I'm in the middle of reading Code Complete, and towards the end of the book, in the chapter about refactoring, the author lists a bunch of things you should do to improve the quality of your code while refactoring.
One of his points was to always return as specific types of data as possible, especially when returning collections, iterators etc. So, as I've understood it, instead of returning, say, Collection<String>, you should return HashSet<String>, if you use that data type inside the method.
This confuses me, because it sounds like he's encouraging people to break the rule of information hiding. Now, I understand this when talking about accessors, that's a clear cut case. But, when calculating and mangling data, and the level of abstraction of the method implies no direct data structure, I find it best to return as abstract a datatype as possible, as long as the data doesn't fall apart (I wouldn't return Object instead of Iterable<String>, for example).
So, my question is: is there a deeper philosophy behind Code Complete's advice of always returning as specific a data type as possible, and allow downcasting, instead of maintaining a need-to-know-basis, that I've just not understood?
I think it is simply wrong for the most cases. It has to be:
be as lenient as possible, be as specific as needed
In my opinion, you should always return List rather than LinkedList or ArrayList, because the difference is more an implementation detail and not a semantic one. The guys from the Google collections api for Java taking this one step further: they return (and expect) iterators where that's enough. But, they also recommend to return ImmutableList, -Set, -Map etc. where possible to show the caller he doesn't have to make a defensive copy.
Beside that, I think the performance of the different list implementations isn't the bottleneck for most applications.
Most of the time one should return an interface or perhaps an abstract type that represents the return value being returned. If you are returning a list of X, then use List. This ultimately provides maximum flexibility if the need arises to return the list type.
Maybe later you realise that you want to return a linked list or a readonly list etc. If you put a concrete type your stuck and its a pain to change. Using the interface solves this problem.
#Gishu
If your api requires that clients cast straight away most of the time your design is suckered. Why bother returning X if clients need to cast to Y.
Can't find any evidence to substantiate my claim but the idea/guideline seems to be:
Be as lenient as possible when accepting input. Choose a generalized type over a specialized type. This means clients can use your method with different specialized types. So an IEnumerable or an IList as an input parameter would mean that the method can run off an ArrayList or a ListItemCollection. It maximizes the chance that your method is useful.
Be as strict as possible when returning values. Prefer a specialized type if possible. This means clients do not have to second-guess or jump through hoops to process the return value. Also specialized types have greater functionality. If you choose to return an IList or an IEnumerable, the number of things the caller can do with your return value drastically reduces - e.g. If you return an IList over an ArrayList, to get the number of elements returned - use the Count property, the client must downcast. But then such downcasting defeats the purpose - works today.. won't tomorrow (if you change the Type of returned object). So for all purposes, the client can't get a count of elements easily - leading him to write mundane boilerplate code (in multiple places or as a helper method)
The summary here is it depends on the context (exceptions to most rules). E.g. if the most probable use of your return value is that clients would use the returned list to search for some element, it makes sense to return a List Implementation (type) that supports some kind of search method. Make it as easy as possible for the client to consume the return value.
I could see how, in some cases, having a more specific data type returned could be useful. For example knowing that the return value is a LinkedList rather than just List would allow you to do a delete from the list knowing that it will be efficient.
I think, while designing interfaces, you should design a method to return the as abstract data type as possible. Returning specific type would make the purpose of the method more clear about what they return.
Also, I would understand it in this way:
Return as abstract a data type as possible = return as specific a data type as possible
i.e. when your method is supposed to return any collection data type return collection rather than object.
tell me if i m wrong.
A specific return type is much more valuable because it:
reduces possible performance issues with discovering functionality with casting or reflection
increases code readability
does NOT in fact, expose more than is necessary.
The return type of a function is specifically chosen to cater to ALL of its callers. It is the calling function that should USE the return variable as abstractly as possible, since the calling function knows how the data will be used.
Is it only necessary to traverse the structure? is it necessary to sort the structure? transform it? clone it? These are questions only the caller can answer, and thus can use an abstracted type. The called function MUST provide for all of these cases.
