I am working on testing a flux. I don't know how many elements exactly the flux has. Initially I have tried with StepVerifier and faced issues as i do not know the elements. Later I have referred this question and tried the same but I am getting the below error:
java.lang.AssertionError: expectation "expectComplete" failed (expected: onComplete(); actual: onNext
My understanding is that, my code is expecting a complete signal but the flux has some more elements left(so it gives onNext() instead of onComplete()). Please help me to understand where I am missing things. Below is my code:
StepVerifier.create(flux)
.recordWith(ArrayList::new)
.consumeRecordedWith(elements-> {assertThat(elements.size()).isGreaterThan(0);})
.verifyComplete();
You're not actually consuming your Flux, you're just setting up what happens when it's consumed. Your verifyComplete(); call then fails, understandably, because the Flux hasn't been consumed at all, and it's thus not complete!
You need to add a thenConsumeWhile() call to actually consume it.
If you really need to use AssertJ as you do above, then you can do:
StepVerifier.create(flux)
.recordWith(ArrayList::new)
.thenConsumeWhile(x -> true)
.consumeRecordedWith(elements -> {
assertThat(elements.isEmpty()).isFalse();
})
.verifyComplete();
However, there's no need for AssertJ here - the reactor test package is enough, and adding additional testing frameworks makes the testing code much less clear IMHO. So if you're not wedded to AssertJ, just do:
StepVerifier.create(flux)
.recordWith(ArrayList::new)
.thenConsumeWhile(x -> true)
.expectRecordedMatches(elements -> !elements.isEmpty())
.verifyComplete();
Note that in real-world use, you'd probably want to adjust the predicate in thenConsumeWhile so that it runs a check against each element in turn, too. I've also adjusted the above code to use isEmpty() rather than checking if size()>0, as it's semantically clearer while achieving the same purpose.
From the same issue, with something new: I had so many entries in my flux that it couldn't fit into the memory (yes, those test case fixtures were designed that way)...
So buffering everything into a List wasn't an option.
And I tried different API methods on StepVerifier and found the following to work:
StepVerifier.create( myFlux )
.thenConsumeWhile( Predicate<T>, Consumer<T> )
.verifyComplete();
I literally did
StepVerifier.create( myFlux )
.thenConsumeWhile( __ -> true, entry -> {
// assertions
} )
.verifyComplete();
I'm new to Kotlin and there's a common pattern that I'm not sure how to deal with most correctly. Take this code, for example, which doesn't compile:
git_repo?.add().addFilepattern()
add() is a call in the JGit library which is purely Java, so its return type is AddCommand!.
I have two options:
git_repo?.add()!!.addFilepattern("test.txt")
and
git_repo?.add()?.addFilepattern("test.txt")
Both work fine and given that
I don't know the intricacies of the library implementation,
the documentation of the JGit library doesn't specify whether add() can return null, and
within this context I'd typically expect add() to not return a null
Which version is more idiomatically correct to write in Kotlin? It seems that this would be a fairly common issue to deal with since basically every non-Kotlin library would introduce this issue.
I would use the ?. safe operator and then put your own exception at the end after an ?: Elvis operator. This way you get a message that is meaningful. Using just !! isn't a very helpful message to someone down the road who has no idea what the intricacies were either.
val cmd = gitRepo.add()?.addFilepattern("test.txt") ?: throw IllegalStateException("assert: gitRepo.add() returned an unexpected null")
cmd.doSomething() // never is null guaranteed
If the value is every null you will have a custom error.
assert: gitRepo.add() returned an unexpected null
And after this line, you will not have to null check because the result of the expression is guaranteed never to be null.
If you inspect the code of the other library and ensure it would never ever be null no matter what, then a !! is appropriate. But if not sure, do one better with the custom message.
Note I dropped the first ?. from your example because I'm assuming git_repo itself is not nullable. Plus I renamed it not to have an underscore which isn't normal Kotlin naming convention.
If you are sure that git_repo will always return a value!! is fine in that case.
It is ugly but !! will always be there when you use Java libraries, and you can't avoid it.
The only reason i would use git_repo?.add()?.addFilepattern("test.txt"), would be if you are returning a value, and you want the value to be nullable so that your calling method can handle the nullable.
fun nullableMethod(): string? {
return git_repo?.add()?.addFilepattern("test.txt")
}
fun callingMethod() {
if(this.nullableMethod() != null) {
}
//Else
}
If you are guaranteed it is never going to null, use !!
