I'm trying to help a coworker come to terms with OO, and I'm finding that for some cases, it's hard to find solid real-world examples for the concept of a tag (or marker) interface. (An interface that contains no methods; it is used as a tag or marker or label only). While it really shouldn't matter for the sakes of our discussions, we're using PHP as the platform behind the discussions (because it's a common language between us). I'm probably not the best person to teach OO since most of my background is highly theoretical and about 15 years old, but I'm what he's got.
In any case, the dearth of discussions I've found regarding tag interfaces leads me to believe it's not even being used enough to warrant discussion. Am I wrong here?
Tag interfaces are used in Java (Serializable being the obvious example). C# and even Java seem to be moving away from this though in favor of attributes, which can accomplish the same thing but also do much more.
I still think there's a place for them in other languages that don't have the attribute concept that .NET and Java have.
ETA:
You would typically use this when you have an interface that implies an implementation, but you don't want the class that implements the interface to actually have to provide that implementation.
Some real world examples:
Serializable is a good example - it implies that there is an implementation (somewhere) that can serialize the object data, but since a generic implementation for that is available, there is no need to actually have the object implement that functionality itself.
Another example might be a web page caching system. Say you have a "Page" object and a "RequestHandler" object. The RequestHandler takes a request for a page, locates/creates the corresponding Page object, calls a Render() method on the Page object, and sends the results to the browser.
Now, say you wanted to implement caching for rendered pages. But the hitch is that some pages are dynamic, so they can't be cached. One way to implement this would be to have the cacheable Page objects implement an ICacheable "tag" interface (Or vice-versa, you could have an INotCacheable interface). The RequestHandler would then check if the Page implemented ICacheable, and if it did, it would cache the results after calling Render() and serve up those cached results on subsequent requests for that page.
In .Net Tag interfaces can be great for use with reflection and extension methods. Tag interfaces are typically interfaces without any methods. They allow you to see if an object is of a certain type without having the penatilty of reflecting over your objects.
An examples in the .Net Framework INamingContainer is part of ASP.Net
I'd call myself an OO programmer, and I've never heard of a tag interface.
I think tag interfaces are worth discussing, because they're an interesting corner case of the concept of an interface. Their infrequent use is also worth noting, though!
I've used tag interfaces a couple of times in an object model representing a SQL database. In these instances, it's a subtype of the root interface for particular types of objects. It's easier to check for a tag interface than an attribute ('obj is IInterface' rather than using reflection)
The .NET Style guide says to use Attributes rather than tag/marker interfaces.
alt text http://www.freeimagehosting.net/uploads/th.4528577db5.jpg
Click for Full image
source: http://www.informit.com/articles/article.aspx?p=423349&seqNum=6
or any number of other exposure points of Cwalina's recommendations, like the book.
I've used a tag interface twice in the past month. They can cut through some nasty problems when refactoring to make a function more generic.
That said, another thing I just discovered is using a tag interface as a parent class to a bunch of related interfaces with methods. The object can be passed around, and various preprocessors can check to see if they need to deal with a particular object. A way of keeping the processing in a separate object where it belongs, but the implementation details of processed objects in their definitions where they belong.
Related
I've been on an object-oriented design binge lately in an effort to better my design skills. This question is about a particular design choice that I see somewhat frequently, and don't understand the rationale. I know design choices tend toward the subjective, but I'd like to know what others think about this to find out if my design instincts are getting better or worse.
I was watching Robert C Martin(Uncle Bob) -Clean Architecture and Design-2012 COHAA The Path to Agility Conference. During the talk he tells a story about developing Fitnesse. I'm unfamiliar with the software, so I look it up and find the project hosted on github.
While looking through the project, one thing catches my attention in the WikiPage interface: the PageCrawler getPageCrawler(); method. So I look up the PageCrawler interface to see what that looks like. Upon examining this interface I think to myself that the methods in PageCrawler look like they would belong to a WikiPage, and that WikiPage could reasonably implement the interface.
