Given the following code:
Class User{
Task m_Task;
public function getTask("Do work") { return m_Task; }
}
Class Project{
Task m_Task;
public function getTask("Do work") { return m_Task; }
}
Class Task {
private m_Name;
public Task(name) { m_Name = name; }
}
Class Evil {
new Task = Error
}
In a language that does not support multiple inheritance, nested classes, or private classes, (private constructor not an option), how can you design a requirement that Task is only ever instantiated through User or Project? Ideally using only language constructs instead of code. Is there a design pattern?
Not quite an answer, you can construct a task from elsewhere but you need a valid task instance to do it..
public class Task {
public Task (User u, String name)
{
if (u==null) throw new UnsupportedOperationException("that's cheating");
this.name=name;
}
}
Utlimately, if you're creating the requirement that Task be external to user then something else needs to be able to load task's constructor so I don't think it's possible.
What's the purpose of this question? Does this relate in any way to some real-world situation?
It seems somewhat quixotic, but I suppose you write Task a constructor that requires a User as an argument and does some handshaking back and forth to link the User to the Task, requiring that the User does not already have a Task.
Related
Imagine you have some Entity class and want another class that groups multiple instances of Entity.
How do you name it?
CompoundEntity?
CompositeEntity?
something else?
This is a common thing I do, and my colleagues use different naming convention. I have no idea what is better and also I'm not a native English speaker.
Concrete example:
public final class CompoundSetUpAction: SetUpAction {
private let setUpActions: [SetUpAction]
public init(
setUpActions: [SetUpAction])
{
self.setUpActions = setUpActions
}
public func setUp() -> TearDownAction {
return CompoundTearDownAction(
tearDownActions: Array(setUpActions.map { $0.setUp() }.reversed())
)
}
}
I am looking for a way to dynamically select the correct dependency during runtime using google guice.
My usecase is a kotlin application which can work with either sqlite or h2 databases depending on the configuration file provided.
The file is read when the application is executed and if the database is not found, the correct one is created and migrated into.
My database structure contains the Database (Interface), H2Database: Database, SQLiteDatabase: Database and the module binding class which looks like this:
class DatabaseModule: KotlinModule() {
override fun configure() {
bind<Database>().annotatedWith<configuration.H2>().to<H2Database>()
bind<Database>().annotatedWith<configuration.SQLite>().to<SQLiteDatabase>()
}
}
So far, with SQlite alone, I would simply request the dependency using:
#Inject
#SQLite
private lateinit var database: Database
How would I make this selection during runtime?
Without knowing too much about the specific of your code, I'll offer three general approaches.
(Also, I have never used Kotlin. I hope Java samples are enough for you to figure things out.)
First Approach
It sounds like you need some non-trivial logic to determine which Database implementation is the right one to use. This is a classic case for a ProviderBinding. Instead binding Database to a specific implementation, you bind Database to a class that is responsible providing instances (a Provider). For example, you might have this class:
public class MyDatabaseProvider.class implements Provider<Database> {
#Inject
public MyDatabaseProvider.class(Provider<SQLiteDatabase> sqliteProvider, Provider<H2Database> h2Provider) {
this.sqliteProvider = sqliteProvider;
this.h2Provider = h2Provider;
}
public Database get() {
// Logic to determine database type goes here
if (isUsingSqlite) {
return sqliteProvider.get();
} else if (isUsingH2) {
return h2Provider.get();
} else {
throw new ProvisionException("Could not determine correct database implementation.");
}
}
}
(Side note: This sample code gets you a new instance every time. It is fairly straightforward to make this also return a singleton instance.)
Then, to use it, you have two options. In your module, you would bind Database not to a specific implementation, but to your DatabaseProvider. Like this:
protected void configure() {
bind(Database.class).toProvider(MyDatabaseProvider.class);
}
The advantage of this approach is that you don't need to know the correct database implementation until Guice tries to construct an object that requires Database as one of its constructor args.
