Say FrameworkA consumes a FrameworkA.StandardLogger class for logging. I want to replace the logging library by another one (the SuperLogger class).
To make that possible, there are interfaces: FrameworkA will provide a FrameworkA.Logger interface that other libraries have to implement.
But what if other libraries don't implement that interface? FrameworkA might be a not popular enough framework to make SuperLogger care about its interface.
Possible solutions are:
have a standardized interface (defined by standards like JSR, PSR, ...)
write adapters
What if there is no standardized interface, and you want to avoid the pain of writing useless adapters if classes are compatible?
Couldn't there be another solution to ensure a class meets a contract, but at runtime?
Imagine (very simple implementation in pseudo-code):
namespace FrameworkA;
interface Logger {
void log(message);
}
namespace SuperLoggingLibrary;
class SupperLogger {
void log(message) {
// ...
}
}
SupperLogger is compatible with Logger if only it implemented Logger interface. But instead of having a "hard-dependency" to FrameworkA.Logger, its public "interface" (or signature) could be verified at runtime:
// Something verify that SupperLogger implements Logger at run-time
Logger logger = new SupperLogger();
// setLogger() expect Logger, all works
myFrameworkAConfiguration.setLogger(logger);
In the fake scenario, I expect the Logger logger = new SupperLogger() to fail at run-time if the class is not compatible with the interface, but to succeed if it is.
Would that be a valid thing in OOP? If yes, does it exist in any language? If no, why is it not valid?
My question stands for statically-typed languages (Java, ...) or dynamically typed languages (PHP, ...).
For PHP & al: I know when there is no type-check you can use any object you want even if it doesn't implement the interface, but I'd be interested in something that actually checks that the object complies with the interface.
This is called duck typing, a concept that you will find in Ruby ("it walks like a duck, it quacks like a duck, it must be a duck")
In other dynamically typed languages you can simulate it, for example in PHP with method_exists. In statically typed languages there might be workarounds with reflection, a search for "duck typing +language" will help to find them.
This is more of a statically typed issue than a OOP one. Both Java and Ruby are OO languages, but Java woudlnt allow what you want (as its statically typed) but Ruby would (as its dynamically typed).
From a statically typed language point of view one of the major (if not the major) advantage is knowing at compile time if an assignment is safe and valid. What you're looking for is provided by dynamically typed languages (such as Ruby), but isnt possible in a statically typed language - and this is by design (compile time safety).
You can, but it is ugly, do something like (in Java):
Object objLogger = new SupperLogger();
Logger logger = (Logger)objLogger;
This would pass at compile time but would fail at runtime if the assignment was invalid.
That said, the above is pretty ugly and isnt something I would do - it doesnt give you much and risks an unpleasant (and possibly suprising) exception at runtime.
I guess the best you could hope for in a language like Java would be to abstract the creation away from where you want to use it:
Logger logger = getLogger();
With the internals of getLogger deciding what to return. This however just defers the actual creation to further down - you'll still have to do so in a statically typed safe way.
Related
consider a SAM defined in Java
public interface Transform {
public String apply(String str);
}
This interface supports lambda to type conversion in Kotlin automatically
fun run(transform: Transform) {
println(transform.apply("world"))
}
run { x -> "Hello $x!!" } // runs fine without any issues
But now consider a Kotlin interface
interface Transform2 {
fun apply(str: String): String
}
Now the only way to invoke the run function would be by creating an anonymous instance of Transform2
run(object : Transform2 {
override fun transform(str: String): String = "hello $str!!"
})
but if we make the Transform2 interface a functional interface then the below is possible
run { str -> "hello $str!!" }
Why the Kotlin compiler cannot automatically type cast lambdas to matching interfaces (just as it does with Java interfaces) without needing to explicitly mark the said interfaces as a functional interface.
I've found some kind of a rationale in a comment in KT-7770:
... treating all the applicable interfaces as SAM might be too
unexpected/implicit: one having a SAM-applicable interface may not
assume that it will be used for SAM-conversions. Thus, adding another
method to the interface becomes more painful since it might require
changing syntax on the call sites (e.g. transforming callable
reference to object literal).