I would think that separating the two might cause WikiPage to expose internals so that the information needed to crawl the page is accessible to the objects that crawl it. Also, the BaseWikiPage abstract class just returns a new PageCrawlerImpl, and there are no other PageCrawler implementations in the project from what I can see.
I've seen this type of code in other projects where a method of one interface/class returns an object of another interface/class with methods that can reasonably belong to the first class. In trying to see the intention of the Fitnesse developers, the only reason for this design that I came up with, is that developers who create new wiki pages by implementing WikiPage aren't required to re-implement the crawling functionality, i.e. the crawling functionality should be the same regardless of the wiki page's implementation. Is this the purpose for such a design, or am I missing something?
I found the Implementing an interface vs. providing an interface question on SO, but it wasn't quite the same and didn't give much insight into when you might design something like this.
What you're seeing is the Interface Segregation Principle in action. You say PageCrawler's interface makes you think "these are all things that belong in a WikiPage," but that's looking at things from an implementation point of view, not the point of view of someone calling WikiPage.
EDIT:
Maybe it's less about the Interface Segregation Principle and more about the Single Responsibility Principle.
In some cases, some functions that are "associated" with an object will need to hold state which should be separate from the object itself. In .NET, IEnumerator<T> methods are prime examples of this. Their meaning generally derives from an associated IEnumerable<T>, but each enumerator has a state which should be separate from that of the underlying collection (especially as a typical implementation of IEnumerable<T> will have no way of knowing how many enumerators, each with an independent state, may be associated with it simultaneously.
A few more advantages of splitting off interface implementations:
The split-off implementation can include members whose names mirror those of the underlying type but whose functionality is different. For example, a Dictionary<TKey,TValue> implements IEnumerable<KeyValuePair<TKey,TValue>>, but has a Keys property which implements IEnumerable<TKey>. Both the Dictionary and the thing returned by Keys have a GetEnumerator method, but one enumerates key-value pairs while the other just enumerates keys.
If the split-off implementation wraps the underlying object, and doesn't expose a reference to it but exposes some of its functionality, it may be safely exposed to code which is trusted with that limited functionality but not with an unfettered reference to the object. In some cases, having an object to hold a reference to a single wrapper object, which could then be used to satisfy any number of requests, may be more efficient than having clients use the wrapper object's constructor to to create a new wrapper object for every request.
Although neither Java nor .NET allows for double inheritance, it may be possible for each of the tightly-associated objects to inherit from a different class.
I'm not familiar with the particular types you mention, so I don't know the particular reasons they behave as they do, but the reasons mentioned are all common ones.
I have a Component which has API exposed with some 10 functionality in all. I can think of two ways to achieve it:
Give out all these functionality as separate functions.
Expose only one function which takes an XML as input. Based on request_Type specified and the parameters passed in the XML, I internally call one of the respective functions.
Q1. Will the second design be more loosely coupled than the first ?
I always read about how I should try my components to be loosely coupled, should I really go to this extent to achieve lose coupling ?
Q2. Which one of these would be a better design in terms of OOP and why?
Edit:
If I am exposing this API over D-Bus for others to use, will type checking still be a consideration to compare the two approaches? From what I understand type checking is done at compile time, but in case when this function is exposed over some IPC, issue of type checking comes into picture ?
The two alternatives you propose do not differ in the (obviously quite large) number of "functions" you want to offer from your API. However, the second seems to have many disadvantages because you are loosing any strong type checking, it will become much harder to document the functionality etc. (The only advantage I see is that you don't need to change your API if you add functionality. But at the disadvantage that users will not be able to figure out API changes like deleted functions until run-time.)
What is more related with this question is the Single Responsiblity Principle (http://en.wikipedia.org/wiki/Single_responsibility_principle). As you are talking about OOP, you should not expose your tens of functions within one class but split them among different classes, each with a single responsibility. Defining good "responsibilities" and roles requires some practice, but following some basic guidelines will help you to get started quickly. See Are there any rules for OOP? for a good starting point.
