We have an API service call that returns a bunch of validation messages. In each message there is a string that contains an error code.
Our implementation converts the validation string into an enum value and then we process the enumeration as there are some error code we just don't care about.
The question becomes, how to handle the loop of messages in a Kotlin way:
response.validationErrors?.forEach {
val mediaFailure = decodeValidationMessage(it.message)
if (mediaFailure != MediaFailure.Unknown) {
return when (mediaFailure) {
MediaFailure.Encrypted -> DomainResponse(ErrorReasonCode.ERR_DOCUMENT_ENCRYPTED)
MediaFailure.NotSupported -> Response.validationFailed()
MediaFailure.InternalError -> Response.serviceFailed()
else -> throw NotImplementedError()
}
}
}
Here we loop through all the messages, then once the message error is not "Unknown" it returns the necessary response to the caller.
However, IntelliJ wants the else path, even though the if prevents that from happening.
Is there a proper Kotlin way of implementing this kind of loop?
From what I understood, you want to return a response for the first mediaFailure which is not MediaFailure.Unknown and you don't want that throw NotImplementedError() part in your function.
One way to fix this is to remove the if condition and continue the forEach loop when MediaFailure.Unknown is found.
response.validationErrors?.forEach {
val mediaFailure = decodeValidationMessage(it.message)
return when (mediaFailure) {
MediaFailure.Encrypted -> DomainResponse(ErrorReasonCode.ERR_DOCUMENT_ENCRYPTED)
MediaFailure.NotSupported -> Response.validationFailed()
MediaFailure.InternalError -> Response.serviceFailed()
MediaFailure.Unknown -> return#forEach // continue the loop
}
}
I think this is one of the many cases when it pays to step back from the code a bit and try to look at the big picture. To ask “What's the ultimate goal here? What am I trying to achieve with this code?”
(In traditional, lower-level languages, almost anything you want to do with a list or array requires a loop, so you get into the habit of reaching for a for or while without thinking. But there are often alternative approaches in Kotlin that can be more concise, clearer, and harder to get wrong. They tend to be more about what you're trying to achieve, rather than how.)
In this case, it looks you want to find the first item which decodes to give a known type (i.e. not MediaFailure.Unknown), and return a value derived from that.
So here's an attempt to code that:
val message = response.validationErrors?.asSequence()
?.map{ decodeValidationMessage(it.message) }
?.firstOrNull{ it != MediaFailure.Unknown }
return when (message) {
MediaFailure.Encrypted -> DomainResponse(ErrorReasonCode.ERR_DOCUMENT_ENCRYPTED)
MediaFailure.NotSupported -> Response.validationFailed()
MediaFailure.InternalError, null -> Response.serviceFailed()
else -> throw NotImplementedError()
}
This is still fairly similar to your code, and it's about as efficient. (Thanks to the asSequence(), it doesn't decode any more messages than it needs to.) But the firstOrNull() makes clear what you're looking for; and it's obvious that you go on to process only that one message — a fact which is rather lost in the original version.
(If there are no valid messages, message will be null and so this will return serviceFailed(), as per comments.)
There are of course many ways to skin a cat, and I can think of several variations. (It's often a worthwhile exercise to come up with some — if nothing else, it gives you more confidence in the version you end up with!) Try to pick whichever seems clearest, simplest, and best matches the big picture of what you're doing; that tends to work out best in the long run.
As I learn new components in Kotlin, I came accross requireNotNull and checkNotNull but the only difference I've found is that requireNotNull can throw an IllegalArgumentException while checkNotNull can throw an IllegalStateException. Is this the only reason why there are two methods, or I'm missing some under-the-hood implementation detail?
The exception types are the only practical difference, as far as the compiler is concerned — but there's a big difference in intent, for anyone reading the code:
• require…() functions are for checking parameters, to confirm that a function's input fulfils its contract. So you'd normally call them first thing in a function. (Of course, Kotlin's non-nullable types mean that you wouldn't need to call requireNotNull() for a single parameter; but you might need to check a more complex condition on a combination of parameters or their sub-objects.) That's why they throw IllegalArgumentException: it's checking that the arguments are legal.
