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So.. I can't understand why should I even use the Singleton pattern in ActionScript 3. Can anyone explain me this? Maybe I just don't understand the purpose of it. I mean how it differs from other patterns? How it works?
I checked the PureMVC source and it's full of Singletons. Why are they using them in the View, Module, Controller?
I have next to no practical experience with PureMVC so I can't argue for or against their use of Singletons. Hence, I'll try to keep my answer more generic.
A singleton is a type of object that can only be instantiated once and is globally accessible.
Typically, this kind of pattern is used in order to have easy access to services of some kind, perhaps a service facade used to retrieve data from a server or an application model that holds information about settings or such.
The singleton pattern is by many considered to be an anti-pattern for a number of reasons, a few of which are mentioned below:
They carry state, making certain tasks such as unit testing virtually impossible.
They inherently violate the Single Responsibility Principle.
They promote tight coupling between classes due to them being globally accessible.
I won't list all of the reasons why a singleton may be an anti pattern, there are plenty of resources on the subject.
The singleton pattern restricts the instantiation of an object to only one instance. Sometimes in systems this pattern is used so an object that controls parts of the system can't be just created at-will. If you have some object that manages settings, for example, you would want something that changes settings to only modify that one object, and not create a new one.
Take Java syntax as an example, though the question itself is language independent. If the following snippet takes an object MyAbstractEmailTemplate as input argument in the method setTemplate, the class MyGateway will then become tightly-coupled with the object MyAbstractEmailTemplate, which lessens the re-usability of the class MyGateway.
A compromise is to use dependency-injection to ease the instantiation of MyAbstractEmailTemplate. This might solve the coupling problem
to some extent, but the interface is still rigid, hardly providing enough flexibility to
other developers/ applications.
So if we only use primitive data type (or even plain XML in web service) as the input/ output of a method, it seems the coupling problem no longer exists. So what do you think?
public class MyGateway {
protected MyAbstractEmailTemplate template;
public void setTemplate(MyAbstractEmailTemplate template) {
this.template = template;
}
}
It's pretty difficult to understand what you are really asking, but going the route of typing everything to Object does not lead to loose coupling because you can't do anything with the input without downcasting, which would break the Liskov Substituion Principle.
Taken to the extreme it leads you here:
public class MyClass
{
public object Invoke(object obj);
}
This is not loose coupling, it's just obscure and hard-to-maintain code.
The name MyAbstractEmailTemplate makes me believe that you are talking about an abstract class.
You should always program against interfaces, so instead of having MyGateway depend on MyAbstractEmailTemplate, it should depend on an EmailTemplate interface, where MyAbstractEmailTemplate implements EmailTemplate. Then, you can pass your custom implementations around as you want to, without further tight coupling.
Combine this with DI and you've got yourself a pretty decent solution.
Not exactly sure what you mean with "the interface is still rigid", but obviously you should design your interface in such a way that it provides the functionality you need.
MyGateway has to assume something about the inputs. Even if it used XML, it would have to assume something about the structure and content of the XML. Coupling isn't an evil in its own right; expresses the contract between two pieces of code. The oft-repeated advice to avoid tight coupling is really just saying that coupling should express the essence of a contract, not more and not less. Passing a specific type (particularly an interface type) is a very good way to achieve this balance.
The first problem you will run into is that a lot of types are simply not representable by a primitive data type (It's a Java problem that there are primitive types at all.).
The coupling should be reduced by using a proper inheritance hierarchy. What means proper? The method should take exactly that part of the interface as a parameter that is need. Not more not less.
After all you won't be able to avoid dependencies. Methods have to know about what they can do with their input or have to able to make assumptions (see C++ concepts) about the capabilities of the input.
IMHO there is nothing inherently wrong in using objects (wth small cap, not Objects) as method parameters and/or class members. Yes, these create dependencies. You can manage this in (at least) two ways:
acknowledge that by creating this dependency, the two classes become tightly coupled. This is entirely appropriate in many cases, where two (or more) classes in fact form a component, which is a meaningful unit of reuse in itself, and its parts may not make much sense or be interchangeable.
if there are multiple interchangeable candidates for a method parameter, these are obvious candidates to form a class hierarchy. Then you program for the interface and can pass any object of any class implementing that interface as parameter to your method. Note that the phrase "there are multiple interchangeable candidates for a method parameter" is a loose rephrasing of the Liskov Substitution Principle, which is the foundation of polymorphism.
in some languages, e.g. C++, the third way would be using templates. Then you need no common interface, only specific methods/members need to resolvable when the template is instantiated. However, since instantiation happens at compile time, this is entirely static binding.
sThe problem is I would say, that the best java can offer are interfaces and people start to see that they are too rigid. It would be interesting to use something like what is in Go language, that allows flexible checking for all methods of an interface to be present in the type, you do not have to be explicit about implementing some interface. We also need something better than interfaces to specify the constraints - maybe some sort of contracts. Another thing is the interface evolution.