Because of it, current vision is adding some kind of modifier for
interfaces that when being applied:
Adds a check that the interface is a valid SAM
Allows SAM-conversions on call sites for it
Something like this:
fun interface MyRunnable {
fun run()
}
Basically, he is saying that if the SAM conversion were done implicitly by default, and I add some new methods to the interface, the SAM conversions would no longer be performed, and every place that used the conversion needs to be changed. The word "fun" is there to tell the compiler to check that the interface indeed has only one abstract method, and also to tell the call site that this is indeed a SAM interface, and they can expect the author to not suddenly add new abstract methods to the interface, suddenly breaking their code.
The thread goes on to discuss why can't the same argument can't be applied to Java, and the reason essentially boils down to "Java is not Kotlin".
This is speculation, but I strongly suspect one reason is to avoid encouraging the use of functional interfaces over Kotlin's more natural approach.
Functional interfaces are Java's solution to the problem of adding lambdas to the Java language in a way that involved the least change and risk, and the greatest compatibility with what had been best practice in the nearly 20 years that Java had existed without them: the use of anonymous classes implementing named interfaces. It needs umpteen different named interfaces such as Supplier, BiFunction, DoublePredicate… each with their own method and parameter names, each incompatible with all the others — and with all the other interfaces people have developed over the years. (For example, Java has a whole host of interfaces that are effectively one-parameter functions — Function, UnaryOperator, Consumer, Predicate, ActionListener, AWTEventListener… — but are all unrelated and incompatible.) And all this is to make up for the fact that Java doesn't have first-class functions.
Kotlin has first-class functions, which are a much more general, more elegant, and more powerful approach. (For example, you can write a lambda (or function, or function literal) taking a single parameter, and use it anywhere that you need a function taking a single parameter, without worrying about its exact interface. You don't have to choose between similar-looking interfaces, or write your own if there isn't one. And there are none of the hidden gotchas that occur when Java can't infer the correct interface type.) All the standard library uses function types, as does most other Kotlin code people write. And because they're so widely-used, they're widely supported: as part of the Kotlin ecosystem, everyone benefits.
So Kotlin supports functional interfaces mainly for compatibility with Java. Compared to first-class functions, they're basically a hack. A very ingenious and elegant hack, and arguably a necessary one given how important backward compatibility is to the Java platform — but a hack nonetheless. And so I suspect that JetBrains want to encourage people to use function types in preference to them where possible.
In Kotlin, you have to explicitly request features which improve Java compatibility but can lead to worse Kotlin code (such as #JvmStatic for static methods, or casting to java.lang.Object in order to call wait()/notify()). So it fits into the same pattern that you also have to explicitly request a functional interface (by using fun interface).
(See also my previous answer on the subject.)
Is it true that for C++ to work similarly in terms of modern OOP as in Java, Ruby, Python, the function (or methods) must be declared virtual and if not, what "strange" behaviors may occur?
I think it is true that for Java, Ruby, Python, and possibly other OOP languages that are late comers such as PHP and Lua, and even Smalltalk and Objective-C, all methods are just what is known as "virtual functions"?
"Method" is an unfortunately overloaded term that can mean many things. There's a reason C++ prefers different terminology, and that's because not only does it do something different from other languages, but it intends to do something different from what other languages do.
In C++ you call a member function. i.e. you externally make a call to a function associated with an object. Whether that function is virtual or not is secondary; what matters is the intended ordering of your actions - you're reaching into the object's scope, and commanding it to take a specific action. It might be that the object can specialize the action, but if so, it warned you in advance that it would do this.
In Smalltalk and the languages that imitate it (Objective-C most closely), you send a message to an object. A message is constructed on your side of the call consisting of a task name (i.e. method selector), arguments, etc., and the packed up and sent to the object, for the object to deal with as it sees fit. Semantically, it's entirely the object's decision what to do upon receipt of the message - it can examine the task name and decide which implementation to apply dynamically, according to a user-implemented choice process, or even do nothing at all. The outside calling code doesn't get to say what the object will do, and certainly doesn't get any say in which procedure actually runs.
Some languages fall in the middle ground, e.g. Java is inspired by the latter, but doesn't give the user any way to specify unusual dynamic responses - for the sake of simplicity every message does result in a call, but which call is still hidden from the external code, because it's entirely the object's business. C++ was never built on this philosophy of messages in the first place, so it has a different default assumption about how its member functions should operate.