• check…() functions are for checking the relevant properties, to confirm that the object or whatever is in a valid state for this function to be called now. (Again, any properties that were never null would be typed accordingly, so checkNotNull() is more appropriate for cases where a property, combination, and/or sub-property can be null, but this function mustn't be called when they are.) So they throw IllegalStateException: they're checking that the object's current state allows the function to be called.
In both cases, you could of course write a standard if check (as you would in Java). Or you could use the Elvis operator ?: to do the check the first time the possibly-null value is used. But these functions give you an alternative that's in a more declarative form: you'd normally put them at the top of the function, where they spell out what the function's contract is, in a way that's obvious to anyone glancing at the code.
As a linked answer points out, there are also assert…() functions, which again have more of a semantic difference than a practical one. Those are for detecting programming errors away from the boundary of a function call: for confirming invariants and other conditions, and for all the checks in unit tests and other automated tests.
(Assertions have another important difference: they can be enabled and disabled from the command-line. Though in my experience, that's not a very good thing. If a check is important, it should always be run: be mandatory; if not, then it should be removed, or at least moved to automated tests, once the code is debugged.)
It is a semantic difference and hence it throws different exceptions. RequireNotNull is used to check input values, typically at the beginning of a method, while checkNotNull is used anywhere to check the current state.
If you're looking for differences in implementation, the best place to go would be the source code. In this case it seems like there are no differences aside from the different exception thrown, the source for both methods is otherwise identical.
checkNotNull
[...]
if (value == null) {
val message = lazyMessage()
throw IllegalStateException(message.toString())
} else {
return value
}
requireNotNull
[...]
if (value == null) {
val message = lazyMessage()
throw IllegalArgumentException(message.toString())
} else {
return value
}
Therefore the difference is purely semantic. The answer from #gidds details some good scenarios for using them both.
There are some ways to fulfill a null-checking in Kotlin:
1.
if(myVar != null) {
foo(myVar)
}
2.
myVar?.let {
foo(it)
}
3.
myVar?.run {
foo(this)
}
What are the difference between these ways?
Are there any reasons (performance, best practice, code style etc.) why I should prefer on way over the other?
!! is to tell the compiler that I am sure the value of the variable is not null, and if it is null throw a null pointer exception (NPE) where as ?. is to tell the compiler that I am not sure if the value of the variable is null or not, if it is null do not throw any null pointer.
Another way of using a nullable property is safe call operator ?.
This calls the method if the property is not null or returns null if that property is null without throwing an NPE (null pointer exception).
nullableVariable?.someMethodCall()
All three code are behave same null check in operation-wise.
?. is used for chain operations.
bob?.department?.head?.name // if any of the properties in it is null it returns null
To perform a chain operation only for non-null values, you can use the safe call operator together with let
myVar?.let {
foo(it)
}
the above code is good for code style and performance
more details refer Null Safety
The ways 2 and 3 are more idiomatic for Kotlin. Both functions are quite similar. There is little difference with argument passing.
For example, we have a nullable variable:
var canBeNull: String? = null
When you working with T.run you work with extension function calling and you pass this in the closure.
canBeNull?.run {
println(length) // `this` could be omitted
}
When you call T.let you can use it like lambda argument it.
canBeNull?.let {
myString -> println(myString.length) // You could convert `it` to some other name
}
A good article about Kotlin standard functions.
All three are roughly equivalent.
The if case is more like most other languages, and so many developers may find it easier to read.
However, one difference is that the if case will read the value of myVar twice: once for the check, and again when passing it to foo(). That makes a difference, because if myVar is a property (i.e. something that could potentially be changed by another thread), then the compiler will warn that it could have been set to null after the check. If that's a problem (e.g. because foo() expects a non-null parameter), then you'll need to use one of the other cases.
For that reason, the let case has become fairly common practice in Kotlin. (The run case does just about the same thing, but for some reason isn't as popular for this sort of thing. I don't know why.)
Another way around it is to assign myVar to a temporary value, test that, and then use that. That's also more like other languages, but it's more verbose; many people prefer the conciseness of the let case — especially when myVar is actually a complicated expression.
The examples in your question don't show the true reason to decide.
First of all, since you're not using the return value of foo, you should use neither let nor run. Your choice is between also and apply.