Using Linq to SQL, and a DDD style Domain Layer with de-coupled repositories, does anyone have any good ideas on how to implement a specification pattern without bleeding L2S concerns up into the domain layer, that is still understandable? :)
We have complex business logic surrounding the selection of a set of transaction data, and would like those rules/specifications to be owned by the Domain. We've also done a good job of keeping our domain persistence ignorant.
This presents a problem, because in order to implement a Specification, the domain (as far as I can tell) needs to see the types being queried (L2S types).
Any ideas?
Also, nHibernate is out of the question for reasons I don't want to explain.. :)
Have you considered mapping your generic Specifications into an Expression tree that would translate into proper L2S syntax? It seems that is the main problem you are hitting here. The Specification pattern isn't the problem, but the mapping to L2S is.
Linq-To-Sql classes can be partial. This means that you can extend them by implementing a partial that implements a common interface. That Interface can be shared between layers without the "bleeding" problem you are describing. The rest is just the details of your "IsStatisfiedBy" which should be easy to encapsulate.
I recently had the same issue. Different pattern, but still LINQ to SQL (L2S). I tried two different ways to avoid the leakage.
First we tried using DTOs and a mapping layer. So we wrote super simple objects that had a one to one mapping to the tables. They were all decorated with L2S attributes. We then wrote a mapping layer to map the DTOs to our business objects. All of this was hidden via the Repository pattern from Doman Driven Design. So consumers of the business objects had no idea the L2S was under the hood.
Next, mostly for variety. We tried using the XML mapping features of L2S so the objects themselves needed no attributes. For collections we exposed IEnumerable instead of any of L2S collections. If you looked at the internals of the business classes you could still detect some usage of L2S (EntitySet or Ref). But consumers of the class had no idea. So some bits of leakage but nothing drastic.
In the end we stuck with the first pattern. The second worked and we could have replaced L2S without changing the interface of the business layer, but I was never happy with XML mapping. The first pattern had a much cleaner separation between the database and the business objects. It took more code. The first one also worked better for us because it allowed us to evolve the business objects differently than the tables. In the early days of the project the xml mapping worked because our objects were pretty much one to one with the tables.
So in the end we put a layer between L2S and the domain. It worked. It took more code, but it was really simple stuff. And it was all very testable.
If you want to avoid referencing Linq2Sql from your domain layer, you must work against interfaces that represent your entities instead of working with the actual entities themselves. You then need a mapping layer between your interfaces and your entities.
I've worked this way and found it to be a severe hindrance. I switched to NHibernate for new projects and for the older projects I simply stopped worrying about the domain referencing Linq2Sql entities directly. Overcoming that restriction is simply too much of a time-cost in my opinion.
This question already has answers here:
Why use getters and setters/accessors?
(37 answers)
Closed 7 years ago.
Allen Holub wrote the following,
You can't have a program without some coupling. Nonetheless, you can minimize coupling considerably by slavishly following OO (object-oriented) precepts (the most important is that the implementation of an object should be completely hidden from the objects that use it). For example, an object's instance variables (member fields that aren't constants), should always be private. Period. No exceptions. Ever. I mean it. (You can occasionally use protected methods effectively, but protected instance variables are an abomination.)
Which sounds reasonable, but he then goes on to say,
You should never use get/set functions for the same reason—they're just overly complicated ways to make a field public (though access functions that return full-blown objects rather than a basic-type value are reasonable in situations where the returned object's class is a key abstraction in the design).
Which, frankly, just sounds insane to me.
I understand the principle of information hiding, but without accessors and mutators you couldn't use Java beans at all. I don't know how you would follow a MVC design without accessors in the model, since the model can not be responsible for rendering the view.
However, I am a younger programmer and I learn more about Object Oriented Design everyday. Perhaps someone with more experience can weigh in on this issue.