If,in fact, the most specific use case you have right now is Iterable< string >, then that's fine. But more often than not - your callers will eventually need to have more details, so start with a specific return type - it doesn't cost anything.
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A colleague of mine states that booleans as method arguments are not acceptable. They shall be replaced by enumerations. At first I did not see any benefit, but he gave me an example.
What's easier to understand?
file.writeData( data, true );
Or
enum WriteMode {
Append,
Overwrite
};
file.writeData( data, Append );
Now I got it! ;-)
This is definitely an example where an enumeration as second parameter makes the code much more readable.
So, what's your opinion on this topic?
Boolean's represent "yes/no" choices. If you want to represent a "yes/no", then use a boolean, it should be self-explanatory.
But if it's a choice between two options, neither of which is clearly yes or no, then an enum can sometimes be more readable.
Enums also allow for future modifications, where you now want a third choice (or more).
Use the one that best models your problem. In the example you give, the enum is a better choice. However, there would be other times when a boolean is better. Which makes more sense to you:
lock.setIsLocked(True);
or
enum LockState { Locked, Unlocked };
lock.setLockState(Locked);
In this case, I might choose the boolean option since I think it's quite clear and unambiguous, and I'm pretty sure my lock is not going to have more than two states. Still, the second choice is valid, but unnecessarily complicated, IMHO.
To me, neither using boolean nor enumeration is a good approach. Robert C. Martin captures this very clearly in his Clean Code Tip #12: Eliminate Boolean Arguments:
Boolean arguments loudly declare that the function does more than one thing. They are confusing and should be eliminated.
If a method does more than one thing, you should rather write two different methods, for example in your case: file.append(data) and file.overwrite(data).
Using an enumeration doesn't make things clearer. It doesn't change anything, it's still a flag argument.
Remember the question Adlai Stevenson posed to ambassador Zorin at the U.N. during the cuban missile crisis?
"You are in the courtroom of world
opinion right now, and you can answer
yes or no. You have denied that [the missiles]
exist, and I want to know whether I
have understood you correctly.... I am
prepared to wait for my answer until
hell freezes over, if that's your
decision."
If the flag you have in your method is of such a nature that you can pin it down to a binary decision, and that decision will never turn into a three-way or n-way decision, go for boolean. Indications: your flag is called isXXX.
Don't make it boolean in case of something that is a mode switch. There is always one more mode than you thought of when writing the method in the first place.
The one-more-mode dilemma has e.g. haunted Unix, where the possible permission modes a file or directory can have today result in weird double meanings of modes depending on file type, ownership etc.
There are two reasons I've run into this being a bad thing:
Because some people will write methods like:
ProcessBatch(true, false, false, true, false, false, true);
This is obviously bad because it's too easy to mix up parameters, and you have no idea by looking at it what you're specifying. Just one bool isn't too bad though.
Because controlling program flow by a simple yes/no branch might mean you have two entirely different functions that are wrapped up into one in an awkard way. For instance:
public void Write(bool toOptical);
Really, this should be two methods
public void WriteOptical();
public void WriteMagnetic();
because the code in these might be entirely different; they might have to do all sorts of different error handling and validation, or maybe even have to format the outgoing data differently. You can't tell that just by using Write() or even Write(Enum.Optical) (though of course you could have either of those methods just call internal methods WriteOptical/Mag if you want).
I guess it just depends. I wouldn't make too big of a deal about it except for #1.
I think you almost answered this yourself, I think the end aim is to make the code more readable, and in this case the enum did that, IMO its always best to look at the end aim rather than blanket rules, maybe think of it more as a guideline i.e. enums are often more readable in code than generic bools, ints etc but there will always be exceptions to the rule.
Enums are better but I wouldn't call boolean params as "unacceptable". Sometimes it's just easier to throw one little boolean in and move on (think private methods etc.)
Booleans may be OK in languages that have named parameters, like Python and Objective-C, since the name can explain what the parameter does:
file.writeData(data, overwrite=true)
or:
[file writeData:data overwrite:YES]
Enums have a definite benefit, but you should't just go replacing all your booleans with enums. There are many places where true/false is actually the best way to represent what is going on.