Second, since you already have the result you want to null-check in a variable, the difference fades. This is a better motivating example:
complexCall(calculateArg1(), calculateArg2())?.also {
results.add(it)
}
as opposed to
val result = complexCall(calculateArg1(), calculateArg2())
if (result != null) {
results.add(result)
}
The second example declares an identifier, result, which is now available to the rest of the lexical scope, even though you're done with it in just one line.
The first example, on the other hand, keeps everything self-contained and when you go on reading the rest of the code, you are 100% confident that you don't have to keep in mind the meaning of result.
Kotlin have new features with NullPoint-Exception as Compare to Java.
Basically When we do Coding in Java , then we have to Check with !! in every Flied.
But in Kotlin, it is Easy way to Implement First
as Like,
Suppose, in Kotlin
var response:Json?=Null
response:Json?.let {
this part will handle automatic if response is Not Null....then this Block start Executing }?.run {
This is Nullable But, where we Can put Warring } So, I am Suggest you Guys to Start Work in Kotlin with this Features Provided by Kotlin.
(Flied)?.let { Not Null Value Comes Under }?.run{ Null Value Code }
This will Handle to NullPoint Exception or Protect You App for Crash
What you want to achieve
What you want to achieve is that the Kotlin compiler does a smart cast on the variable you are working with.
In all of your three examples, the compiler can do that.
Example:
if(myVar != null) {
foo(myVar) // smart cast: the compiler knows, that myVar can never be null here
}
The choice
Which one of the options to use, is really a matter of style. What you should not do is mix it up to often. Use one and stick to it.
You don't need to worry about performance since let and run are inlined (see inline function). This means that their code (body) is copied to the call site at compile time so there is no runtime overhead.
Closed. This question is opinion-based. It is not currently accepting answers.
Want to improve this question? Update the question so it can be answered with facts and citations by editing this post.
Closed 4 years ago.
Improve this question
I've heard some voices saying that checking for a returned null value from methods is bad design. I would like to hear some reasons for this.
pseudocode:
variable x = object.method()
if (x is null) do something
The rationale behind not returning null is that you do not have to check for it and hence your code does not need to follow a different path based on the return value. You might want to check out the Null Object Pattern which provides more information on this.
For example, if I were to define a method in Java that returned a Collection I would typically prefer to return an empty collection (i.e. Collections.emptyList()) rather than null as it means my client code is cleaner; e.g.
Collection<? extends Item> c = getItems(); // Will never return null.
for (Item item : c) { // Will not enter the loop if c is empty.
// Process item.
}
... which is cleaner than:
Collection<? extends Item> c = getItems(); // Could potentially return null.
// Two possible code paths now so harder to test.
if (c != null) {
for (Item item : c) {
// Process item.
}
}
Here's the reason.
In Clean Code by Robert Martin he writes that returning null is bad design when you can instead return, say, empty array. Since expected result is an array, why not? It'll enable you to iterate over result without any extra conditions. If it's an integer, maybe 0 will suffice, if it's a hash, empty hash. etc.
The premise is to not force calling code to immediately handle issues. Calling code may not want to concern itself with them. That's also why in many cases exceptions is better than nil.
Good uses of returning null:
If null is a valid functional result, for example: FindFirstObjectThatNeedsProcessing() can return null if not found and the caller should check accordingly.
Bad uses: Trying to replace or hide exceptional situations such as:
catch(...) and return null
API dependency initialization failed
Out of disk space
Invalid input parameters (programming error, inputs must be sanitized by the caller)
etc
In those cases throwing an exception is more adequate since:
A null return value provides no meaningful error info
The immediate caller most likely cannot handle the error condition
There is no guarantee that the caller is checking for null results
However, Exceptions should not be used to handle normal program operation conditions such as:
Invalid username/password (or any user-provided inputs)
Breaking loops or as non-local gotos
Yes, returning NULL is a terrible design, in object-oriented world. In a nutshell, NULL usage leads to:
ad-hoc error handling (instead of exceptions)
ambiguous semantic
slow instead of fast failing
computer thinking instead of object thinking
mutable and incomplete objects
Check this blog post for a detailed explanation: http://www.yegor256.com/2014/05/13/why-null-is-bad.html. More in my book Elegant Objects, Section 4.1.
Who says this is bad design?
Checking for nulls is a common practice, even encouraged, otherwise you run the risk of NullReferenceExceptions everywhere. Its better to handle the error gracefully than throw exceptions when you don't need to.