Allen Holub's articles for reference
Why Extends Is Evil
Why Getter And Setter Methods Are Evil
Related Questions:
Java: Are Getters and Setters evil?
Is it really that wrong not using setters and getters?
Are get and set functions popular with C++ programmers?
Should you use accessor properties from within the class, or just from outside of the class?
I don't have a problem with Holub telling you that you should generally avoid altering the state of an object but instead resort to integrated methods (execution of behaviors) to achieve this end. As Corletk points out, there is wisdom in thinking long and hard about the highest level of abstraction and not just programming thoughtlessly with getters/setters that just let you do an end-run around encapsulation.
However, I have a great deal of trouble with anyone who tells you that you should "never" use setters or should "never" access primitive types. Indeed, the effort required to maintain this level of purity in all cases can and will end up causing more complexity in your code than using appropriately implemented properties. You just have to have enough sense to know when you are skirting the rules for short-term gain at the expense of long-term pain.
Holub doesn't trust you to know the difference. I think that knowing the difference is what makes you a professional.
Read through that article carefully. Holub is pushing the point that getters and setters are an evil "default antipattern", a bad habit that we slip into when designing a system; because we can.
The thought process should be along the lines; What does this object do? What are its responsibilities? What are its behaviours? What does it know? Thinking long and hard on these questions leads you naturally towards designing classes which expose the highest-level interface possible.
A car is a good example. It exposes a well-defined, standardised high-level interface. I don't concern myself with setSpeed(60)... is that MPH or km/h? I just accelerate, cruise, decelerate. I don't have to think about the details in setSteeringWheelAngle(getSteeringWheelAngle()+Math.rad(-1.5)), I just turn(-1.5), and the details are taken care of under the hood.
It boils down to "You can and should figure out what every class will be used for, what it does, what it represents, and expose the highest level interface possible which fulfills those requirements. Getters and setters are usually a cop-out, when the programmer is just to lazy to do the analysis required to determine exactly what each class is and is-not, and so we go down the path of "it can do anything". Getters and setters are evil!
Sometimes the actual requirements for a class are unknowable ahead of time. That's cool, just cop-out and use getter/setter antipattern for now, but when you do know, through experience, what the class is being used for, you'll probably want to comeback and cleanup the dirty low level interface. Refactoring based on "stuff you wish you knew when you write the sucker in the first place" is par for the course. You don't have to know everything in order to make a start, it's just that the more you do know, the less rework is likely to be required upon the way.
That's the mentality he's promoting. Getters and setters are an easy trap to fall into.
Yes, beans basically require getters and setters, but to me a bean is a special case. Beans represent nouns, things, tangible identifiable (if not physical) objects. Not a lot of objects actually have automatic behaviours; most times things are manipulated by external forces, including humans, to make them productive things.
daisy.setColor(Color.PINK) makes perfect sense. What else can you do? Maybe a Vulcan mind-meld, to make the flower want to be pink? Hmmm?
Getters and setters have their ?evil? place. It's just, like all really good OO things, we tend to overuse them, because they are safe and familiar, not to mention simple, and therefore it might be better if noobs didn't see or hear about them, at least until they'd mastered the mind-meld thing.
I think what Allen Holub tried to say, rephrased in this article, is the following.
Getters and setters can be useful for variables that you specifically want to encapsulate, but you don't have to use them for all variables. In fact, using them for all variables is nasty code smell.
The trouble programmers have, and Allen Holub was right in pointing it out, is that they sometimes use getters/setters for all variables. And the purpose of encapsulation is lost.
(note I'm coming at this from a .NET "property" angle)
Well, simply - I don't agree with him; he makes a big fuss about the return type of properties being a bad thing because it can break your calling code - but exactly the same argument would apply to method arguments. And if you can't use methods either?
OK, method arguments could be changed as widening conversions, but.... just why... Also, note that in C# the var keyword could mitigate a lot of this perceived pain.
Accessors are not an implementation detail; they are the public API / contract. Yup, if you break the contracft you have trouble. When did that become a surprise? Likewise, it is not uncommon for accessors to be non-trivial - i.e. they do more than just wrap fields; they perform calculations, logic checks, notifications, etc. And they allow interface based abstractions of state. Oh, and polymorphism - etc.
Re the verbose nature of accessors (p3?4?) - in C#: public int Foo {get; private set;} - job done.