Reply to the question edit
I haven't used D-Bus, so this might be totally wrong. But from a quick look at the tutorial I read
Each object supports one or more interfaces. Think of an interface as
a named group of methods and signals, just as it is in GLib or Qt or
Java. Interfaces define the type of an object instance.
DBus identifies interfaces with a simple namespaced string, something
like org.freedesktop.Introspectable. Most bindings will map these
interface names directly to the appropriate programming language
construct, for example to Java interfaces or C++ pure virtual classes.
As far as I understand, D-Bus has the concept of differnt objects which provide interfaces consisting of several methods. This means (to me) that my answer above still applies. The "D-Bus native" way of specifying your API would mean to exhibit interfaces and I don't see any reason why good OOP design guidelines shouldn't be valid, here. As D-Bus seems to map these even to native language constructs, this is even more likely.
Of course, nobody keeps you from just building your own API description language in XML. However, things like are some kind of abuse of underlying techniques. You should have good reasons for doing such things.
When do you encourage programming against an interface and not directly to a concrete class?
A guideline that I follow is to create abstractions whenever code requires to cross a logical/physical boundary, most especially when infrastructure-related concerns are involved.
Another checkpoint would be if a dependency will likely change in the future, due to possible additional concerns code (such as caching, transactional awareness, invoking a webservice instead of in-process execution) or if such dependencies have direct references to infrastructure integration points.
If code depends on something that does not require control to cross a logical/physical boundary, I more or less don't create abstractions to interact with those.
Am I missing anything?
Also, use interfaces when
Multiple objects will need to be acted upon in a particular fashion, but are not fundamentally related. Perhaps many of your business objects access a particular utility object, and when they do they need to give a reference of themselves to that utility object so the utility object can call a particular method. Have that method in an interface and pass that interface to that utility object.
Passing around interfaces as parameters can be very helpful in unit testing. Even if you have just one type of object that sports a particular interface, and hence don't really need a defined interface, you might define/implement an interface solely to "fake" that object in unit tests.
related to the first 2 bullets, check out the Observer pattern and the Dependency Injection. I'm not saying to implement these patterns, but they illustrate types of places where interfaces are really helpful.
Another twist on this is for implementing a couple of the SOLID Principals, Open Closed principal and the Interface Segregation principle. Like the previous bullet, don't get stressed about strictly implementing these principals everywhere (right away at least), but use these concepts to help move your thinking away from just what objects go where to thinking more about contracts and dependency
In the end, let's not make it too complicated: we're in a strongly typed world in .NET. If you need to call a method or set a property but the object you're passing/using could be fundamentally different, use an interface.
I would add that if your code is not going to be referenced by another library (for a while at least), then the decision of whether to use an interface in a particular situation is one that you can responsibly put off. The "extract interface" refactoring is easy to do these days. In my current project, I've got an object being passed around that I'm thinking maybe I should switch to an interface; I'm not stressing about it.
Interfaces abstraction are convenient when doing unit test. It helps for mocking test objects. It very useful in TDD for developing without actually using data from your database.
If you don't need any features of the class that aren't found in the Interface...then why not always prefer the Interface implementation?
It will make your code easier to modify in the future and easier to test (mocking).
you have the right idea, already. i would only add a couple of notes to this...
first, abstraction does not mean 'interface'. for example, a "connection string" is an abstraction, even though it's just a string... it's not about the 'type' of the thing in question, it's about the intention of use for that thing.
and secondly, if you are doing test automation of any kind, look for the pain and friction that are exposed by writing the tests. if you find yourself having to set up too many external conditions for a test, it's a sign that you need a better abstraction between the thing your testing and the things it interacts with.
I think you've said it pretty well. Much of this will be a stylistic thing. There are open source projects I've looked at where everything has an interface and an implementation, and it's kind of frustrating, but it might make iterative development a little easier, since any objects implementation can break but dummies will still work. But honestly, I can dummy any class that doesn't overuse the final keyword by inheritance.
I would add to your list this: anything which can be thought of as a black box should be abstracted. This includes some of the things you've mentioned, but it also includes hairy algorithms, which are likely to have multiple useful implementations with different advantages for different situation.
