a decision about the implementation, some questions are these-
Do you expect newer implementations of Vehicle to be added later?: If yes, you might have to provide a factory that could register newer types without you making changes in the factory again and again. This provides an example. That way you avoid many if-else as well.
Do you want to decide what Vehicle to return at runtime?: Then Strategy Pattern helps as pointed in other answers.
Hope this helps a little bit.
Let's say you need to build an application that manages cheques. Each cheque contains data about the amount of money, the date, the payee and an additional payment date which may or may not be present. Additionally, each cheque must be related to a current account which belongs to a certain bank.
Now, our application should allow cheques printing under these conditions:
Each bank managed by the app has a different cheque layout (i.e. each field has a different x,y position).
The cheque layout changes slightly if the payment date is present, even with the same related bank object. But, from bank to bank these changes may not be the same (e.g. bank A may vary position for the date field, while bank B changes position for the payee field)
With these restrictions in place, it's difficult to come up with a simple inheritance schema as there is no consistent behavior to factor out accross the different types of cheques there are. One possible solution would be to avoid inheritance and create a class for every cheque - bank combination:
class ChequeNoPaymentDateForBankA
class ChequeWithPaymentDateForBankA
class ChequeNoPaymentDateForBankB
class ChequeWithPaymentDateForBankB, etc.
Each of these classes implement the print() method which takes the fields positions from a Bank object and builds up the cheque layout. So far so good, but this approach leaves me with a strange feeling as there is no room for code reuse. I wonder if I'm misinterpreting the problem and perhaps there is a better way. As this is not a new problem domain at all, I'm sure this is a reinvent-the-wheel effort. Any insights will be kindly appreciated.
Usually in these situations I move from inheritance to delegation. That is, instead of putting the common code in a superclass (which, as you say, is problematic becuase there are two dimensions), I put the common in a field (one field per dimension) and delegate to that field.
Assuming you're speaking about Java:
public interface Bank {
public void print();
}
public class BankA implements Bank {
public void print() { ... }
}
public class BankB implements Bank {
public void print() { ... }
}
public interface PaymentSchedule {
public void print();
}
public class WithPaymentDate implements PaymentSchedule {
public void print() { ... }
}
public class NoPaymentDate implements PaymentSchedule {
public void print() { ... }
}
public class Cheque {
private final Bank bank;
private final PaymentSchedule schedule;
public Cheque(Bank b, PaymentSchedule s) {
bank = b;
schedule = s;
}
public void print() {
bank.print();
schedule.print();
}
}
That's the general structure of the solution.
Depending on the exact details of your print() algorithm you may need to pass some more data into the print methods and/or to pass this data into the constructors of the classes (of the Bank or PaymentSchedule subclasses) and store it in fields.
I would start by separating the domain model (cheques, banks, etc) from the view (the way the cheques are printed). This is the basic idea behind the MVC pattern and one of its aims is to allow the same domain model to be displayed in different ways (which seems to be your case). So, I would first create the domain classes, something like:
class Cheque
{
protected $bank;
protected $money;
...
}
class Bank {...}
Note that these classes are the "M" of the MVC triad and implement the logic of your domain model, not the behavior related to the rendering process. The next step would be to implement the View classes used to render a cheque. Which approach to take heavily depends on how complex your rendering is, but I would start by having a ChequeView class that renders the common parts and that delegates to other sub-view the specific parts that can change (in this case the date):
abstract class ChequeView
{
protected $cheque;
protected $dateView;
public function __construct($cheque)
{
$this->cheque = $cheque;
$this->dateView = //Whatever process you use to decide if the payment date is shown or not
}
public function render()
{
$this->coreRender();
$this->dateView->render();
}
abstract protected function coreRender();
}
class BankACheckView extends ChequeView
{
protected function coreRender() {...}
}
class BankBCheckView extends ChequeView
{
protected function coreRender() {...}
}
abstract class DateView
{
abstract function render()
}
class ShowDateView extends DateView
{
function render() {...}
}
class NullDateView extends DateView
{
function render() {...}
}
And, if there is code to reuse across subclasses, you can of course factor them in ChequeView and make coreRender() call them.
In case your rendering turns to be too complex, this design may not scale. In that case I would go for splitting your view in meaningful subparts (e.g. HeaderView, AmountView, etc) so that rendering a cheque becomes basically rendering its different sub-parts. In this case the ChequeView may end basically working as a Composite. Finally, if you reach this case and setting up the ChequeView turns out to be a complex task you may want to use a Builder.