Ultimately, all of code is a means to express our intent to the compiler. Properties let me do that in a type-safe, contract-based, verifiable, extensible, polymorphic way - thanks. Why do I need to "fix" this?
Getters and setters are used as little more than a mask to make a private variable public.
There's no point repeating what Holub said already but the crux of it is that classes should represent behaviour and not just state.
Some opposing views are in italics:
Though getIdentity starts with "get," it's not an accessor because it doesn't just return a field. It returns a complex object that has reasonable behavior
Oh but wait... then it's okay to use accessors as long as you return objects instead of primitive types? Now that's a different story, but it's just as dumb to me. Sometimes you need an object, sometimes you need a primitive type.
Also, I notice that Allen has radically softened his position since his previous column on the same topic, where the mantra "Never use accessors" didn't suffer one single exception. Maybe he realized after a few year that accessors do serve a purpose after all...
Bear in mind that I haven't actually put any UI code into the business logic. I've written the UI layer in terms of AWT (Abstract Window Toolkit) or Swing, which are both abstraction layers.
Good one. What if you are writing your application on SWT? How "abstract" is really AWT in that case? Just face it: this advice simply leads you to write UI code in your business logic. What a great principle. After all, it's only been like at least ten years since we've identified this practice as one of the worst design decisions you can make in a project.
My problem is as a novice programmer is sometimes stumbling onto articles on the internet and give them more credence then I should. Perhaps this is one of those cases.
When ideas like these are presented to me, I like to take a look at libraries and frameworks I use and which I like using.
For example, although some will disagree, I like the Java Standard API. I also like the Spring Framework. Looking at the classes in these libraries, you will notice that very rarely there are setters and getters which are there just to expose some internal variable. There are methods named getX, but that does not mean it is a getter in the conventional sense.
So, I think he has a point, and it is this: every time you press choose "Generate getters/setters" in Eclipse (or your IDE of choice), you should take a step back and wonder what you are doing. Is it really appropriate to expose this internal representation, or did I mess up my design at some step?
I don't believe he's saying never use get/set, but rather that using get/set for a field is no better than just making the field public (e.g. public string Name vs. public string Name {get; set; }).
If get/set is used it limits the information hiding of OO which can potentially lock you into a bad interface.
In the above example, Name is a string; what if we want to change the design later to add multiple Names? The interface exposed only a single string so we can’t add more without breaking existing implementation.
However, if instead of using get/set you initially had a method such as Add(string name), internally you could process name singularly or add to a list or what not and externally call the Add method as many times as you want to add more Names.
The OO goal is to design with a level of abstraction; don’t expose more detail than you absolutely have to.
Chances are if you’ve just wrapped a primitive type with a get/set you’ve broken this tenet.
Of course, this is if you believe in the OO goals; I find that most don't, not really, they just use Objects as a convienient way to group functional code.
Public variables make sense when the class is nothing more than a bundle of data with no real coherency, or when it's really, really elementary (such as a point class). In general, if there's any variable in a class that you think probably shouldn't be public, that means that the class has some coherence, and variables have a certain relation that should be maintained, so all variables should be private.
Getters and setters make sense when they reflect some sort of coherent idea. In a polygon class, for example, the x and y coordinates of given vertices have a meaning outside the class boundary. It probably makes sense to have getters, and it likely makes sense to have setters. In a bank account class, the balance is probably stored as a private variable, and almost certainly should have a getter. If it has a setter, it needs to have logging built in to preserve auditability.
There are some advantages of getters and setters over public variables. They provide some separation of interface and implementation. Just because a point has a .getX() function doesn't mean there has to be an x, since .getX() and .setX() can be made to work just fine with radial coordinates. Another is that it's possible to maintain class invariants, by doing whatever's necessary to keep the class consistent within the setter. Another is that it's possible to have functionality that triggers on a set, like the logging for the bank account balance.
However, for more abstract classes, the member variables lose individual significance, and only make sense in context. You don't need to know all the internal variables of a C++ stream class, for example. You need to know how to get elements in and out, and how to perform various other actions. If you counted on the exact internal structure, you'd be bogged down in detail that could arbitrarily vary between compilers or versions.
So, I'd say to use private variables almost exclusively, getters and setters where they have a real meaning in object behavior, and not otherwise.
Just because getters and setters are frequently overused doesn't mean they're useless.