Second Approach
You could create a DatabaseRoutingProxy class which implements Database and then delegates to the correct database implementation. (I've used this pattern professionally. I don't think there's an "official" name for this design pattern, but you can find a discussion here.) This approach is based on lazy loading with Provider using the Providers that Guice automatically creates(1) for every bound type.
public class DatabaseRoutingProxy implements Database {
private Provider<SqliteDatabse> sqliteDatabaseProvider;
private Provider<H2Database> h2DatabaseProvider;
#Inject
public DatabaseRoutingProxy(Provider<SqliteDatabse> sqliteDatabaseProvider, Provider<H2Database> h2DatabaseProvider) {
this.sqliteDatabaseProvider = sqliteDatabaseProvider;
this.h2DatabaseProvider = h2DatabaseProvider;
}
// Not an overriden method
private Database getDatabase() {
boolean isSqlite = // ... decision logic, or maintain a decision state somewhere
// If these providers don't return singletons, then you should probably write some code
// to call the provider once and save the result for future use.
if (isSqlite) {
return sqliteDatabaseProvider.get();
} else {
return h2DatabaseProvider.get();
}
}
#Override
public QueryResult queryDatabase(QueryInput queryInput) {
return getDatabase().queryDatabase(queryInput);
}
// Implement rest of methods here, delegating as above
}
And in your Guice module:
protected void configure() {
bind(Database.class).to(DatabaseRoutingProxy.class);
// Bind these just so that Guice knows about them. (This might not actually be necessary.)
bind(SqliteDatabase.class);
bind(H2Database.class);
}
The advantage of this approach is that you don't need to be able to know which database implementation to use until you actually make a database call.
Both of these approaches have been assuming that you cannot instantiate an instance of H2Database or SqliteDatabase unless the backing database file actually exists. If it's possible to instantiate the object without the backing database file, then your code becomes much simpler. (Just have a router/proxy/delegator/whatever that takes the actual Database instances as the constructor args.)
Third Approach
This approach is completely different then the last two. It seems to me like your code is actually dealing with two questions:
Does a database actually exist? (If not, then make one.)
Which database exists? (And get the correct class to interact with it.)
If you can solve question 1 before even creating the guice injector that needs to know the answer to question 2, then you don't need to do anything complicated. You can just have a database module like this:
public class MyDatabaseModule extends AbstractModule {
public enum DatabaseType {
SQLITE,
H2
}
private DatabaseType databaseType;
public MyDatabaseModule(DatabaseType databaseType) {
this.databaseType = databaseType;
}
protected void configure() {
if (SQLITE.equals(databaseType)) {
bind(Database.class).to(SqliteDatabase.class);
} else if (H2.equals(databaseType)) {
bind(Database.class).to(H2Database.class);
}
}
}
Since you've separated out questions 1 & 2, when you create the injector that will use the MyDatabaseModule, you can pass in the appropriate value for the constructor argument.
Notes
The Injector documentation states that there will exist a Provider<T> for every binding T. I have successfully created bindings without creating the corresponding provider, therefore Guice must be automatically creating a Provider for configured bindings. (Edit: I found more documentation that states this more clearly.)
I'm trying to find a method of passing a constructor argument to the constructors of child classes.
These objects are immutable so I'd prefer to use constructor arguments.
The issue I have encountered is that ConstructorArgument does not inherit to child instantiations and the following statements are not interchangeable:
_parsingProcessor = _kernel.Get<IParsingProcessor>(new ConstructorArgument("dataFilePath", dataFilePath);
and
_parsingProcessor = _kernel.Get<IParsingProcessor>(new Parameter("dataFilePath", dataFilePath, true);
So, how can get an inheritable ConstructorArgument and when does it makes sense, if ever, to new the Parameter class?
Yes, you can do this, but it's probably not what you really want. If the container is not actually responsible for instantiating its own dependencies, then its dependencies probably shouldn't be sharing its constructor arguments - it just doesn't make sense.