Additionally, interfaces come in handy very often with composite objects. That's the only way something like java's swing library gets anything done, but it can also be useful for more mundane objects. (I personally like having an interface like ValidityChecker with ways to and-compose or or-compose subordinate ValidityCheckers.)
Most of the useful things that come with the Interface passing have been already said. However I would add:
implementing an interface to an object, or later multiple objects, FORCES all the implementers to follow an IDENTICAL pattern to implement contract with the object. This can be useful in case you have not so OOP-experienced-programmers actually writing the implementation code.
in some languages you can add attributes on the interface itself, which can be different from the actual object implementation attribute as sense and intent
From what I have read best practice is to have classes based on an interface and loosely couple the objects, in order to help code re-use and unit test.
Is this correct and is it a rule that should always be followed?
The reason I ask is I have recently worked on a system with 100’s of very different objects. A few shared common interfaces but most do not and wonder if it should have had an interface mirroring every property and function in those classes?
I am using C# and dot net 2.0 however I believe this question would fit many languages.
It's useful for objects which really provide a service - authentication, storage etc. For simple types which don't have any further dependencies, and where there are never going to be any alternative implementations, I think it's okay to use the concrete types.
If you go overboard with this kind of thing, you end up spending a lot of time mocking/stubbing everything in the world - which can often end up creating brittle tests.
Not really. Service components (class that do things for your application) are a good fit for interfaces, but as a rule I wouldn't bother having interfaces for, say, basic entity classes.
For example:
If you're working on a domain model, then that model shouldn't be interfaces. However if that domain model wants to call service classes (like data access, operating system functions etc) then you should be looking at interfaces for those components. This reduces coupling between the classes and means it's the interface, or "contract" that is coupled.
In this situation you then start to find it much easier to write unit tests (because you can have stubs/mocks/fakes for database access etc) and can use IoC to swap components without recompiling your applications.
I'd only use interfaces where that level of abstraction was required - i.e. you need to use polymorphic behaviour. Common examples would be dependency injection or where you have a factory-type scenario going on somewhere, or you need to establish a "multiple inheritance" type behaviour.
In my case, with my development style, this is quite often (I favour aggregation over deep inheritance hierarchies for most things other than UI controls), but I have seen perfectly fine apps that use very little. It all depends...
Oh yes, and if you do go heavily into interfaces - beware web services. If you need to expose your object methods via a web service they can't really return or take interface types, only concrete types (unless you are going to hand-write all your own serialization/deserialization). Yes, that has bitten me big time...
A downside to interface is that they can't be versioned. Once you shipped the interface you won't be making changes to it. If you use abstract classes then you can easily extend the contract over time by adding new methods and flagging them as virtual.
As an example, all stream objects in .NET derive from System.IO.Stream which is an abstract class. This makes it easy for Microsoft to add new features. In version 2 of the frameworkj they added the ReadTimeout and WriteTimeout properties without breaking any code. If they used an interface(say IStream) then they wouldn't have been able to do this. Instead they'd have had to create a new interface to define the timeout methods and we'd have to write code to conditionally cast to this interface if we wanted to use the functionality.
Interfaces should be used when you want to clearly define the interaction between two different sections of your software. Especially when it is possible that you want to rip out either end of the connection and replace it with something else.
For example in my CAM application I have a CuttingPath connected to a Collection of Points. It makes no sense to have a IPointList interface as CuttingPaths are always going to be comprised of Points in my application.
However I uses the interface IMotionController to communicate with the machine because we support many different types of cutting machine each with their own commend set and method of communications. So in that case it makes sense to put it behind a interface as one installation may be using a different machine than another.
Our applications has been maintain since the mid 80s and went to a object oriented design in late 90s. I have found that what could change greatly exceeded what I originally thought and the use of interfaces has grown. For example it used to be that our DrawingPath was comprised of points. But now it is comprised of entities (splines, arcs, ec) So it is pointed to a EntityList that is a collection of Object implementing IEntity interface.
