I am a big fan of StructureMap and use it in just about everything I do. I have only ever used it with interfaces though. I was wondering if anyone had any experience using with abstract classes? or...does it not support that type of wiring? If you got this to work can you post an example?
Thanks!
Yes, abstract classes work exactly the same way as interfaces.
If WorkerBase is an abstract class, and RealWorker is an implementation, then:
var container = new Container(x => x.For<WorkerBase>().Use<RealWorker>());
var worker = container.GetInstance<WorkerBase>();
Related
I am new to Swift and OOP. For example, I have a class that manages the system-wide configurations.
class system_conf {
init()
getValue1()
getValue2()
...
setValue1()
setValue2()
...
reloadValues()
activateX()
activeteY()
...
}
This class should have only one instance and many other classes will use it. What's the recommended way for this case?
Should I pass around this instance?
Should I consider to use Singleton?
Should I use static functions directly?
Should I create a global instance, so every other class can access it directly?
or?
It seems your class is a configuration class. If you intend to pass it to a bunch of classes, you should wonder if you need to write unit tests for them.
If so, assuming you are either using a singleton or static methods or a global var, take a moment to think about how you would mock this configuration class for each of your tests. It's not easy, is it?
If your class is a kind of mediator, a global var or static methods are fine (or any other alternative you suggested). However, in your case, it would be better to pass your object in any initializer/constructor of each class using it. Then, testing would definitely be easier. Also, passing it via an interface is even better: you can mock it super easily (mock up libraries mostly work with interfaces only).
So there is no unique answer to your question. It is just a matter of compromises and scaling. If your app is small, any of the method you listed above is perfectly fine. However, if you app tends to get bigger, a proxy solution would be better for maintainability and testability.
If you fancy reading, you should glance at this article from Misko Hevery, especially this chapter.
I writing a small Snake game in Lazarus, and Lazarus complains when I write
type
ISegment = interface(IRenderable, IMover)
end;
When I'm trying to achieve is to make ISegment a combined interface, but it doesn't seem to work. Does Lazarus not support multiple interface inheritance?
There is no multiple inheritance supported in the language. A class cannot be derived from multiple base classes. An interface cannot be derived from multiple base interfaces.
What you can do however, is have a class that implements multiple interfaces. Like this:
type
TMyClass = class(TInterfacedObject, IFoo, IBar)
....
end;
It does, you just need a better reading skill to understand this (look at the syntax diagram, in the heritage part). class type identifier is not stated as optional, but implemented interface does. It's roughly read as:
"A class may extend a base class and implements as many interfaces as possible. When an interface is about to be implemented, the base class must also be specified. The other way around does not apply, you can perfectly have a class extends a base class without specifying any interface"
The answer is no, Pascal is not supposed to support multiple inheritance so I don't see why it should do a different thing for interfaces then
As explained in previous answer, you still can implement several interfaces in a class
We cannot create an instance of an interface.
But why does Arrays.asList(Object[] a) in the Java API, return a List (List being an interface)?
Thank you!
It creates an instance of a class which implements the interface.
You don't know what that class is; it could even use a different class every other Tuesday (it doesn't).
You just use the class through the interface.
Java and OOO programming in general lets you define how an object should be used (that´s the interface of the object) so only the library implementor needs to worry about the gory details of how things actually work. That´s why it is good practice to never return a class itself but just an interface, in addition to better maintanibility it will also let you use mocks or stubs objects when coding tests for your applications.
Java in particular let´s you create an interface implementation on fly. i.e you can do something like
return new List() {
boolean add() {...}
void addAll {...}
...
}
This is of course an overkill for complex interfaces like List but actually very handy for smaller interfaces.
With reference to UML diagrams, what is an interface? and can someone explain in more simpler words. I cant understand anything from googling it.
An interface is like a template design for a class that contains no data or implemetnation; only definitions for methods, properties etc. These are abstract and cannot be instantiated but can be inherited from at which point all specified methods etc must be implemented by the concrete class inheriting the interface.
An interface is a design item describing a behaviour.
Classes implementing the interface will/must behave according to its definition.