I'm pretty sure I know what you're trying to do, and the recommended approach is to create a unique binding specifically for your one container, and use the WhenInjectedInto conditional binding syntax, as in the example below:
public class Hello : IHello
{
private readonly string name;
public Hello(string name)
{
this.name = name;
}
public void SayHello()
{
Console.WriteLine("Hello, {0}!", name);
}
}
This is the class that takes a constructor argument which we want to modify, depending on who is asking for an IHello. Let's say it's this boring container class:
public class MyApp : IApp
{
private readonly IHello hello;
public MyApp(IHello hello)
{
this.hello = hello;
}
public virtual void Run()
{
hello.SayHello();
Console.ReadLine();
}
}
Now, here's how you do up the bindings:
public class MainModule : NinjectModule
{
public override void Load()
{
Bind<IApp>().To<MyApp>();
Bind<IHello>().To<Hello>()
.WithConstructorArgument("name", "Jim");
Bind<IHello>().To<Hello>()
.WhenInjectedInto<MyApp>()
.WithConstructorArgument("name", "Bob");
}
}
Basically all this binding is doing is saying the name should be "Jim" unless it's being requested by Hello, which in this case it is, so instead it will get the name "Bob".
If you are absolutely certain that you truly want cascading behaviour and understand that this is very dangerous and brittle, you can cheat using a method binding. Assuming that we've now added a name argument to the MyApp class for some unspecified purpose, the binding would be:
Bind<IHello>().ToMethod(ctx =>
ctx.Kernel.Get<Hello>(ctx.Request.ParentContext.Parameters
.OfType<ConstructorArgument>()
.Where(c => c.Name == "name")
.First()));
Please, please, make sure you are positive that this is what you want before doing it. It looks easy but it is also very likely to break during a simple refactoring, and 95% of the "customized dependency" scenarios I've seen can be addressed using the WhenInjectedInto binding instead.
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.
To show an example what is this question about:
I have currently a dilemma in PHP project I'm working on. I have in mind a method that will be used by multiple classes (UIs in this case - MVC model), but I'm not sure how to represent such methods in OO design. The first thing that came into my mind was to create a class with static functions that I'd call whenever I need them. However I'm not sure if it's the right thing to do.
To be more precise, I want to work, for example, with time. So I'll need several methods that handle time. I was thinking about creating a Time class where I'd be functions that check whether the time is in correct format etc.
Some might say that I shouldn't use class for this at all, since in PHP I can still use procedural code. But I'm more interested in answer that would enlighten me how to approach such situations in OOP / OOD.
So the actual questions are: How to represent such methods? Is static function approach good enough or should I reconsider anything else?
I would recommend creating a normal class the contains this behavior, and then let that class implement an interface extracted from the class' members.
Whenever you need to call those methods, you inject the interface (not the concrete class) into the consumer. This lets you vary the two independently of each other.
This may sound like more work, but is simply the Strategy design pattern applied.
This will also make it much easier to unit test the code, because the code is more loosely coupled.
Here's an example in C#.
Interface:
public interface ITimeMachine
{
IStopwatch CreateStopwatch();
DateTimeOffset GetNow();
}
Production implementation:
public class RealTimeMachine : ITimeMachine
{
#region ITimeMachine Members
public IStopwatch CreateStopwatch()
{
return new StopwatchAdapter();
}
public DateTimeOffset GetNow()
{
return DateTimeOffset.Now;
}
#endregion
}
and here's a consumer of the interface:
public abstract class PerformanceRecordingSession : IDisposable
{
private readonly IStopwatch watch;
protected PerformanceRecordingSession(ITimeMachine timeMachine)
{
if (timeMachine == null)
{
throw new ArgumentNullException("timeMachine");
}
this.watch = timeMachine.CreateStopwatch();
this.watch.Start();
}
public abstract void Record(long elapsedTicks);
public virtual void StopRecording()
{
this.watch.Stop();
this.Record(this.watch.ElapsedTicks);
}
}
Although you say you want a structure for arbitrary, unrelated functions, you have given an example of a Time class, which has many related functions. So from an OO point of view you would create a Time class and have a static function getCurrentTime(), for example, which returns an instance of this class. Or you could define that the constuctors default behaviour is to return the current time, whichever you like more. Or both.
class DateTime {
public static function getNow() {
return new self();
}
public function __construct() {
$this->setDateTime('now');
}
public function setDateTime($value) {
#...
}
}
But apart from that, there is already a builtin DateTime class in PHP.
Use a class as a namespace. So yes, have a static class.
class Time {
public static function getCurrentTime() {
return time() + 42;
}
}
I don't do PHP, but from an OO point of view, placing these sorts of utility methods as static methods is fine. If they are completely reusable in nature, consider placing them in a utils class.