However, using them as method arguments is a bit suspect, simply because you can't see without digging into things what they are supposed to do, as they let you see what the true/false actually means
[Edit for the current state in 2022]
In modern C#, or other languages that support this, the nicest way to do it is with named arguments:
var worker = new BackgroundWorker(workerReportsProgress: true);
If your language doesn't allow for named arguments, then you may find properties to be a reasonable solution as well
[Original Answer from 2008 left for posterity]
Properties (especially with C#3 object initializers) or keyword arguments (a la ruby or python) are a much better way to go where you'd otherwise use a boolean argument.
C# example:
var worker = new BackgroundWorker { WorkerReportsProgress = true };
Ruby example
validates_presence_of :name, :allow_nil => true
Python example
connect_to_database( persistent=true )
The only thing I can think of where a boolean method argument is the right thing to do is in java, where you don't have either properties or keyword arguments. This is one of the reasons I hate java :-(
I would not agree that it is a good rule. Obviously, Enum makes for a better explicit or verbose code at some instances, but as a rule it seems way over reaching.
First let me take your example:
The programmers responsibility (and ability) to write good code is not really jeopardized by having a Boolean parameter. In your example the programmer could have written just as verbose code by writing:
dim append as boolean = true
file.writeData( data, append );
or I prefer more general
dim shouldAppend as boolean = true
file.writeData( data, shouldAppend );
Second:
The Enum example you gave is only "better" because you are passing a CONST. Most likely in most application at least some if not most of the time parameters that are passed to functions are VARIABLES. in which case my second example (giving variables with good names) is much better and Enum would have given you little benefits.
While it is true that in many cases enums are more readable and more extensible than booleans, an absolute rule that "booleans are not acceptable" is daft. It is inflexible and counter-productive - it does not leave room for human judgement. They're a fundamental built in type in most languages because they're useful - consider applying it to other built-in-types: saying for instance "never use an int as a parameter" would just be crazy.
This rule is just a question of style, not of potential for bugs or runtime performance. A better rule would be "prefer enums to booleans for reasons of readability".
Look at the .Net framework. Booleans are used as parameters on quite a few methods. The .Net API is not perfect, but I don't think that the use of boolean as parameters is a big problem. The tooltip always gives you the name of the parameter, and you can build this kind of guidance too - fill in your XML comments on the method parameters, they will come up in the tooltip.
I should also add that there is a case when you should clearly refactor booleans to an enumeration - when you have two or more booleans on your class, or in your method params, and not all states are valid (e.g. it's not valid to have them both set true).
For instance, if your class has properties like
public bool IsFoo
public bool IsBar
And it's an error to have both of them true at the same time, what you've actually got is three valid states, better expressed as something like:
enum FooBarType { IsFoo, IsBar, IsNeither };
Some rules that your colleague might be better adhering to are:
Don't be dogmatic with your design.
Choose what fits most appropriately for the users of your code.
Don't try to bash star-shaped pegs into every hole just because you like the shape this month!
A Boolean would only be acceptable if you do not intend to extend the functionality of the framework. The Enum is preferred because you can extend the enum and not break previous implementations of the function call.
The other advantage of the Enum is that is easier to read.
If the method asks a question such as:
KeepWritingData (DataAvailable());
where
bool DataAvailable()
{
return true; //data is ALWAYS available!
}
void KeepWritingData (bool keepGoing)
{
if (keepGoing)
{
...
}
}
boolean method arguments seem to make absolutely perfect sense.
It depends on the method. If the method does something that is very obviously a true/false thing then it is fine, e.g. below [though not I am not saying this is the best design for this method, it's just an example of where the usage is obvious].
CommentService.SetApprovalStatus(commentId, false);
However in most cases, such as the example you mention, it is better to use an enumeration. There are many examples in the .NET Framework itself where this convention is not followed, but that is because they introduced this design guideline fairly late on in the cycle.