Currently working on a small pet project, a Math Parser/Lexer and eventually solver for fun/learning. I've bashed out a basic prototype and am now looking to convert this into TypeScript to clean things up. Prototype can be found here https://github.com/chips5k/math-solver-prototype for those interested.
I am trying to come up with a clean interface based approach to dealing with my Tokens. I suspect i am looking at this the wrong way, but hopefully someone can offer useful advice.
In my TS design i have several interfaces, the base interface being Token, with NumericToken and FunctionToken extending these. I then have several classes that implement these interfaces such as: AdditionToken, VariableToken, NumberToken, PiToken, SineToken etc...
My problem is that in order to work with these interfaces i end up requiring methods to check the basic type e.g isNumeric, isFunction, isVariable or alternatively a direct type === TokenFactory.NUMERIC etc... This, to me, feels wrong as it basically voids the point of using an interface. I suspect that there is a nicer/cleaner more polymorphic approach i could take but unfortunately i'm out of ideas and have been unable to find info on what i am doing.
An example of where things fall apart shows itself when attempting to solve a series of tokens:
if(t.isFunction()) {
var a = stack.unshift();
var b = stack.unshift();
if(a.isNumeric() && b.isNumeric()){
result.push(tokenFactory.create(t.evaluate<number>(a.evaluate<number>, b.evaluate<number>));
} else {
//return to stack and move on, e.g can't solve x + 1 directly
}
} else {
stack.push(t);
}
Basically looking for what is considered the ideal approach for handling a scenario like this, and i suspect it may be an alternate approach to the design.
TIA!
basic type e.g isNumeric, isFunction, isVariable or alternatively a direct type === TokenFactory.NUMERIC etc... This, to me, feels wrong
Nope. This is fairly idiomatic as the type controls what functionality is there.
E.g you will see the TypeScript checker.ts littered with check on .kind (SyntaxKind) which is at TypeScript AST nodes discriminator.
Also you might want to consider adding a visitor that is recursive e.g.
function visit(item){
if (item.addition) {
return visit(item.left) + visit(item.right)
}
if (item.literal) {
return literal.value();
}
// ETC.
}
Assert.assertFalse(driver.findElement(By.xpath("element's xpath")).isDisplayed(),
"Bug!! The element is appeared");
When i run my code with the above script, my scripts are failing with the statement like "can't find the element, (Which is mentioned int he element's xpath)"
Now what i have to do? My notion is to verify the element is not present in the screen
findElement throws an exception if no matching element is found, so you tests will fail with the selenium exception, not JUnit.
you can use findElements (which doesn't throws exceptions) and check that the returned list length is equals to 0
See details in the Selenium documentation
I see that this has been asked a long while ago, but perhaps someone who is looking for a solution to the similar problem will find it useful now.
I have a certain way to deal with such problems, not sure if it's a good way to do this, but at least it makes sense and does the job.
Firstly, I usually define a separate method for finding such elements:
private static WebElement element = null;
public static WebElement findYourElement(WebDriver driver) {
try {
element = driver.findElement(By.xpath("your_xpath_here"));
}
catch(NoSuchElementException e) {
System.out.println(e.getMessage());
element = null;
return element;
}
return element;
}
You can see here that the possible NoSuchElementException is being caught should it be thrown. In case of exception, the value for "element" is also set to null, since in my program there might be values from previous elements stored here and we don't to have any other non-relevant elements here.
Next comes the assertion:
Assert.assertFalse(findYourElement != null, "Bug!! The element is there!"
Here the assertFalse method fails if the element returned by the findYourElement is not empty, yielding a True result from element != null comparison and making assertFalse fail, as it is expecting to have a false value in order to continue.
You could just as well go:
Assert.assertTrue(findYourElement = null, "Bug!! The element is there!"
I think that such solution helps to keep things well organized, but on the other hand it may be larger in terms of code volume and more complicated too. It also depends how good are you with Java and if you are willing to mess with the exceptions.
I am no Java guru myself, so any contribution towards improving my current solution will be much appreciated!
Do this small change, it definitely works
Assert.assertTrue(driver.findElements(By.xpath("element's xpath")).size()<1;,
"Bug!! The element is appeared");
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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,