Based on what you've said so far, I think there's not enough information.
Returning null from a CreateWidget()method seems bad.
Returning null from a FindFooInBar() method seems fine.
Its inventor says it is a billion dollar mistake!
It depends on the language you're using. If you're in a language like C# where the idiomatic way of indicating the lack of a value is to return null, then returning null is a good design if you don't have a value. Alternatively, in languages such as Haskell which idiomatically use the Maybe monad for this case, then returning null would be a bad design (if it were even possible).
If you read all the answers it becomes clear the answer to this question depends on the kind of method.
Firstly, when something exceptional happens (IOproblem etc), logically exceptions are thrown. When exactly something is exceptional is probably something for a different topic..
Whenever a method is expected to possibly have no results there are two categories:
If it is possible to return a neutral value, do so.
Empty enumrables, strings etc are good examples
If such a neutral value does not exist, null should be returned.
As mentioned, the method is assumed to possibly have no result, so it is not exceptional, hence should not throw an exception. A neutral value is not possible (for example: 0 is not especially a neutral result, depending on the program)
Untill we have an official way to denote that a function can or cannot return null, I try to have a naming convention to denote so.
Just like you have the TrySomething() convention for methods that are expected to fail, I often name my methods SafeSomething() when the method returns a neutral result instead of null.
I'm not fully ok with the name yet, but couldn't come up with anything better. So I'm running with that for now.
I have a convention in this area that served me well
For single item queries:
Create... returns a new instance, or throws
Get... returns an expected existing instance, or throws
GetOrCreate... returns an existing instance, or new instance if none exists, or throws
Find... returns an existing instance, if it exists, or null
For collection queries:
Get... always returns a collection, which is empty if no matching[1] items are found
[1] given some criteria, explicit or implicit, given in the function name or as parameters.
Exceptions are for exceptional circumstances.
If your function is intended to find an attribute associated with a given object, and that object does has no such attribute, it may be appropriate to return null. If the object does not exist, throwing an exception may be more appropriate. If the function is meant to return a list of attributes, and there are none to return, returning an empty list makes sense - you're returning all zero attributes.
It's not necessarily a bad design - as with so many design decisions, it depends.
If the result of the method is something that would not have a good result in normal use, returning null is fine:
object x = GetObjectFromCache(); // return null if it's not in the cache
If there really should always be a non-null result, then it might be better to throw an exception:
try {
Controller c = GetController(); // the controller object is central to
// the application. If we don't get one,
// we're fubar
// it's likely that it's OK to not have the try/catch since you won't
// be able to really handle the problem here
}
catch /* ... */ {
}
It's fine to return null if doing so is meaningful in some way:
public String getEmployeeName(int id){ ..}
In a case like this it's meaningful to return null if the id doesn't correspond to an existing entity, as it allows you to distinguish the case where no match was found from a legitimate error.
People may think this is bad because it can be abused as a "special" return value that indicates an error condition, which is not so good, a bit like returning error codes from a function but confusing because the user has to check the return for null, instead of catching the appropriate exceptions, e.g.
public Integer getId(...){
try{ ... ; return id; }
catch(Exception e){ return null;}
}
For certain scenarios, you want to notice a failure as soon as it happens.
Checking against NULL and not asserting (for programmer errors) or throwing (for user or caller errors) in the failure case can mean that later crashes are harder to track down, because the original odd case wasn't found.
Moreover, ignoring errors can lead to security exploits. Perhaps the null-ness came from the fact that a buffer was overwritten or the like. Now, you are not crashing, which means the exploiter has a chance to execute in your code.
What alternatives do you see to returning null?
I see two cases:
findAnItem( id ). What should this do if the item is not found
In this case we could: Return Null or throw a (checked) exception (or maybe create an item and return it)
listItemsMatching (criteria) what should this return if nothing is found?
In this case we could return Null, return an empty list or throw an Exception.
I believe that return null may be less good than the alternatives becasue it requires the client to remember to check for null, programmers forget and code
x = find();
x.getField(); // bang null pointer exception
In Java, throwing a checked exception, RecordNotFoundException, allows the compiler to remind the client to deal with case.
I find that searches returning empty lists can be quite convenient - just populate the display with all the contents of the list, oh it's empty, the code "just works".