It does make things a bit more explicit, but does start to massively extend the complexity of your interfaces - in a sheer boolean choice such as appending/overwriting it seems like overkill. If you need to add a further option (which I can't think of in this case), you can always perform a refactor (depending on the language)
Enums can certainly make the code more readable. There are still a few things to watch out for (in .net at least)
Because the underlying storage of an enum is an int, the default value will be zero, so you should make sure that 0 is a sensible default. (E.g. structs have all fields set to zero when created, so there's no way to specify a default other than 0. If you don't have a 0 value, you can't even test the enum without casting to int, which would be bad style.)
If your enum's are private to your code (never exposed publicly) then you can stop reading here.
If your enums are published in any way to external code and/or are saved outside of the program, consider numbering them explicitly. The compiler automatically numbers them from 0, but if you rearrange your enums without giving them values you can end up with defects.
I can legally write
WriteMode illegalButWorks = (WriteMode)1000000;
file.Write( data, illegalButWorks );
To combat this, any code that consumes an enum that you can't be certain of (e.g. public API) needs to check if the enum is valid. You do this via
if (!Enum.IsDefined(typeof(WriteMode), userValue))
throw new ArgumentException("userValue");
The only caveat of Enum.IsDefined is that it uses reflection and is slower. It also suffers a versioning issue. If you need to check the enum value often, you would be better off the following:
public static bool CheckWriteModeEnumValue(WriteMode writeMode)
{
switch( writeMode )
{
case WriteMode.Append:
case WriteMode.OverWrite:
break;
default:
Debug.Assert(false, "The WriteMode '" + writeMode + "' is not valid.");
return false;
}
return true;
}
The versioning issue is that old code may only know how to handle the 2 enums you have. If you add a third value, Enum.IsDefined will be true, but the old code can't necessarily handle it. Whoops.
There's even more fun you can do with [Flags] enums, and the validation code for that is slightly different.
I'll also note that for portability, you should use call ToString() on the enum, and use Enum.Parse() when reading them back in. Both ToString() and Enum.Parse() can handle [Flags] enum's as well, so there's no reason not to use them. Mind you, it's yet another pitfall, because now you can't even change the name of the enum without possibly breaking code.
So, sometimes you need to weigh all of the above in when you ask yourself Can I get away with just an bool?
IMHO it seems like an enum would be the obvious choice for any situation where more than two options are possible. But there definitely ARE situations where a boolean is all you need. In that case I would say that using an enum where a bool would work would be an example of using 7 words when 4 will do.
Booleans make sense when you have an obvious toggle which can only be one of two things (i.e. the state of a light bulb, on or off). Other than that, it's good to write it in such a way that it's obvious what you're passing - e.g. disk writes - unbuffered, line-buffered, or synchronous - should be passed as such. Even if you don't want to allow synchronous writes now (and so you're limited to two options), it's worth considering making them more verbose for the purposes of knowing what they do at first glance.
That said, you can also use False and True (boolean 0 and 1) and then if you need more values later, expand the function out to support user-defined values (say, 2 and 3), and your old 0/1 values will port over nicely, so your code ought not to break.
Sometimes it's just simpler to model different behaviour with overloads. To continue from your example would be:
file.appendData( data );
file.overwriteData( data );
This approach degrades if you have multiple parameters, each allowing a fixed set of options. For example, a method that opens a file might have several permutations of file mode (open/create), file access (read/write), sharing mode (none/read/write). The total number of configurations is equal to the Cartesian products of the individual options. Naturally in such cases multiple overloads are not appropriate.
Enums can, in some cases make code more readable, although validating the exact enum value in some languages (C# for example) can be difficult.
Often a boolean parameter is appended to the list of parameters as a new overload. One example in .NET is:
Enum.Parse(str);
Enum.Parse(str, true); // ignore case
The latter overload became available in a later version of the .NET framework than the first.
If you know that there will only ever be two choices, a boolean might be fine. Enums are extensible in a way that won't break old code, although old libraries might not support new enum values so versioning cannot be completely disregarded.
EDIT
In newer versions of C# it's possible to use named arguments which, IMO, can make calling code clearer in the same way that enums can. Using the same example as above:
Enum.Parse(str, ignoreCase: true);
Where I do agree that Enums are good way to go, in methods where you have 2 options (and just two options you can have readability without enum.)
e.g.
public void writeData(Stream data, boolean is_overwrite)
Love the Enums, but boolean is useful too.