The trouble with getters/setters is they try to fake encapsulation but they actually break it by exposing their internals. Secondly they are trying to do two separate things - providing access to and controlling their state - and end up doing neither very well.
It breaks encapsulation because when you call a get/set method you first need to know the name (or have a good idea) of the field you want to change, and second you have to know it's type eg. you couldn't call
setPositionX("some string");
If you know the name and type of the field, and the setter is public, then anyone can call the method as if it were a public field anyway, it's just a more complicated way of doing it, so why not just simplify it and make it a public field in the first place.
By allowing access to it's state but trying to control it at the same time, a get/set method just confuses things and ends up either being useless boiler-plate, or misleading, by not actually doing what it says it does by having side-effects the user might not expect. If error checking is needed, it could be called something like
public void tryPositionX(int x) throws InvalidParameterException{
if (x >= 0)
this.x = x;
else
throw new InvalidParameterException("Holy Negatives Batman!");
}
or if additional code is needed it could be called a more accurate name based on what the whole method does eg.
tryPositionXAndLog(int x) throws InvalidParameterException{
tryPositionX(x);
numChanges++;
}
IMHO needing getters/setters to make something work is often a symptom of bad design.
Make use of the "tell, don't ask" principle, or re-think why an object needs to send it's state data in the first place. Expose methods that change an object's behaviour instead of it's state. Benefits of that include easier maintenance and increased extensibility.
You mention MVC too and say a model can't be responsible for it's view, for that case Allen Holub gives an example of making an abstraction layer by having a "give-me-a-JComponent-that-represents-your-identity class" which he says would "isolate the way identities are represented from the rest of the system." I'm not experienced enough to comment on whether that would work or not but on the surface it sounds a decent idea.
Public getters/setters are bad if they provide access to implementation details. Yet, it is reasonable to provide access to object's properties and use getters/setters for this. For example, if Car has the color property, it's acceptable to let clients "observe" it using a getter. If some client needs the ability to recolor a car, the class can provide a setter ('recolor' is more clear name though). It is important to do not let clients know how properties are stored in objects, how they are maintained, and so on.
Ummmm...has he never heard of the concept of Encapsulation. Getter and Setter methods are put in place to control access to a Class' members. By making all fields publicly visible...anybody could write whatever values they wanted to them thereby completely invalidating the entire object.
Just in case anybody is a little fuzzy on the concept of Encapsulation, read up on it here:
Encapsulation (Computer Science)
...and if they're really evil, would .NET build the Property concept into the language? (Getter and Setter methods that just look a little prettier)
EDIT
The article does mention Encapsulation:
"Getters and setters can be useful for variables that you specifically want to encapsulate, but you don't have to use them for all variables. In fact, using them for all variables is nasty code smell."
Using this method will lead to extremely hard to maintain code in the long run. If you find out half way through a project that spans years that a field needs to be Encapsulated, you're going to have to update EVERY REFERENCE of that field everywhere in your software to get the benefit. Sounds a lot smarter to use proper Encapsulation up front and safe yourself the headache later.
I think that getters and setters should only be used for variables which one needs to access or change outside a class. That being said, I don't believe variables should be public unless they're static. This is because making variables public which aren't static can lead to them being changed undesirably. Let's say you have a developer who is carelessly using public variables. He then accesses a variable from another class and without meaning to, changes it. Now he has an error in his software as a result of this mishap. That's why I believe in the proper use of getters and setters, but you don't need them for every private or protected variable.
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One of the biggest advantages of object-oriented programming is encapsulation, and one of the "truths" we've (or, at least, I've) been taught is that members should always be made private and made available via accessor and mutator methods, thus ensuring the ability to verify and validate the changes.
I'm curious, though, how important this really is in practice. In particular, if you've got a more complicated member (such as a collection), it can be very tempting to just make it public rather than make a bunch of methods to get the collection's keys, add/remove items from the collection, etc.
Do you follow the rule in general? Does your answer change depending on whether it's code written for yourself vs. to be used by others? Are there more subtle reasons I'm missing for this obfuscation?
It depends. This is one of those issues that must be decided pragmatically.
Suppose I had a class for representing a point. I could have getters and setters for the X and Y coordinates, or I could just make them both public and allow free read/write access to the data. In my opinion, this is OK because the class is acting like a glorified struct - a data collection with maybe some useful functions attached.
