IntelliJ is showing me context hints that my variables are of type (String..String?). I cannot find any mention of it on the internet, what is this type?
(String..String?) represents a flexible type with lower bound String and upperbound String? (nullable string). This is not valid Kotlin code (it's not denotable) but it is used in the compiler internals and thus in IntelliJ's hints sometimes.
(On the JVM we often see platform types using ! as in String!, which are a more specific case of flexible types)
It's Kotlin's way of saying it doesn't know whether the String type declared for payload.email is nullable or not (for instance if this is declared in Java, which doesn't distinguish those), and yet it doesn't want to enforce either of those, for convenience (hence "flexible").
As the name suggests, flexible types are flexible — a value of type (L..U) can be used in any context, where one of the possible types between L and U is needed
This means that even though the actual type of the value is "somewhere between String and String?", values of this type can be used even in places expecting String, even though the real type of that value may be String? and thus the value could be null.
This is useful because assuming it is String would mean that null checks would be marked as redundant, and assuming it is String? would force the developer to write null checks everywhere, even though they might know that this particular Java method cannot return null.
In general, it's a good practice to explicitly declare the type of a variable that you get from Java, to avoid the propagation of the platform type and the uncertainty (and unsafety) that comes with it:
val email: String = payload.email // if you know it cannot be null
val email: String? = payload.email // if you don't know
fun myfunction(a:String) //this is valid
fun myfunction(var a:String) //this is invalid
fun myfunction(val a:String) //this is invalid
The support for var was removed way back from kotlin with the following reason:
The main reason is that this was confusing: people tend to think that this means passing a parameter by reference, which we do not support (it is costly at runtime). Another source of confusion is primary constructors: “val” or “var” in a constructor declaration means something different from the same thing if a function declarations (namely, it creates a property). Also, we all know that mutating parameters is no good style, so writing “val” or “var” infront of a parameter in a function, catch block of for-loop is no longer allowed.
More details in https://blog.jetbrains.com/kotlin/2013/02/kotlin-m5-1/.
So I have 2 overloaded score functions, one of which takes a performance param of type Performance, and the other which takes a performances param, which is a List<Performance>. The former returns a double, while the latter returns a double array. (Another team owns the Scorer class, so fixing it to not overload like this isn't really doable rn; breaking changes and all).
I want to test 2 branches, one with each implementation, how can I mock these using kotlin's mockk?
The former can be mockked using ofType(Performance::class),
scorer: Scorer = mockk()
every { scorer.score(????) } returns doubleArrayOf(0.9)
What goes there?
ofType(List<Performance>::class) doesn't work because apparently that can't be done with generics.
not(ofType(CandidateFeature::class)) results in a compile-time error Type mismatch: inferred type is DoubleArray but Double was expected
How do I explicitly choose which overriden signature I'm trying to call?
You can use withArg
every { scorer.score(withArg<Performance> {} ) returns mockData
every { scorer.score(withArg<List<Performance>> {} ) returns mockData
I am a newbie in Kotlin, I just started to learn it,
I get the following code example about literal/high order function:
fun myHigherOrderFun(functionArg: (Int)->String) = functionArg(5)
println ( myHigherOrderFun { "The Number is $it" })
prints "The Number is 5"
Which I have difficulty to understand: the function myHigherOrderFun get a lambda function as parameter but i can't understand, where is the (Int) input parameter? I see is passed in functionArg(5)... but i can't realize how is possible that?
Thanks in advance.
To start from the beginning, in Kotlin functions are first-class types, just like numbers and Strings and stuff. So a function can take another function as a parameter, and/or return a function as its result. A function which does this is called a ‘higher-order function’.
And that's what you have in your example! The line:
fun myHigherOrderFun(functionArg: (Int)->String) = functionArg(5)
defines myHigherOrderFun() as a function which takes one parameter, which is itself a function taking a single Int parameter and returning a String. (myHigherOrderFun() doesn't specify an explicit return type, so it's inferred to be a String too.)
The next line is probably where things are less clear:
println(myHigherOrderFun{ "The Number is $it" })
The first non-obvious thing is that it's calling myHigherOrderFun() with a parameter. Because that parameter is a lambda, Kotlin lets you omit the usual (…), and use only the braces.
The other non-obvious thing is the lambda itself: { "The Number is $it" }. This is a literal function taking one parameter (of unspecified type).
Normally, you'd have to specify any parameters explicitly, e.g.: { a: Char, b: Int -> /* … */ }. But if there's exactly one parameter, and you aren't specifying its type, then you can skip that and just refer to the parameter as it. That's what's happening here.
(If the lambda didn't reference it, then it would be a function taking no parameters at all.)