The thing is that C++ is like the great grand father. It has many features, which often requires huge code definition.
Consider an example:
class A
{
virtual void fn() = 0;
};
class B: A
{
void fn();
};
#include "a.hpp"
#include "b.hpp"
int main()
{
A *a = new B();
a->fn();
}
This would implement overriding in C++.
Note that virtual void fn()=0 makes the class A abstract, and a pointer to base class (A) is essential.
In Java, the process is even simpler
abstract class A
{
abstract void fn();
}
class B extends A
{
void fn() {
//Some insane function :)
}
}
public static void main(String[] args) {
B ob = new B();
ob.fn();
}
Well, the effect is same; but the process is largely different. In short, C++ does have many features implemented in languages like Java, Ruby etc. but it is simply implemented using some (often complicated) techniques.
Regarding Php, since it is directly based on C++, there exists some syntax similarities between C++ and Php.
It is true that (for example) in Java all methods are virtual by default. In C++ it is possible to overload a non-virtual function (as opposed to overriding a virtual function) in a subclass, leading to possible counter-intutive behaviour, when only the base function is actually executed via a pointer or reference to the base class (i.e. when polymorphic behavior would normally be expected).
Because C++ is a value-based (as opposed to reference-based) language, then even when a function has been declared as virtual, the well known
object slicing problem can still arise: the superclass method is invoked when the type of a value object of a subclass is `cut down' to that of the base class (e.g. when the subclass is passed to a function which takes a base class argument by value).
For this reason, it is recommended to make all non-leaf classes abstract, something which is often achieved by providing a virtual destructor, even if such would otherwise be gratuitous.
I am not a program designer by any means but I would really like to start getting a better grasp of how to do it and a better understanding of the .NET languages in general (VB, C#). I was reading a book by Wrox - Professional Visual Basic 2008. In it I believed it mentioned that Modules are slowly going out of existence. I can see why most coding would go into a class object but I would assume modules would always be necessary to at least keep the code clean.
Could anybody clarify this up for me? Also, I have been searching for a good source on software design but I can't seem to find any recent books published. I might be searching in the wrong places but I would really like to get my hands on one.
Thank you.
While in general they don't quite fit with OOP, they are still used and are required in some cases.
In VB.Net, if you wish to write extension methods, you are going to have to use a Module - The compiler will only allow Extension Methods to be defined in one.
You could of course get round not using Modules - an Non Inheritable Class with a private constructor and nothing but Shared Methods will achieved the same thing as a Module.
Like everything in programming (and many other things), they have their uses, and as long as they are not miss-used there is no problem with them. Right tool for the job!
The Module keyword in VB.NET primarily exists for compatibility with VB6 and earlier. Back then, most VB code was procedural with free-standing non-class Subs and Functions. The language acquired the Class keyword somewhere around VB4. Not true classes in the OOP sense, it didn't support inheritance. A feature missing from the underlying COM architecture.
It doesn't fit very well with the execution model provided by the CLR. There is no support for free functions, every method must be a member of a class. The VB.NET compiler emulates modules by declaring a class, the module procedures become Shared methods of that class. You can see this with Ildasm.exe:
.class private auto ansi sealed ConsoleApplication1.Module1
extends [mscorlib]System.Object
{
.custom instance void [Microsoft.VisualBasic]Microsoft.VisualBasic.CompilerServices.StandardModuleAttribute::.ctor() = ( 01 00 00 00 )
} // end of class ConsoleApplication1.Module1
Note how it is private, so that code can't get a reference to it, and sealed, so that no code can derive a class from a module.
The C# compiler does the exact same thing with a "static class", the CLR doesn't have a notion of static classes either. There are plenty of good reasons for static classes, the idea of "Module" isn't obsolete. You could accomplish the same by declaring a NotInheritable Class in VB.NET code, having only Shared methods. The VB.NET compiler however doesn't enforce methods to be Shared like the C# compiler does and doesn't allow you to declare the class private. As such, a Module is just fine.
Modules are the closest thing VB has to static classes, which can be very useful, even when programming in an object-oriented environment.
And since VB has no static classes, modules are as far as I know the only way to create extension methods.
You need modules in order to define your own Extension methods
This question already has answers here:
What is reflection and why is it useful?