But that change was propelled by the realization that a DrawingPath could be drawn using many different methods. Once that it was realized that a variety of drawing methods was needed then the need for a interface as opposed to a fixed relationship to a Entity Object was indicated.
Note that in our system DrawingPaths are rendered down to a low level cutting path which are always series of point segments.
I tried to take the advice of 'code to an interface' literally on a recent project. The end result was essentially duplication of the public interface (small i) of each class precisely once in an Interface (big I) implementation. This is pretty pointless in practice.
A better strategy I feel is to confine your interface implementations to verbs:
Print()
Draw()
Save()
Serialize()
Update()
...etc etc. This means that classes whose primary role is to store data - and if your code is well-designed they would usually only do that - don't want or need interface implementations. Anywhere you might want runtime-configurable behaviour, for example a variety of different graph styles representing the same data.
It's better still when the thing asking for the work really doesn't want to know how the work is done. This means you can give it a macguffin that it can simply trust will do whatever its public interface says it does, and let the component in question simply choose when to do the work.
I agree with kpollock. Interfaces are used to get a common ground for objects. The fact that they can be used in IOC containers and other purposes is an added feature.
Let's say you have several types of customer classes that vary slightly but have common properties. In this case it is great to have a ICustomer interface to bound them together, logicaly. By doing that you could create a CustomerHander class/method that handels ICustomer objects the same way instead of creating a handerl method for each variation of customers.
This is the strength of interfaces.
If you only have a single class that implements an interface, then the interface isn't to much help, it just sits there and does nothing.
What can be reasons to prevent a class from being inherited? (e.g. using sealed on a c# class)
Right now I can't think of any.
Because writing classes to be substitutably extended is damn hard and requires you to make accurate predictions of how future users will want to extend what you've written.
Sealing your class forces them to use composition, which is much more robust.
How about if you are not sure about the interface yet and don't want any other code depending on the present interface? [That's off the top of my head, but I'd be interested in other reasons as well!]
Edit:
A bit of googling gave the following:
http://codebetter.com/blogs/patricksmacchia/archive/2008/01/05/rambling-on-the-sealed-keyword.aspx
Quoting:
There are three reasons why a sealed class is better than an unsealed class:
Versioning: When a class is originally sealed, it can change to unsealed in the future without breaking compatibility. (…)
Performance: (…) if the JIT compiler sees a call to a virtual method using a sealed types, the JIT compiler can produce more efficient code by calling the method non-virtually.(…)
Security and Predictability: A class must protect its own state and not allow itself to ever become corrupted. When a class is unsealed, a derived class can access and manipulate the base class’s state if any data fields or methods that internally manipulate fields are accessible and not private.(…)
I want to give you this message from "Code Complete":
Inheritance - subclasses - tends to
work against the primary technical
imperative you have as a programmer,
which is to manage complexity.For the sake of controlling complexity, you should maintain a heavy bias against inheritance.
The only legitimate use of inheritance is to define a particular case of a base class like, for example, when inherit from Shape to derive Circle. To check this look at the relation in opposite direction: is a Shape a generalization of Circle? If the answer is yes then it is ok to use inheritance.
So if you have a class for which there can not be any particular cases that specialize its behavior it should be sealed.
Also due to LSP (Liskov Substitution Principle) one can use derived class where base class is expected and this is actually imposes the greatest impact from use of inheritance: code using base class may be given an inherited class and it still has to work as expected. In order to protect external code when there is no obvious need for subclasses you seal the class and its clients can rely that its behavior will not be changed. Otherwise external code needs to be explicitly designed to expect possible changes in behavior in subclasses.
A more concrete example would be Singleton pattern. You need to seal singleton to ensure one can not break the "singletonness".
This may not apply to your code, but a lot of classes within the .NET framework are sealed purposely so that no one tries to create a sub-class.
There are certain situations where the internals are complex and require certain things to be controlled very specifically so the designer decided no one should inherit the class so that no one accidentally breaks functionality by using something in the wrong way.