And because the lambda is being passed to something expecting a function taking an Int parameter, Kotlin knows that it must be an Int, which is why we can get away without specifying that.
So, Kotlin passes that lambda to the myHigherOrderFun(), which executes the lambda, passing 5 as it. That interpolates it into a string, which it returns as the argument to println().
Many lambdas take a single parameter, so it gets used quite a lot in Kotlin; it's more concise (and usually more readable) than the alternative. See the docs for more info.
I know how to read/interpret Java code and I can write it. However being new to kotlin I find code like below hard to read. Perhaps I am missing key concepts in the language.
But, how would you go about interpreting this code? Where do you propose one to start reading it in order to understand this piece of code quickly and efficiently? Left to right? Right to left? Break down parameters first? Look at return values?
inline fun <T : Any, R> ifNotNull(input: T?, callback: (T) -> R): R? {
return input?.let(callback)
}
So, like Java this is a generic function. It has two type parameters T which is of type 'Any' ('Any' is like 'Object' in Java) and R. The input parameter is a nullable T, as denoted by the question mark. Nullable types mean that the value can be null. The other function parameter is a function that takes in a T (non nullable type) and returns R. The return type of the function is a nullable R. The body of the function says that if input is not null, call and pass that to the callback and return that value. If input is null, then null is what gets returned.
Let's dissect the function definition piece by piece:
inline: Indicates that the code of the function will be copied directly to the call site, rather than being called like a normal function.
fun: We're defining a function.
<T : Any, R>: The function takes two generic type parameters, T and R. The T type is restricted to the Any type (which is Kotlin's Object-type). That might seem redundant, but what it actually says is that T cannot be a nullable type (Any?).
ifNotNull: The name of the function.
input: T?: The first parameter of type T?. We can put the ? on the T type here because we restricted it to non-nullable types in the type declaration.
callback: (T) -> R: The second parameter is of type (T) -> R, which is a function type. It's the type of a function that takes a T as input and returns an R.
: R?: The function returns a value of type R or null.
return input?.let(callback): The function body. The let function takes a function parameter, calls it with its receiver (input), and then returns the result of the function. The ? after input says that let will be called only if input is not null. If input is null, then the expression will return null.
The function is equivalent to this Java method (except for the inlining and nullable types):
public <T, R> R ifNotNull(final T input, final Function<T, R> callback) {
if (input == null) {
return null;
}
return callback.apply(input);
}
Matt's answer explains everything well in one go; I'll try to look at how you might go about reading such code.
Skipping over the first word for now, the most important thing is the second word: fun. So the whole thing is defining a function. That tells you what to expect from the rest.
The braces tell you that it's a block function, not a one-liner, so the basic structure you're expecting is: fun name(params): returnType { code }. The rest is filling in the blanks! (This fits the general pattern of Kotlin declarations, where the type comes second, after a colon. The Java equivalent would of course be more like returnType name(params) { code }.)
As with Java, the stuff in angle brackets is giving generic parameters, so we can skip that for now and go straight to the next most important bit, which is the name of the function being defined: ifNotNull.
Armed with those, we can read the rest. inline is a simple modifier, telling you that the function will be inlined by the compiler. (That enables a few things and restricts a few others, but I wouldn't worry about that now.)
The <T : Any, R> gives the generic parameter types that the function uses. The first is T, which must be Any or a subtype; the second is R, which is unrestricted.
(Any is like Java's Object, but can't be null; the topmost type is the related Any?, which also allows null. So except for the nullability, that's equivalent to the Java <T extends Object, R>.)
Going on, we have the function parameters in parentheses. Again, there are two: the first is called input, and it's of type T?, which means it accepts any value of type T, and also accepts null. The second parameter is called callback, and has a more complicated type, (T) -> R: it's a function which takes a T as its parameter, and returns an R. (Java doesn't have function types as such, so that probably looks strangest. Java's nearest equivalent is Function<R, T>.)
After the parentheses comes the return type of this function itself, R?, which means it can return either an R or null.
Finally, in braces is the actual code of the function. That has one line, which returns the value of an expression. (Its effect is to check whether the value of input is null: if so, it returns the null directly. Otherwise, it calls the callback function given in the parameter, passing input as its parameter, and returns its result.)
Although that's a short declaration, it's quite abstract and packs a lot in, so it's no wonder you're finding it hard going! (The format is similar to a Java method declaration — but Kotlin's quite expressive, so equivalent code tends to be quite a bit shorter than Java. And the generics make it more complex.) If you're just starting to learn Kotlin, I'd suggest something a bit easier :-)
(The good news is that, as in Java, you don't often need to read the stdlib code. Although Kotlin's doc comments are rarely up to the exemplary level of Java's, they're still usually enough.)