However, there are plenty of circumstances where you do not want to provide full access to your internal data and rely on the methods provided by the class to interact with the object. An example would be an HTTP request and response. In this case it's a bad idea to allow anybody to send anything over the wire - it must be processed and formatted by the class methods. In this case, the class is conceived of as an actual object and not a simple data store.
It really comes down to whether or not verbs (methods) drive the structure or if the data does.
As someone having to maintain several-year-old code worked on by many people in the past, it's very clear to me that if a member attribute is made public, it is eventually abused. I've even heard people disagreeing with the idea of accessors and mutators, as that's still not really living up to the purpose of encapsulation, which is "hiding the inner workings of a class". It's obviously a controversial topic, but my opinion would be "make every member variable private, think primarily about what the class has got to do (methods) rather than how you're going to let people change internal variables".
Yes, encapsulation matters. Exposing the underlying implementation does (at least) two things wrong:
Mixes up responsibilities. Callers shouldn't need or want to understand the underlying implementation. They should just want the class to do its job. By exposing the underlying implementation, you're class isn't doing its job. Instead, it's just pushing the responsibility onto the caller.
Ties you to the underlying implementation. Once you expose the underlying implementation, you're tied to it. If you tell callers, e.g., there's a collection underneath, you cannot easily swap the collection for a new implementation.
These (and other) problems apply regardless of whether you give direct access to the underlying implementation or just duplicate all the underlying methods. You should be exposing the necessary implementation, and nothing more. Keeping the implementation private makes the overall system more maintainable.
I prefer to keep members private as long as possible and only access em via getters, even from within the very same class. I also try to avoid setters as a first draft to promote value style objects as long as it is possible. Working with dependency injection a lot you often have setters but no getters, as clients should be able to configure the object but (others) not get to know what's acutally configured as this is an implementation detail.
Regards,
Ollie
I tend to follow the rule pretty strictly, even when it's just my own code. I really like Properties in C# for that reason. It makes it really easy to control what values it's given, but you can still use them as variables. Or make the set private and the get public, etc.
Basically, information hiding is about code clarity. It's designed to make it easier for someone else to extend your code, and prevent them from accidentally creating bugs when they work with the internal data of your classes. It's based on the principle that nobody ever reads comments, especially ones with instructions in them.
Example: I'm writing code that updates a variable, and I need to make absolutely sure that the Gui changes to reflect the change, the easiest way is to add an accessor method (aka a "Setter"), which is called instead of updating data is updated.
If I make that data public, and something changes the variable without going through the Setter method (and this happens every swear-word time), then someone will need to spend an hour debugging to find out why the updates aren't being displayed. The same applies, to a lesser extent, to "Getting" data. I could put a comment in the header file, but odds are that no-one will read it till something goes terribly, terribly wrong. Enforcing it with private means that the mistake can't be made, because it'll show up as an easily located compile-time bug, rather than a run-time bug.
From experience, the only times you'd want to make a member variable public, and leave out Getter and Setter methods, is if you want to make it absolutely clear that changing it will have no side effects; especially if the data structure is simple, like a class that simply holds two variables as a pair.
This should be a fairly rare occurence, as normally you'd want side effects, and if the data structure you're creating is so simple that you don't (e.g a pairing), there will already be a more efficiently written one available in a Standard Library.
With that said, for most small programs that are one-use no-extension, like the ones you get at university, it's more "good practice" than anything, because you'll remember over the course of writing them, and then you'll hand them in and never touch the code again. Also, if you're writing a data structure as a way of finding out about how they store data rather than as release code, then there's a good argument that Getters and Setters will not help, and will get in the way of the learning experience.
It's only when you get to the workplace or a large project, where the probability is that your code will be called to by objects and structures written by different people, that it becomes vital to make these "reminders" strong. Whether or not it's a single man project is surprisingly irrelevant, for the simple reason that "you six weeks from now" is as different person as a co-worker. And "me six weeks ago" often turns out to be lazy.
A final point is that some people are pretty zealous about information hiding, and will get annoyed if your data is unnecessarily public. It's best to humour them.
C# Properties 'simulate' public fields. Looks pretty cool and the syntax really speeds up creating those get/set methods
Keep in mind the semantics of invoking methods on an object. A method invocation is a very high level abstraction that can be implemented my the compiler or the run time system in a variety of different ways.