Make them call another method after the fact to figure out if the previous call was null. ;-) Hey, it was good enough for JDBC
Well, it sure depends of the purpose of the method ... Sometimes, a better choice would be to throw an exception. It all depends from case to case.
Sometimes, returning NULL is the right thing to do, but specifically when you're dealing with sequences of different sorts (arrays, lists, strings, what-have-you) it is probably better to return a zero-length sequence, as it leads to shorter and hopefully more understandable code, while not taking much more writing on API implementer's part.
The base idea behind this thread is to program defensively. That is, code against the unexpected.
There is an array of different replies:
Adamski suggests looking at Null Object Pattern, with that reply being up voted for that suggestion.
Michael Valenty also suggests a naming convention to tell the developer what may be expected.
ZeroConcept suggests a proper use of Exception, if that is the reason for the NULL.
And others.
If we make the "rule" that we always want to do defensive programming then we can see that these suggestions are valid.
But we have 2 development scenarios.
Classes "authored" by a developer: The Author
Classes "consumed" by another(maybe) developer: the Developer
Regardless of whether a class returns NULL for methods with a return value or not,
the Developer will need to test if the object is valid.
If the developer cannot do this, then that Class/method is not deterministic.
That is, if the "method call" to get the object does not do what it "advertises" (eg getEmployee) it has broken the contract.
As an author of a class, I always want to be as kind and defensive ( and deterministic) when creating a method.
So given that either NULL or the NULL OBJECT (eg if(employee as NullEmployee.ISVALID)) needs to be checked
and that may need to happen with a collection of Employees, then the null object approach is the better approach.
But I also like Michael Valenty's suggestion of naming the method that MUST return null eg getEmployeeOrNull.
An Author who throws an exception is removing the choice for the developer to test the object's validity,
which is very bad on a collection of objects, and forces the developer into exception handling
when branching their consuming code.
As a developer consuming the class, I hope the author gives me the ability to avoid or program for the null situation
that their class/methods may be faced with.
So as a developer I would program defensively against NULL from a method.
If the author has given me a contract that always returns a object (NULL OBJECT always does)
and that object has a method/property by which to test the validity of the object,
then I would use that method/property to continue using the object, else the object is not valid
and I cannot use it.
Bottom line is that the Author of the Class/Methods must provide mechanisms
that a Developer can use in their defensive programming. That is, a clearer intention of the method.
The Developer should always use defensive programming to test the validity of the objects returned
from another class/method.
regards
GregJF
Other options to this, are:
returning some value that indicates success or not (or type of an error), but if you just need boolean value that will indicate success / fail, returning null for failure, and an object for success wouldn't be less correct, then returning true/false and getting the object through parameter.
Other approach would to to use exception to indicates failures, but here - there are actually many more voices, that say this is a BAD practice (as using exceptions may be convenient but has many disadvantages).
So I personally don't see anything bad in returning null as indication that something went wrong, and checking it later (to actually know if you have succeeded or not). Also, blindly thinking that your method will not return NULL, and then base your code on it, may lead to other, sometimes hard to find, errors (although in most cases it will just crash your system :), as you will reference to 0x00000000 sooner or later).
Unintended null functions can arise during the development of a complex programs, and like dead code, such occurrences indicate serious flaws in program structures.
A null function or method is often used as the default behavior of a revectorable function or overrideable method in an object framework.
Null_function #wikipedia
If the code is something like:
command = get_something_to_do()
if command: # if not Null
command.execute()
If you have a dummy object whose execute() method does nothing, and you return that instead of Null in the appropriate cases, you don't have to check for the Null case and can instead just do:
get_something_to_do().execute()
So, here the issue is not between checking for NULL vs. an exception, but is instead between the caller having to handle special non-cases differently (in whatever way) or not.
For my use case I needed to return a Map from method and then looking for a specific key. But if I return an empty Map, then it will lead to NullPointerException and then it wont be much different returning null instead of an empty Map.
But from Java8 onward we could use Optional. The above is the very reason Optional concept was introduced.
G'day,
Returning NULL when you are unable to create a new object is standard practise for many APIs.
Why the hell it's bad design I have no idea.
Edit: This is true of languages where you don't have exceptions such as C where it has been the convention for many years.
HTH
'Avahappy,