(23 answers)
Closed 6 years ago.
I was just curious, why should we use reflection in the first place?
// Without reflection
Foo foo = new Foo();
foo.hello();
// With reflection
Class cls = Class.forName("Foo");
Object foo = cls.newInstance();
Method method = cls.getMethod("hello", null);
method.invoke(foo, null);
We can simply create an object and call the class's method, but why do the same using forName, newInstance and getMthod functions?
To make everything dynamic?
Simply put: because sometimes you don't know either the "Foo" or "hello" parts at compile time.
The vast majority of the time you do know this, so it's not worth using reflection. Just occasionally, however, you don't - and at that point, reflection is all you can turn to.
As an example, protocol buffers allows you to generate code which either contains full statically-typed code for reading and writing messages, or it generates just enough so that the rest can be done by reflection: in the reflection case, the load/save code has to get and set properties via reflection - it knows the names of the properties involved due to the message descriptor. This is much (much) slower but results in considerably less code being generated.
Another example would be dependency injection, where the names of the types used for the dependencies are often provided in configuration files: the DI framework then has to use reflection to construct all the components involved, finding constructors and/or properties along the way.
It is used whenever you (=your method/your class) doesn't know at compile time the type should instantiate or the method it should invoke.
Also, many frameworks use reflection to analyze and use your objects. For example:
hibernate/nhibernate (and any object-relational mapper) use reflection to inspect all the properties of your classes so that it is able to update them or use them when executing database operations
you may want to make it configurable which method of a user-defined class is executed by default by your application. The configured value is String, and you can get the target class, get the method that has the configured name, and invoke it, without knowing it at compile time.
parsing annotations is done by reflection
A typical usage is a plug-in mechanism, which supports classes (usually implementations of interfaces) that are unknown at compile time.
You can use reflection for automating any process that could usefully use a list of the object's methods and/or properties. If you've ever spent time writing code that does roughly the same thing on each of an object's fields in turn -- the obvious way of saving and loading data often works like that -- then that's something reflection could do for you automatically.
The most common applications are probably these three:
Serialization (see, e.g., .NET's XmlSerializer)
Generation of widgets for editing objects' properties (e.g., Xcode's Interface Builder, .NET's dialog designer)
Factories that create objects with arbitrary dependencies by examining the classes for constructors and supplying suitable objects on creation (e.g., any dependency injection framework)
Using reflection, you can very easily write configurations that detail methods/fields in text, and the framework using these can read a text description of the field and find the real corresponding field.
e.g. JXPath allows you to navigate objects like this:
//company[#name='Sun']/address
so JXPath will look for a method getCompany() (corresponding to company), a field in that called name etc.
You'll find this in lots of frameworks in Java e.g. JavaBeans, Spring etc.
It's useful for things like serialization and object-relational mapping. You can write a generic function to serialize an object by using reflection to get all of an object's properties. In C++, you'd have to write a separate function for every class.
I have used it in some validation classes before, where I passed a large, complex data structure in the constructor and then ran a zillion (couple hundred really) methods to check the validity of the data. All of my validation methods were private and returned booleans so I made one "validate" method you could call which used reflection to invoke all the private methods in the class than returned booleans.
This made the validate method more concise (didn't need to enumerate each little method) and garuanteed all the methods were being run (e.g. someone writes a new validation rule and forgets to call it in the main method).
After changing to use reflection I didn't notice any meaningful loss in performance, and the code was easier to maintain.
in addition to Jons answer, another usage is to be able to "dip your toe in the water" to test if a given facility is present in the JVM.
Under OS X a java application looks nicer if some Apple-provided classes are called. The easiest way to test if these classes are present, is to test with reflection first
some times you need to create a object of class on fly or from some other place not a java code (e.g jsp). at that time reflection is useful.
Specifically, when you create an interface/implementor pair, and there is no overriding organizational concern (such as the interface should go in a different assembly ie, as recommended by the s# architecture) do you have a default way of organizing them in your namespace/naming scheme?
This is obviously a more opinion based question but I think some people have thought about this more and we can all benefit from their conclusions.
The answer depends on your intentions.
If you intend the consumer of your namespaces to use the interfaces over the concrete implementations, I would recommend having your interfaces in the top-level namespace with the implementations in a child namespace
If the consumer is to use both, have them in the same namespace.