This is a late entry on an old post, and it's so far down the page that nobody will ever read it, but since nobody has said it already....
An inline comment goes a long way to solving the unexpected bool problem. The original example is particularly heinous: imagine trying to name the variable in the function declearation! It'd be something like
void writeData( DataObject data, bool use_append_mode );
But, for the sake of example, let's say that's the declaration. Then, for an otherwise unexplained boolean argument, I put the variable name in an inline comment. Compare
file.writeData( data, true );
with
file.writeData( data, true /* use_append_mode */);
It really depends on the exact nature of the argument. If it is not a yes/no or true/false then a enum makes it more readable. But with an enum you need to check the argument or have acceptable default behaviour since undefined values of the underlying type can be passed.
The use of enums instead of booleans in your example does help make the method call more readable. However, this is a substitute for my favorite wish item in C#, named arguments in method calls. This syntax:
var v = CallMethod(pData = data, pFileMode = WriteMode, pIsDirty = true);
would be perfectly readable, and you could then do what a programmer should do, which is choose the most appropriate type for each parameter in the method without regard to how it looks in the IDE.
C# 3.0 allows named arguments in constructors. I don't know why they can't do this with methods as well.
Booleans values true/false only. So it is not clear what it represent. Enum can have meaningful name, e.g OVERWRITE, APPEND, etc. So enums are better.
The title may not really explain what I'm really trying to get at, couldn't really think of a way to describe what I mean.
I was wondering if it is good practice to check the arguments that a function accepts for nulls or empty before using them. I have this function which just wraps some hash creation like so.
Public Shared Function GenerateHash(ByVal FilePath As IO.FileInfo) As String
If (FilePath Is Nothing) Then
Throw New ArgumentNullException("FilePath")
End If
Dim _sha As New Security.Cryptography.MD5CryptoServiceProvider
Dim _Hash = Convert.ToBase64String(_sha.ComputeHash(New IO.FileStream(FilePath.FullName, IO.FileMode.Open, IO.FileAccess.Read)))
Return _Hash
End Function
As you can see I just takes a IO.Fileinfo as an argument, at the start of the function I am checking to make sure that it is not nothing.
I'm wondering is this good practice or should I just let it get to the actual hasher and then throw the exception because it is null.?
Thanks.
In general, I'd suggest it's good practice to validate all of the arguments to public functions/methods before using them, and fail early rather than after executing half of the function. In this case, you're right to throw the exception.
Depending on what your method is doing, failing early could be important. If your method was altering instance data on your class, you don't want it to alter half of the data, then encounter the null and throw an exception, as your object's data might them be in an intermediate and possibly invalid state.
If you're using an OO language then I'd suggest it's essential to validate the arguments to public methods, but less important with private and protected methods. My rationale here is that you don't know what the inputs to a public method will be - any other code could create an instance of your class and call it's public methods, and pass in unexpected/invalid data. Private methods, however, are called from inside the class, and the class should already have validated any data passing around internally.
One of my favourite techniques in C++ was to DEBUG_ASSERT on NULL pointers. This was drilled into me by senior programmers (along with const correctness) and is one of the things I was most strict on during code reviews. We never dereferenced a pointer without first asserting it wasn't null.
A debug assert is only active for debug targets (it gets stripped in release) so you don't have the extra overhead in production to test for thousands of if's. Generally it would either throw an exception or trigger a hardware breakpoint. We even had systems that would throw up a debug console with the file/line info and an option to ignore the assert (once or indefinitely for the session). That was such a great debug and QA tool (we'd get screenshots with the assert on the testers screen and information on whether the program continued if ignored).
I suggest asserting all invariants in your code including unexpected nulls. If performance of the if's becomes a concern find a way to conditionally compile and keep them active in debug targets. Like source control, this is a technique that has saved my ass more often than it has caused me grief (the most important litmus test of any development technique).
Yes, it's good practice to validate all arguments at the beginning of a method and throw appropriate exceptions like ArgumentException, ArgumentNullException, or ArgumentOutOfRangeException.