Edit based on the OP comments
The Builder is mostly used when the instantiation of the final object is a complex process (e.g. there are many things to sync between the sub-parts in order to get a consistent whole). There is generally one builder class and different clients, that send messages (potentially in different orders and with different arguments) to create a variety of final objects. So, while not prohibited, it is not usual to have one builder per type of object that you want to build.
If you are looking for a class that represents the creation of a particular instance you may want to check the Factory family of patterns (maybe the Abstract Factory resembles closely to what you had in mind).
HTH
OOP interfaces.
In my own experience I find interfaces very useful when it comes to design and implement multiple inter-operating modules with multiple developers. For example, if there are two developers, one working on backend and other on frontend (UI) then they can start working in parallel once they have interfaces finalized. Thus, if everyone follows the defined contract then the integration later becomes painless. And thats what interfaces precisely do - define the contract!
Basically it avoids this situation :
Interfaces are very useful when you need a class to operate on generic methods implemented by subclasses.
public class Person
{
public void Eat(IFruit fruit)
{
Console.WriteLine("The {0} is delicious!",fruit.Name);
}
}
public interface IFruit
{
string Name { get; }
}
public class Apple : IFruit
{
public string Name
{
get { return "Apple"; }
}
}
public class Strawberry : IFruit
{
public string Name
{
get { return "Strawberry"; }
}
}
Interfaces are very useful, in case of multiple inheritance.
An Interface totally abstracts away the implementation knowledge from the client.
It allows us to change their behavior dynamically. This means how it will act depends on dynamic specialization (or substitution).
It prevents the client from being broken if the developer made some changes
to implementation or added new specialization/implementation.
It gives an open way to extend an implementation.
Programming language (C#, java )
These languages do not support multiple inheritance from classes, however, they do support multiple inheritance from interfaces; this is yet another advantage of an interface.
Basically Interfaces allow a Program to change the Implementation without having to tell all clients that they now need a "Bar" Object instead of a "Foo" Object. It tells the users of this class what it does, not what it is.
Example:
A Method you wrote wants to loop through the values given to it. Now there are several things you can iterate over, like Lists, Arrays and Collections.
Without Interfaces you would have to write:
public class Foo<T>
{
public void DoSomething(T items[])
{
}
public void DoSomething(List<T> items)
{
}
public void DoSomething(SomeCollectionType<T> items)
{
}
}
And for every new iteratable type you'd have to add another method or the user of your class would have to cast his data. For example with this solution if he has a Collection of FooCollectionType he has to cast it to an Array, List or SomeOtherCollectionType.
With interfaces you only need:
public class Foo<T>
{
public void DoSomething(IEnumerable<T> items)
{
}
}
This means your class only has to know that, whatever the user passes to it can be iterated over. If the user changes his SomeCollectionType to AnotherCollectionType he neither has to cast nor change your class.
Take note that abstract base classes allow for the same sort of abstraction but have some slight differences in usage.
I would like some help with a OOD query.
Say I have the following Customer class:
public class Customer
{
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
It's a simple placeholder for customer related data. Next comes the CustomerFactory, which is used to fetch customers:
public static class CustomerFactory
{
public static Customer GetCustomer(int id)
{
return null; // Pretend this goes off to get a customer.
}
}
Now, if I wanted to write a routine named UpdateCustomer(Customer customer) can someone suggest where I could place this method?
Obviously I don't want to use the Customer class since that would be against SRP (Single Responsibility Principle), also I don't see it as a good idea to attach the method to the CustomerFactory class, since it's only role is to get customers from the database.
So it looks like I'm going to need another class, but I don't know what to name it.
Cheers.
Jas.
What you have called a Factory isn't a Factory at all. It's a Repository.
A Factory handles the instansiation of various classes sharing a common Interface or Class Hierarchy based on some set of parameters.
A Repository handles the retrieval and management of data.
The Repository would definitely have the UpdateCustomer(Customer customer) method in it as well as the GetCustomer(int id) method.
You are more on less on your way to creating a Repository. Do something like this:
public interface ICustomerRepository
{
Customer SelectCustomer(int id);
void UpdateCustomer(Customer customer);
void DeleteCustomer(int id);
void CreateCustomer(Customer customer);
}
Then create concrete implementations of this interface (the interface is really just because it's good practice to program against interfaces - you could skip it, though, although I would recommend that you keep it).
Wouldn't your UpdateCustomer routine be placed in your DAL (Data Access Layer). You should define a class to handle inserts or updates to the database and then pass a customer object to it.
You could write the DAL class to handle all of this but I don't see any issue in storing it in your CustomerFactory class, although as mentioned it is not really a factory.