If the interface is for predominantly specialized use, like creating new implementations, consider having them in a child namespace such as Design or ComponentModel.
I'm sure there are other options as well, but as with most namespace issues, it comes down to the use-cases of the project, and the classes and interfaces it contains.
I usually keep the interface in the same namespace of as the concrete types.
But, that's just my opinion, and namespace layout is highly subjective.
Animals
|
| - IAnimal
| - Dog
| - Cat
Plants
|
| - IPlant
| - Cactus
You don't really gain anything by moving one or two types out of the main namespace, but you do add the requirement for one extra using statement.
What I generally do is to create an Interfaces namespace at a high level in my hierarchy and put all interfaces in there (I do not bother to nest other namespaces in there as I would then end up with many namespaces containing only one interface).
Interfaces
|--IAnimal
|--IVegetable
|--IMineral
MineralImplementor
Organisms
|--AnimalImplementor
|--VegetableImplementor
This is just the way that I have done it in the past and I have not had many problems with it, though admittedly it might be confusing to others sitting down with my projects. I am very curious to see what other people do.
I prefer to keep my interfaces and implementation classes in the same namespace. When possible, I give the implementation classes internal visibility and provide a factory (usually in the form of a static factory method that delegates to a worker class, with an internal method that allows a unit tests in a friend assembly to substitute a different worker that produces stubs). Of course, if the concrete class needs to be public--for instance, if it's an abstract base class, then that's fine; I don't see any reason to put an ABC in its own namespace.
On a side note, I strongly dislike the .NET convention of prefacing interface names with the letter 'I.' The thing the (I)Foo interface models is not an ifoo, it's simply a foo. So why can't I just call it Foo? I then name the implementation classes specifically, for example, AbstractFoo, MemoryOptimizedFoo, SimpleFoo, StubFoo etc.
(.Net) I tend to keep interfaces in a separate "common" assembly so I can use that interface in several applications and, more often, in the server components of my apps.
Regarding namespaces, I keep them in BusinessCommon.Interfaces.
I do this to ensure that neither I nor my developers are tempted to reference the implementations directly.
Separate the interfaces in some way (projects in Eclipse, etc) so that it's easy to deploy only the interfaces. This allows you to provide your external API without providing implementations. This allows dependent projects to build with a bare minimum of externals. Obviously this applies more to larger projects, but the concept is good in all cases.
I usually separate them into two separate assemblies. One of the usual reasons for a interface is to have a series of objects look the same to some subsystem of your software. For example I have all my Reports implementing the IReport Interfaces. IReport is used is not only used in printing but for previewing and selecting individual options for each report. Finally I have a collection of IReport to use in dialog where the user selects which reports (and configuring options) they want to print.
The Reports reside in a separate assembly and the IReport, the Preview engine, print engine, report selections reside in their respective core assembly and/or UI assembly.
If you use the Factory Class to return a list of available reports in the report assembly then updating the software with new report becomes merely a matter of copying the new report assembly over the original. You can even use the Reflection API to just scan the list of assemblies for any Report Factories and build your list of Reports that way.
You can apply this techniques to Files as well. My own software runs a metal cutting machine so we use this idea for the shape and fitting libraries we sell alongside our software.
Again the classes implementing a core interface should reside in a separate assembly so you can update that separately from the rest of the software.
I give my own experience that is against other answers.
I tend to put all my interfaces in the package they belongs to. This grants that, if I move a package in another project I have all the thing there must be to run the package without any changes.
For me, any helper functions and operator functions that are part of the functionality of a class should go into the same namespace as that of the class, because they form part of the public API of that namespace.
If you have common implementations that share the same interface in different packages you probably need to refactor your project.
Sometimes I see that there are plenty of interfaces in a project that could be converted in an abstract implementation rather that an interface.
So, ask yourself if you are really modeling a type or a structure.
A good example might be looking at what Microsoft does.
Assembly: System.Runtime.dll
System.Collections.Generic.IEnumerable<T>
Where are the concrete types?
Assembly: System.Colleections.dll
System.Collections.Generic.List<T>
System.Collections.Generic.Queue<T>
System.Collections.Generic.Stack<T>
// etc
Assembly: EntityFramework.dll
System.Data.Entity.IDbSet<T>
Concrete Type?