#jjnguy
Another user may want to re-use your code by sub-classing your class. I don't see a reason to stop this.
If they want to use the functionality of my class they can achieve that with containment, and they will have much less brittle code as a result.
Composition seems to be often overlooked; all too often people want to jump on the inheritance bandwagon. They should not! Substitutability is difficult. Default to composition; you'll thank me in the long run.
I am in agreement with jjnguy... I think the reasons to seal a class are few and far between. Quite the contrary, I have been in the situation more than once where I want to extend a class, but couldn't because it was sealed.
As a perfect example, I was recently creating a small package (Java, not C#, but same principles) to wrap functionality around the memcached tool. I wanted an interface so in tests I could mock away the memcached client API I was using, and also so we could switch clients if the need arose (there are 2 clients listed on the memcached homepage). Additionally, I wanted to have the opportunity to replace the functionality altogether if the need or desire arose (such as if the memcached servers are down for some reason, we could potentially hot swap with a local cache implementation instead).
I exposed a minimal interface to interact with the client API, and it would have been awesome to extend the client API class and then just add an implements clause with my new interface. The methods that I had in the interface that matched the actual interface would then need no further details and so I wouldn't have to explicitly implement them. However, the class was sealed, so I had to instead proxy calls to an internal reference to this class. The result: more work and a lot more code for no real good reason.
That said, I think there are potential times when you might want to make a class sealed... and the best thing I can think of is an API that you will invoke directly, but allow clients to implement. For example, a game where you can program against the game... if your classes were not sealed, then the players who are adding features could potentially exploit the API to their advantage. This is a very narrow case though, and I think any time you have full control over the codebase, there really is little if any reason to make a class sealed.
This is one reason I really like the Ruby programming language... even the core classes are open, not just to extend but to ADD AND CHANGE functionality dynamically, TO THE CLASS ITSELF! It's called monkeypatching and can be a nightmare if abused, but it's damn fun to play with!
From an object-oriented perspective, sealing a class clearly documents the author's intent without the need for comments. When I seal a class I am trying to say that this class was designed to encapsulate some specific piece of knowledge or some specific service. It was not meant to be enhanced or subclassed further.
This goes well with the Template Method design pattern. I have an interface that says "I perform this service." I then have a class that implements that interface. But, what if performing that service relies on context that the base class doesn't know about (and shouldn't know about)? What happens is that the base class provides virtual methods, which are either protected or private, and these virtual methods are the hooks for subclasses to provide the piece of information or action that the base class does not know and cannot know. Meanwhile, the base class can contain code that is common for all the child classes. These subclasses would be sealed because they are meant to accomplish that one and only one concrete implementation of the service.
Can you make the argument that these subclasses should be further subclassed to enhance them? I would say no because if that subclass couldn't get the job done in the first place then it should never have derived from the base class. If you don't like it then you have the original interface, go write your own implementation class.
Sealing these subclasses also discourages deep levels of inheritence, which works well for GUI frameworks but works poorly for business logic layers.
Because you always want to be handed a reference to the class and not to a derived one for various reasons:
i. invariants that you have in some other part of your code
ii. security
etc
Also, because it's a safe bet with regards to backward compatibility - you'll never be able to close that class for inheritance if it's release unsealed.
Or maybe you didn't have enough time to test the interface that the class exposes to be sure that you can allow others to inherit from it.
Or maybe there's no point (that you see now) in having a subclass.
Or you don't want bug reports when people try to subclass and don't manage to get all the nitty-gritty details - cut support costs.
Sometimes your class interface just isn't meant to be inheirited. The public interface just isn't virtual and while someone could override the functionality that's in place it would just be wrong. Yes in general they shouldn't override the public interface, but you can insure that they don't by making the class non-inheritable.
The example I can think of right now are customized contained classes with deep clones in .Net. If you inherit from them you lose the deep clone ability.[I'm kind of fuzzy on this example, it's been a while since I worked with IClonable] If you have a true singelton class, you probably don't want inherited forms of it around, and a data persistence layer is not normally place you want a lot of inheritance.