If the method is private such that only you the programmer could pass invalid arguments, then you may choose to assert each argument is valid (Debug.Assert) instead of throw.
If NULL is an inacceptable input, throw an exception. By yourself, like you did in your sample, so that the message is helpful.
Another method of handling NULL inputs is just to respont with a NULL in turn. Depends on the type of function -- in the example above I would keep the exception.
If its for an externally facing API then I would say you want to check every parameter as the input cannot be trusted.
However, if it is only going to be used internally then the input should be able to be trusted and you can save yourself a bunch of code that's not adding value to the software.
You should check all arguments against the set of assumptions that you make in that function about their values.
As in your example, if a null argument to your function doesn't make any sense and you're assuming that anyone using your function will know this then being passed a null argument shows some sort of error and some sort of action taken (eg. throwing an exception). And if you use asserts (as James Fassett got in and said before me ;-) ) they cost you nothing in a release version. (they cost you almost nothing in a debug version either)
The same thing applies to any other assumption.
And it's going to be easier to trace the error if you generate it than if you leave it to some standard library routine to throw the exception. You will be able to provide much more useful contextual information.
It's outside the bounds of this question, but you do need to expose the assumptions that your function makes - for example, through the comment header to your function.
According to The Pragmatic Programmer by Andrew Hunt and David Thomas, it is the responsibility of the caller to make sure it gives valid input. So, you must now choose whether you consider a null input to be valid. Unless it makes specific sense to consider null to be a valid input (e.g. it is probably a good idea to consider null to be a legal input if you're testing for equality), I would consider it invalid. That way your program, when it hits incorrect input, will fail sooner. If your program is going to encounter an error condition, you want it to happen as soon as possible. In the event your function does inadvertently get passed a null, you should consider it to be a bug, and react accordingly (i.e. instead of throwing an exception, you should consider making use of an assertion that kills the program, until you are releasing the program).
Classic design by contract: If input is right, output will be right. If input is wrong, there is a bug. (if input is right but output is wrong, there is a bug. That's a gimme.)
I'll add a couple of elaborations (in bold) to the excellent design by contract advice offerred by Brian earlier...
The priniples of "design by contract" require that you define what is acceptable for the caller to pass in (the valid domain of input values) and then, for any valid input, what the method/provider will do.
For an internal method, you can define NULLs as outside the domain of valid input parameters. In this case, you would immediately assert that the input parameter value is NOT NULL. The key insight in this contract specification is that any call passing in a NULL value IS A CALLER'S BUG and the error thrown by the assert statement is the proper behavior.
Now, while very well defined and parsimonius, if you're exposing the method to external/public callers, you should ask yourself, is that the contract I/we really want?
Probably not. In a public interface, you'd probably accept the NULL (as technically in the domain of inputs that the method accepts), but then decline to process gracefully w/ a return message. (More work to meet the naturally more complex customer-facing requirement.)
In either case, what you're after is a protocol that handles all of the cases from both the perspective of the caller and the provider, not lots of scattershot tests that can make it difficult to assess the completeness or lack of completeness of the contractual condition coverage.
Most of the time, letting it just throw the exception is pretty reasonable as long as you are sure the exception won't be ignored.
If you can add something to it, however, it doesn't hurt to wrap the exception with one that is more accurate and rethrow it. Decoding "NullPointerException" is going to take a bit longer than "IllegalArgumentException("FilePath MUST be supplied")" (Or whatever).
Lately I've been working on a platform where you have to run an obfuscator before you test. Every stack trace looks like monkeys typing random crap, so I got in the habit of checking my arguments all the time.
I'd love to see a "nullable" or "nonull" modifier on variables and arguments so the compiler can check for you.
If you're writing a public API, do your caller the favor of helping them find their bugs quickly, and check for valid inputs.
If you're writing an API where the caller might untrusted (or the caller of the caller), checked for valid inputs, because it's good security.
If your APIs are only reachable by trusted callers, like "internal" in C#, then don't feel like you have to write all that extra code. It won't be useful to anyone.