If the object who's method you are invoking exists in the same process/ memory map then a method could well be optimized by a compiler or VM to directly access the data member. On the other hand if the object lives on another node in a distributed system then there is no way that you can directly access it's internal data members, but you can still invoke its methods my sending it a message.
By coding to interfaces you can write code that doesn't care where the target object exists or how it's methods are invoked or even if it's written in the same language.
In your example of an object that implements all the methods of a collection, then surely that object actually is a collection. so maybe this would be a case where inheritance would be better than encapsulation.
It's all about controlling what people can do with what you give them. The more controlling you are the more assumptions you can make.
Also, theorectically you can change the underlying implementation or something, but since for the most part it's:
private Foo foo;
public Foo getFoo() {}
public void setFoo(Foo foo) {}
It's a little hard to justify.
Encapsulation is important when at least one of these holds:
Anyone but you is going to use your class (or they'll break your invariants because they don't read the documentation).
Anyone who doesn't read the documentation is going to use your class (or they'll break your carefully documented invariants). Note that this category includes you-two-years-from-now.
At some point in the future someone is going to inherit from your class (because maybe an extra action needs to be taken when the value of a field changes, so there has to be a setter).
If it is just for me, and used in few places, and I'm not going to inherit from it, and changing fields will not invalidate any invariants that the class assumes, only then I will occasionally make a field public.
My tendency is to try to make everything private if possible. This keeps object boundaries as clearly defined as possible and keeps the objects as decoupled as possible. I like this because when I have to rewrite an object that I botched the first (second, fifth?) time, it keeps the damage contained to a smaller number of objects.
If you couple the objects tightly enough, it may be more straightforward just to combine them into one object. If you relax the coupling constraints enough you're back to structured programming.
It may be that if you find that a bunch of your objects are just accessor functions, you should rethink your object divisions. If you're not doing any actions on that data it may belong as a part of another object.
Of course, if you're writing a something like a library you want as clear and sharp of an interface as possible so others can program against it.
Fit the tool to the job... recently I saw some code like this in my current codebase:
private static class SomeSmallDataStructure {
public int someField;
public String someOtherField;
}
And then this class was used internally for easily passing around multiple data values. It doesn't always make sense, but if you have just DATA, with no methods, and you aren't exposing it to clients, I find it a quite useful pattern.
The most recent use I had of this was a JSP page where I had a table of data being displayed, defined at the top declaratively. So, initially it was in multiple arrays, one array per data field... this ended in the code being rather difficult to wade through with fields not being next to eachother in definition that would be displayed together... so I created a simple class like above which would pull it together... the result was REALLY readable code, a lot more so than before.
Moral... sometimes you should consider "accepted bad" alternatives if they may make the code simpler and easier to read, as long as you think it through and consider the consequences... don't blindly accept EVERYTHING you hear.
That said... public getters and setters is pretty much equivalent to public fields... at least essentially (there is a tad more flexibility, but it is still a bad pattern to apply to EVERY field you have).
Even the java standard libraries has some cases of public fields.
When I make objects meaningful they are easier to use and easier to maintain.
For example: Person.Hand.Grab(howquick, howmuch);
The trick is not to think of members as simple values but objects in themselves.
I would argue that this question does mix-up the concept of encapsulation with 'information hiding'
(this is not a critic, since it does seem to match a common interpretation of the notion of 'encapsulation')
However for me, 'encapsulation' is either:
the process of regrouping several items into a container
the container itself regrouping the items
Suppose you are designing a tax payer system. For each tax payer, you could encapsulate the notion of child into
a list of children representing the children
a map of to takes into account children from different parents
an object Children (not Child) which would provide the needed information (like total number of children)
Here you have three different kinds of encapsulations, 2 represented by low-level container (list or map), one represented by an object.
By making those decisions, you do not
make that encapsulation public or protected or private: that choice of 'information hiding' is still to be made
make a complete abstraction (you need to refine the attributes of object Children and you may decide to create an object Child, which would keep only the relevant informations from the point of view of a tax payer system)
Abstraction is the process of choosing which attributes of the object are relevant to your system, and which must be completely ignored.
So my point is:
That question may been titled:
Private vs. Public members in practice (how important is information hiding?)
Just my 2 cents, though. I perfectly respect that one may consider encapsulation as a process including 'information hiding' decision.
However, I always try to differentiate 'abstraction' - 'encapsulation' - 'information hiding or visibility'.
