I have a Event hierarchy with some types of it namely WelcomeEvent, LogOffEvent, BilledEvent etc. I want to generate different kind of emails for these events. What I do is have a abstract Email class with types WelcomeEmail, LogoffEmail, BilledEmail.
I have a EmailFactory inside which I have following method:
Email getEmail(Event event) {
if(event.instanceOf(WelcomeEvent))
return new WelcomeEmail(event);
if(event.instanceOf(LogOffEvent))
return new LogoffEmail(event);
if(event.instanceOf(LogOffEvent))
return new BilledEmail(event);
}
My reviewer says that instanceOf should be avoided as much as possible.
Is there a better way of doing this?
Make getEmail an instance method of your Event subclasses, that is abstract in the parent class Event. Pseudocode:
abstract class Event {
abstract Email getEmail();
}
class WelcomeEvent < Event {
Email getEmail() {
return new WelcomeEmail();
}
}
class LogOffEvent < Event {
Email getEmail() {
return new LogOffEmail();
}
}
Then you can simply call event.getEmail() on any event to get a new Email that corresponds to its subclass.
Related
A custom object that takes a parameter of (DocumentSnapShot documentsnapShot). also is an inner object from Firebase that retrieves a snapshot and set the values to my custom model also have its argument (DocumentSnapShot documentsnapShot). However, I wish to get the data from Firebase and pass it to my custom argument because mine takes multiple data not only Firebase. And it's not possible to iterate Firestore without an override.
Here's the code:
public UserSettings getUserSettings(DocumentSnapshot documentSnapshot){
Log.d(TAG, "getUserSettings: retrieving user account settings from firestore");
DocumentReference mSettings = mFirebaseFirestore.collection("user_account_settings").document(userID);
mSettings.get().addOnSuccessListener(new OnSuccessListener<DocumentSnapshot>() {
#Override
public void onSuccess(DocumentSnapshot documentSnapshot) {
UserAccountSettings settings = documentSnapshot.toObject(UserAccountSettings.class);
settings.setDisplay_name(documentSnapshot.getString("display_name"));
settings.setUsername(documentSnapshot.getString("username"));
settings.setWebsite(documentSnapshot.getString("website"));
settings.setProfile_photo(documentSnapshot.getString("profile_photo"));
settings.setPosts(documentSnapshot.getLong("posts"));
settings.setFollowers(documentSnapshot.getLong("followers"));
settings.setFollowing(documentSnapshot.getLong("following"));
}
});
}
You cannot return something now that hasn't been loaded yet. Firestore loads data asynchronously, since it may take some time for this. Depending on your connection speed and the state, it may take from a few hundred milliseconds to a few seconds before that data is available. If you want to pass settings object to another method, just call that method inside onSuccess() method and pass that object as an argument. So a quick fix would be this:
#Override
public void onSuccess(DocumentSnapshot documentSnapshot) {
UserAccountSettings settings = documentSnapshot.toObject(UserAccountSettings.class);
yourMethod(settings);
}
One more thing to mention is that you don't need to set the those values to object that already have them. You are already getting the data from the database as an object.
So remember, onSuccess() method has an asynchronous behaviour, which means that is called even before you are getting the data from your database. If you want to use the settings object outside that method, you need to create your own callback. To achieve this, first you need to create an interface like this:
public interface MyCallback {
void onCallback(UserAccountSettings settings);
}
Then you need to create a method that is actually getting the data from the database. This method should look like this:
public void readData(MyCallback myCallback) {
DocumentReference mSettings = mFirebaseFirestore.collection("user_account_settings").document(userID);
mSettings.get().addOnSuccessListener(new OnSuccessListener<DocumentSnapshot>() {
#Override
public void onSuccess(DocumentSnapshot documentSnapshot) {
UserAccountSettings settings = documentSnapshot.toObject(UserAccountSettings.class);
myCallback.onCallback(settings);
}
});
}
In the end just simply call readData() method and pass an instance of the MyCallback interface as an argument wherever you need it like this:
readData(new MyCallback() {
#Override
public void onCallback(UserAccountSettings settings) {
Log.d("TAG", settings.getDisplay_name());
}
});
This is the only way in which you can use that object of UserAccountSettings class outside onSuccess() method. For more informations, you can take also a look at this video.
Use LiveData as return type and observe the changes of it's value to execute desired operation.
private MutableLiveData<UserAccountSettings> userSettingsMutableLiveData = new MutableLiveData<>();
public MutableLiveData<UserAccountSettings> getUserSettings(DocumentSnapshot documentSnapshot){
DocumentReference mSettings = mFirebaseFirestore.collection("user_account_settings").document(userID);
mSettings.get().addOnSuccessListener(new OnSuccessListener<DocumentSnapshot>() {
#Override
public void onSuccess(DocumentSnapshot documentSnapshot) {
UserAccountSettings settings = documentSnapshot.toObject(UserAccountSettings.class);
settings.setDisplay_name(documentSnapshot.getString("display_name"));
settings.setUsername(documentSnapshot.getString("username"));
settings.setWebsite(documentSnapshot.getString("website"));
settings.setProfile_photo(documentSnapshot.getString("profile_photo"));
settings.setPosts(documentSnapshot.getLong("posts"));
settings.setFollowers(documentSnapshot.getLong("followers"));
settings.setFollowing(documentSnapshot.getLong("following"));
userSettingsMutableLiveData.setValue(settings);
}
});
return userSettingsMutableLiveData;
}
Then from your Activity/Fragment observe the LiveData and inside onChanged do your desired operation.