Assembly: EntityFramework.dll
System.Data.Entity.DbSet<T>
Further examples
Microsoft.Extensions.Logging.ILogger<T>
- Microsoft.Extensions.Logging.Logger<T>
Microsoft.Extensions.Options.IOptions<T>
- Microsoft.Extensions.Options.OptionsManager<T>
- Microsoft.Extensions.Options.OptionsWrapper<T>
- Microsoft.Extensions.Caching.Memory.MemoryCacheOptions
- Microsoft.Extensions.Caching.SqlServer.SqlServerCacheOptions
- Microsoft.Extensions.Caching.Redis.RedisCacheOptions
Some very interesting tells here. When the namespace changes to support the interface, the namespace change Caching is also prefixed to the derived type RedisCacheOptions. Additionally, the derived types are in an additional namespace of the implementation.
Memory -> MemoryCacheOptions
SqlServer -> SqlServerCatchOptions
Redis -> RedisCacheOptions
This seems like a fairly easy pattern to follow most of the time. As an example I (since no example was given) the following pattern might emerge:
CarDealership.Entities.Dll
CarDealership.Entities.IPerson
CarDealership.Entities.IVehicle
CarDealership.Entities.Person
CarDealership.Entities.Vehicle
Maybe a technology like Entity Framework prevents you from using the predefined classes. Thus we make our own.
CarDealership.Entities.EntityFramework.Dll
CarDealership.Entities.EntityFramework.Person
CarDealership.Entities.EntityFramework.Vehicle
CarDealership.Entities.EntityFramework.SalesPerson
CarDealership.Entities.EntityFramework.FinancePerson
CarDealership.Entities.EntityFramework.LotVehicle
CarDealership.Entities.EntityFramework.ShuttleVehicle
CarDealership.Entities.EntityFramework.BorrowVehicle
Not that it happens often but may there's a decision to switch technologies for whatever reason and now we have...
CarDealership.Entities.Dapper.Dll
CarDealership.Entities.Dapper.Person
CarDealership.Entities.Dapper.Vehicle
//etc
As long as we're programming to the interfaces we've defined in root Entities (following the Liskov Substitution Principle) down stream code doesn't care where how the Interface was implemented.
More importantly, In My Opinion, creating derived types also means you don't have to consistently include a different namespace because the parent namespace contains the interfaces. I'm not sure I've ever seen a Microsoft example of interfaces stored in child namespaces that are then implement in the parent namespace (almost an Anti-Pattern if you ask me).
I definitely don't recommend segregating your code by type, eg:
MyNamespace.Interfaces
MyNamespace.Enums
MyNameSpace.Classes
MyNamespace.Structs
This doesn't add value to being descriptive. And it's akin to using System Hungarian notation, which is mostly if not now exclusively, frowned upon.
I HATE when I find interfaces and implementations in the same namespace/assembly. Please don't do that, if the project evolves, it's a pain in the ass to refactor.
When I reference an interface, I want to implement it, not to get all its implementations.
What might me be admissible is to put the interface with its dependency class(class that references the interface).
EDIT: #Josh, I juste read the last sentence of mine, it's confusing! of course, both the dependency class and the one that implements it reference the interface. In order to make myself clear I'll give examples :
Acceptable :
Interface + implementation :
namespace A;
Interface IMyInterface
{
void MyMethod();
}
namespace A;
Interface MyDependentClass
{
private IMyInterface inject;
public MyDependentClass(IMyInterface inject)
{
this.inject = inject;
}
public void DoJob()
{
//Bla bla
inject.MyMethod();
}
}
Implementing class:
namespace B;
Interface MyImplementing : IMyInterface
{
public void MyMethod()
{
Console.WriteLine("hello world");
}
}
NOT ACCEPTABLE:
namespace A;
Interface IMyInterface
{
void MyMethod();
}
namespace A;
Interface MyImplementing : IMyInterface
{
public void MyMethod()
{
Console.WriteLine("hello world");
}
}
And please DON'T CREATE a project/garbage for your interfaces ! example : ShittyProject.Interfaces. You've missed the point!
Imagine you created a DLL reserved for your interfaces (200 MB). If you had to add a single interface with two line of codes, your users will have to update 200 MB just for two dumb signaturs!