#VonC
You might find the International Organisation for Standardization's, "Reference Model of Open Distributed Processing," an interesting read. It defines: "Encapsulation: the property that the information contained in an object is accessible only through interactions at the interfaces supported by the object."
I tried to make a case for information hiding's being a critical part of this definition here:
http://www.edmundkirwan.com/encap/s2.html
Regards,
Ed.
I find lots of getters and setters to be a code smell that the structure of the program is not designed well. You should look at the code that uses those getters and setters, and look for functionality that really should be part of the class. In most cases, the fields of a class should be private implementation details and only the methods of that class may manipulate them.
Having both getters and setters is equal to the field being public (when the getters and setters are trivial/generated automatically). Sometimes it might be better to just declare the fields public, so that the code will be more simple, unless you need polymorphism or a framework requires get/set methods (and you can't change the framework).
But there are also cases where having getters and setters is a good pattern. One example:
When I create the GUI of an application, I try to keep the behaviour of the GUI in one class (FooModel) so that it can be unit tested easily, and have the visualization of the GUI in another class (FooView) which can be tested only manually. The view and model are joined with simple glue code; when the user changes the value of field x, the view calls setX(String) on the model, which in turn may raise an event that some other part of the model has changed, and the view will get the updated values from the model with getters.
In one project, there is a GUI model which has 15 getters and setters, of which only 3 get methods are trivial (such that the IDE could generate them). All the others contain some functionality or non-trivial expressions, such as the following:
public boolean isEmployeeStatusEnabled() {
return pinCodeValidation.equals(PinCodeValidation.VALID);
}
public EmployeeStatus getEmployeeStatus() {
Employee employee;
if (isEmployeeStatusEnabled()
&& (employee = getSelectedEmployee()) != null) {
return employee.getStatus();
}
return null;
}
public void setEmployeeStatus(EmployeeStatus status) {
getSelectedEmployee().changeStatusTo(status, getPinCode());
fireComponentStateChanged();
}
In practice I always follow only one rule, the "no size fits all" rule.
Encapsulation and its importance is a product of your project. What object will be accessing your interface, how will they be using it, will it matter if they have unneeded access rights to members? those questions and the likes of them you need to ask yourself when working on each project implementation.
I base my decision on the Code's depth within a module.
If I'm writting code that is internal to a module, and does not interface with the outside world I don't encapsulate things with private as much because it affects my programmer performance (how fast I can write and rewrite my code).
But for the objects that server as the module's interface with user code, then I adhere to strict privacy patterns.
Certainly it makes a difference whether your writing internal code or code to be used by someone else (or even by yourself, but as a contained unit.) Any code that is going to be used externally should have a well defined/documented interface that you'll want to change as little as possible.
For internal code, depending on the difficulty, you may find it's less work to do things the simple way now, and pay a little penalty later. Of course Murphy's law will ensure that the short term gain will be erased many times over in having to make wide-ranging changes later on where you needed to change a class' internals that you failed to encapsulate.
Specifically to your example of using a collection that you would return, it seems possible that the implementation of such a collection might change (unlike simpler member variables) making the utility of encapsulation higher.
That being said, I kinda like Python's way of dealing with it. Member variables are public by default. If you want to hide them or add validation there are techniques provided, but those are considered the special cases.
I follow the rules on this almost all the time. There are four scenarios for me - basically, the rule itself and several exceptions (all Java-influenced):
Usable by anything outside of the current class, accessed via getters/setters
Internal-to-class usage typically preceded by 'this' to make it clear that it's not a method parameter
Something meant to stay extremely small, like a transport object - basically a straight shot of attributes; all public
Needed to be non-private for extension of some sort
There's a practical concern here that isn't being addressed by most of the existing answers. Encapsulation and the exposure of clean, safe interfaces to outside code is always great, but it's much more important when the code you're writing is intended to be consumed by a spatially- and/or temporally-large "user" base. What I mean is that if you plan on somebody (even you) maintaining the code well into the future, or if you're writing a module that will interface with code from more than a handful of other developers, you need to think much more carefully than if you're writing code that's either one-off or wholly written by you.
Honestly, I know what wretched software engineering practice this is, but I'll oftentimes make everything public at first, which makes things marginally faster to remember and type, then add encapsulation as it makes sense. Refactoring tools in most popular IDEs these days makes which approach you use (adding encapsulation vs. taking it away) much less relevant than it used to be.