getUserSettings().observe(this, new Observer<UserAccountSettings>() {
#Override
public void onChanged(UserAccountSettings userAccountSettings) {
//here, do whatever you want on `userAccountSettings`
}
});
I am trying to clean and refactor my service code which currently looks like this-
public void generateBalance(Receipt receipt) {
if (receipt.getType().equals(X) && receipt.getRegion.equals(EMEA)) {
// do something to the receipt that's passed
} else if (receiptType.equals(Y)) {
// do something to the receipt
} else if (receipt.getRegion.equals(APAC) {
// call an external API and update the receipt
}....
...
// finally
dataStore.save(receipt);
Basically there's a bunch of conditionals that are in this main service which look for certain fields in the object that is being passed. Either it's the type or the region.
I was looking to use this design pattern- https://www.refactoring.com/catalog/replaceConditionalWithPolymorphism.html
However, I am not sure how this would work for a service class. Currently my REST handler calls this particular service. Also how can I do polymorphism for both the "receiptType" and "region"?
Is there a way I can just do all the updates to the receipt once in different services, then finally save the receipt at one location? (maybe a base class?) I am really confused on how to start. TIA!
If your classes should have the same behaviour, then it becomes pretty simple to use polymorpism. The pattern is called as Strategy. Let me show an example.
At first we need to use enum. If you do not have enum, then you can create a method which will return enum value based on your conditions:
if (receipt.getType().equals(X) && receipt.getRegion.equals(EMEA)) // other
// code is omitted for the brevity
So enum will look like this:
public enum ReceiptType
{
Emea, Y, Apac
}
Then we need an abstract class which will describe behaviour for derived classes:
public abstract class ActionReceipt
{
public abstract string Do();
}
And our derived classes will look this:
public class ActionReceiptEmea : ActionReceipt
{
public override string Do()
{
return "I am Emea";
}
}
public class ActionReceiptY : ActionReceipt
{
public override string Do()
{
return "I am Y";
}
}
public class ActionReceiptApac : ActionReceipt
{
public override string Do()
{
return "I am Apac";
}
}
Moreover, we need a factory which will create derived classes based on enum. So we can use Factory pattern with a slight modification:
public class ActionReceiptFactory
{
private Dictionary<ReceiptType, ActionReceipt> _actionReceiptByType =
new Dictionary<ReceiptType, ActionReceipt>
{
{
ReceiptType.Apac, new ActionReceiptApac()
},
{
ReceiptType.Emea, new ActionReceiptEmea()
},
{
ReceiptType.Y, new ActionReceiptY()
}
};
public ActionReceipt GetInstanceByReceiptType(ReceiptType receiptType) =>
_actionReceiptByType[receiptType];
}
And then polymorpism in action will look like this:
void DoSomething(ReceiptType receiptType)
{
ActionReceiptFactory actionReceiptFactory = new ActionReceiptFactory();
ActionReceipt receipt =
actionReceiptFactory.GetInstanceByReceiptType(receiptType);
string someDoing = receipt.Do(); // Output: "I am Emea"
}
UPDATE:
You can create some helper method which will return enum value based on
your logic of region and receiptType:
public class ReceiptTypeHelper
{
public ReceiptType Get(ActionReceipt actionReceipt)
{
if (actionReceipt.GetType().Equals("Emea"))
return ReceiptType.Emea;
else if (actionReceipt.GetType().Equals("Y"))
return ReceiptType.Y;
return ReceiptType.Apac;
}
}
and you can call it like this:
void DoSomething()
{
ReceiptTypeHelper receiptTypeHelper = new ReceiptTypeHelper();
ReceiptType receiptType = receiptTypeHelper
.Get(new ActionReceiptEmea());
ActionReceiptFactory actionReceiptFactory = new
ActionReceiptFactory();
ActionReceipt receipt =
actionReceiptFactory.GetInstanceByReceiptType(receiptType);
string someDoing = receipt.Do(); // Output: "I am Emea"
}
I have the following property and pseudo-code method:
private DataModelMember _PropertyMember = new DataModelMember();
public DataModelMember PropertyMember
{
get { return _PropertyMember; }
}
void GetWCFData()
{
DataModel d = client.getWCFData();
this._PropertyMember = d.result.anotherclass; //*(DataModelMember is anotherclass)
}
So, "this._PropertyMember = value" does in fact change the private member, it however does not update any of the properties inside (as evident with the databinding). It almost seems like my 'DataModelMember' class needs to inherit some other interface to notify of the class copy?