Interfaces are used to promote loose coupling and the base of many IoC patterns (Inversion of Control)
A Interface is just a description of a class nothing concrete.
You use it to create a new class "with the same description" without knowing the concrete implementation.
In one word: it's a contract. Every class that implements this contract (interface) will have to implement the methods defined on it.
I am looking at nServiceBus and came over this interface
namespace NServiceBus
{
public interface IMessage
{
}
}
What is the use of an empty interface?
Usually it's to signal usage of a class. You can implement IMessage to signal that your class is a message. Other code can then use reflection to see if your objects are meant to be used as messages and act accordingly.
This is something that was used in Java a lot before they had annotations. In .Net it's cleaner to use attributes for this.
#Stimpy77 Thanks! I hadn't thought of it that way.
I hope you'll allow me to rephrase your comment in a more general way.
Annotations and attributes have to be checked at runtime using reflection. Empty interfaces can be checked at compile-time using the type-system in the compiler. This brings no overhead at runtime at all so it is faster.
Also known as a Marker Interface:
http://en.wikipedia.org/wiki/Marker_interface_pattern
In java Serializable is the perfect example for this. It defines no methods but every class that "implements" it has to make sure, that it is really serializable and holds no reference to things that cannot be serialized, like database connections, open files etc.
In Java, empty interfaces were usually used for "tagging" classes - these days annotations would normally be used.
It's just a way of adding a bit of metadata to a class saying, "This class is suitable for <this> kind of use" even when no common members will be involved.
Normally it's similar to attributes. Using attributes is a preferred to empty interfaces (at least as much as FxCop is aware). However .NET itself uses some of these interfaces like IRequiresSessionState and IReadOnlySessionState. I think there is performance loss in metadata lookup when you use attributes that made them use interfaces instead.
An empty interface acts simply as a placeholder for a data type no better specified in its interface behaviour.
In Java, the mechanism of the interface extension represents a good example of use. For example, let's say that we've the following
interface one {}
interface two {}
interface three extends one, two {}
Interface three will inherit the behaviour of 'one' and 'two', and so
class four implements three { ... }
has to specify the two methods, being of type 'three'.
As you can see, from the above example, empty interface can be seen also as a point of multiple inheritance (not allowed in Java).
Hoping this helps to clarify with a further viewpoint.
They're called "Mark Interfaces" and are meant to signal instances of the marked classes.
For example... in C++ is a common practice to mark as "ICollectible" objects so they can be stored in generic non typed collections.
So like someone over says, they're to signal some object supported behavior, like ability to be collected, serialized, etc.
Been working with NServiceBus for the past year. While I wouldn't speak for Udi Dahan my understanding is that this interface is indeed used as a marker primarily.
Though I'd suggest you ask the man himself if he'd had thoughts of leaving this for future extension. My bet is no, as the mantra seems to be to keep messages very simple or at least practically platform agnostic.
Others answer well on the more general reasons for empty interfaces.
I'd say its used for "future" reference or if you want to share some objects, meaning you could have 10 classes each implementing this interface.
And have them sent to a function for work on them, but if the interface is empty, I'd say its just "pre"-work.
Empty interfaces are used to document that the classes that implement a given interface have a certain behaviour
For example in java the Cloneable interface in Java is an empty interface. When a class implements the Cloneable interface you know that you can call run the clone() on it.
Empty interfaces are used to mark the class, at run time type check can be performed using the interfaces.
For example
An application of marker interfaces from the Java programming language is the Serializable interface. A class implements this interface to indicate that its non-transient data members can be written to an ObjectOutputStream. The ObjectOutputStream private method writeObject() contains a series of instanceof tests to determine writeability, one of which looks for the Serializable interface. If any of these tests fails, the method throws a NotSerializableException.
An empty interface can be used to classify classes under a specific purpose. (Marker Interface)
Example : Database Entities
public interface IEntity {
}
public class Question implements IEntity {
// Implementation Goes Here
}
public class Answer implements IEntity {
// Implementation Goes Here
}
For Instance, If you will be using Generic Repository(ex. IEntityRepository), using generic constraints, you can prevent the classes that do not implement the IEntity interface from being sent by the developers.