Not everything that's important in a class is asserted easily in code. There can be semantics and relationships present that are easily broken by inheriting and overriding methods. Overriding one method at a time is an easy way to do this. You design a class/object as a single meaningful entity and then someone comes along and thinks if a method or two were 'better' it would do no harm. That may or may not be true. Maybe you can correctly separate all methods between private and not private or virtual and not virtual but that still may not be enough. Demanding inheritance of all classes also puts a huge additional burden on the original developer to foresee all the ways an inheriting class could screw things up.
I don't know of a perfect solution. I'm sympathetic to preventing inheritance but that's also a problem because it hinders unit testing.
I exposed a minimal interface to interact with the client API, and it would have been awesome to extend the client API class and then just add an implements clause with my new interface. The methods that I had in the interface that matched the actual interface would then need no further details and so I wouldn't have to explicitly implement them. However, the class was sealed, so I had to instead proxy calls to an internal reference to this class. The result: more work and a lot more code for no real good reason.
Well, there is a reason: your code is now somewhat insulated from changes to the memcached interface.
Performance: (…) if the JIT compiler sees a call to a virtual method using a sealed types, the JIT compiler can produce more efficient code by calling the method non-virtually.(…)
That's a great reason indeed. Thus, for performance-critical classes, sealed and friends make sense.
All the other reasons I've seen mentioned so far boil down to "nobody touches my class!". If you're worried someone might misunderstand its internals, you did a poor job documenting it. You can't possibly know that there's nothing useful to add to your class, or that you already know every imaginable use case for it. Even if you're right and the other developer shouldn't have used your class to solve their problem, using a keyword isn't a great way of preventing such a mistake. Documentation is. If they ignore the documentation, their loss.
Most of answers (when abstracted) state that sealed/finalized classes are tool to protect other programmers against potential mistakes. There is a blurry line between meaningful protection and pointless restriction. But as long as programmer is the one who is expected to understand the program, I see no hardly any reasons to restrict him from reusing parts of a class. Most of you talk about classes. But it's all about objects!
In his first post, DrPizza claims that designing inheritable class means anticipating possible extensions. Do I get it right that you think that class should be inheritable only if it's likely to be extended well? Looks as if you were used to design software from the most abstract classes. Allow me a brief explanation of how do I think when designing:
Starting from the very concrete objects, I find characteristics and [thus] functionality that they have in common and I abstract it to superclass of those particular objects. This is a way to reduce code duplicity.
Unless developing some specific product such as a framework, I should care about my code, not others (virtual) code. The fact that others might find it useful to reuse my code is a nice bonus, not my primary goal. If they decide to do so, it's their responsibility to ensure validity of extensions. This applies team-wide. Up-front design is crucial to productivity.
Getting back to my idea: Your objects should primarily serve your purposes, not some possible shoulda/woulda/coulda functionality of their subtypes. Your goal is to solve given problem. Object oriented languages uses fact that many problems (or more likely their subproblems) are similar and therefore existing code can be used to accelerate further development.
Sealing a class forces people who could possibly take advantage of existing code WITHOUT ACTUALLY MODIFYING YOUR PRODUCT to reinvent the wheel. (This is a crucial idea of my thesis: Inheriting a class doesn't modify it! Which seems quite pedestrian and obvious, but it's being commonly ignored).
People are often scared that their "open" classes will be twisted to something that can not substitute its ascendants. So what? Why should you care? No tool can prevent bad programmer from creating bad software!
I'm not trying to denote inheritable classes as the ultimately correct way of designing, consider this more like an explanation of my inclination to inheritable classes. That's the beauty of programming - virtually infinite set of correct solutions, each with its own cons and pros. Your comments and arguments are welcome.
And finally, my answer to the original question: I'd finalize a class to let others know that I consider the class a leaf of the hierarchical class tree and I see absolutely no possibility that it could become a parent node. (And if anyone thinks that it actually could, then either I was wrong or they don't get me).