INotifyPropertyChanged is not magic.
You need to raise the PropertyChanged event yourself whenever the property changes.
I am a bit lot about what to do in an OO/DB relation...
Here is the DB model :
CREATE TABLE User
Id
CREATE TABLE Location
userId
// EDIT oups, wrong !
// placeId
// Should be :
seatId
CREATE TABLE Game
locationId
Now some code :
class User
{
private Location locations[]; // need this for several reasons...
public function loadFromDatabase()
{
// Load data from DB
// ...
result = DB::query("SELECT Id FROM Locations WHERE userId="+this->Id);
foreach(result)
{
l = new Location();
l->loadFromDatabase(result);
locations[] = l;
}
}
}
class Location
{
private User user;
public function loadFromDatabase()
{
...
}
}
class Game
{
private Location location;
public loadFromDatabase()
{
/*
Here comes the problem :
how to have a reference to a location
created by the User class ?
*/
}
}
A User play Games in several Locations.
EDIT : And for each location the user plays on seat. Or on another seat...
When I want to know where a game has been played I access Game.location. And when I want to know who played it, I access Game.location.user
Here is my problem : I want the Game.location to be the same reference to one of the User.locations and I do not know how to do this...
And, globally, I feel something wrong about my code...
Any help ?
Thanks
Since you have a placeId in your Location table, I assume there is a Place table which describes what the places actually are, while the Location table simply represents the many-to-many mapping between users and places.
In that case, Location doesn't need to have an Id of its own and doesn't need to be a class, but Place does.
To load just one instance of each object from the database, cache the instances in a static map inside each class.
class Place
{
// Static
private static Place loadedPlaces[];
public static function get(id)
{
if (!loadedPlaces[id])
{
loadedPlaces[id] = new Place(id);
loadedPlaces[id]->loadFromDatabase();
}
return loadedPlaces[id];
}
// Non-static
private id;
public function loadFromDatabase()
{
// ...
}
}
Then to get references to places for the properties of a user or a game, you just access them via the static method.
class User
{
public function loadFromDatabase()
{
result = DB::query("SELECT placeId FROM Locations WHERE userId="+this->Id);
foreach(result)
{
places[] = Place::get(result);
}
}
}
class Game
{
public function loadFromDatabase()
{
place = Place::get(place);
}
}
This uses:
Lazy initialization, because places are only loaded when they are needed.
Multiton pattern, because there is only one instance of each place by id.
Not quite a factory method, because there's no object hierarchy involved.
I don't seem to find this in usage scenarios for the visitor pattern (or maybe I don't get it). It's also not hierarchical.
Let's use an authentication example. A UserAuthenticator authenticates credentials given by a user. It returns a result object. The result object contains the result of the authentication: authentication succeeded, not succeeded because username was not found, not succeeded because illegal characters were used etc. Client code may resort to conditionals to handle this.
In pseudocode:
AuthResult = Userauthenticator.authenticate(Username, Password)
if AuthResult.isAuthenticated: do something
else if AuthResult.AuthFailedBecauseUsernameNotFound: do something else
else if etc...
Would a visitor pattern fit here? :
Authresult.acceptVisitor(AuthVisitor)
Authresult then calls a method on AuthVisitor depending on the result :
AuthVisitor.handleNotAuthenticatedBecauseUsernameNotFound
I would not recommend using patterns for intent they were not made for.
The intents of the visitor patterns are:
Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.
The classic technique for recovering lost type information.
Do the right thing based on the type of two objects.
Double dispatch
This solution would be useful if you had planned to do various authentification methods, but if you plan on only doing one, you'll have to use conditionals anyway.
Visitor is a valuable design when your data doesn't change fast as your behaviour. A typical example is with a parse tree:
your class hierarchy (your data) is frozen
your behaviour varies too much, you don't want to break your classes adding another virtual method
I don't think that a Visitor is a valuable solution here, since each time you add a subclass of AuthResult you break your visitor.
Visitor is about trading encapsulation with double dispatch.
You can try a similar approach:
interface Handler {
void onUsernameNotFound();
void onWrongPassword();
void authOk();
}
interface Authenticator {
void authenticate(String username, String password, Handler handler);
}
class SimpleAuthenticator implements Authetnciator {
void authenticate(String username, String password, Handler handler) {
if (username.equals("dfa")) {
if (password.equals("I'm1337")) {
handler.authOk();
} else {
handler.onWrongPassword();
}
} else {
handler.onUsernameNotFound();
}
}
}
some Handler stategies:
class FatalHandler implements Handler {
void onUsernameNotFound() {
throw new AuthError("auth failed");
}
void onWrongPassword() {
throw new AuthError("auth failed");
}
void authOk() {
/* do something */
}
}
and:
class DebugHandler implements Handler {
void onUsernameNotFound() {
System.out.println("wrong username");
}
void onWrongPassword() {
System.out.println("wrong password");
}
void authOk() {
System.out.println("ok");
}
}
now you can encapsulate error handling and operatorion in your Handlers that is much less code than Visitor since you don't really need double dispatch here.