Kotlin is one of the languages that allow us to easily define behavior for various predefined operators, operation named operator overloading - https://kotlinlang.org/docs/reference/operator-overloading.html
My question is regarding the overloading part of the operation.
From what I see default the language only declares operators for the basic types - https://github.com/JetBrains/kotlin/blob/01a613dca4042dde8d2374ff0e6610cb9eddc415/core/builtins/native/kotlin/Primitives.kt
If I'm reading this correctly our custom types would not have any of this special methods - operators available by default. And indeed if we're to try
class A { }
val a = A()
System.out.println(a + a)
the compiler would try to find a suitable operator plus method to call but ultimately give a compilation error.
But if we do declare our own operator plus method
class A {
operator fun plus(other: A) = "Hello!"
}
val a = A()
System.out.println(a + a)
we would indeed have "Hello!" printed.
The above mechanism is called "operator overloading" but without a previous method with the same name we do not in fact use the OOP method overloading we all are accustomed to - https://en.wikipedia.org/wiki/Function_overloading.
So between the two mechanisms - operator overloading and method overloading there is really no connection, other than an unfortunate name clash?
Looks like you are confused about operators in general.
The thing about operators is that they are just inline functions and the operator keyword is just a language construct to give you the ability to group operators with classes.
Where you can find answers about this is definitely the source code. If we take a look at the tests, we can find the following:
// "Create local variable '-'" "false"
// ACTION: Create extension function 'A.minus'
// ACTION: Create member function 'A.minus'
// ACTION: Replace overloaded operator with function call
Sadly, I cannot find the source code where operator is transformed, but most certainly this must be the procedure where the operator overload is replaced with function call.
Related
This question already has answers here:
Example of when should we use run, let, apply, also and with on Kotlin
(6 answers)
Closed 3 years ago.
We can write the code with or without let as follows.
var str = "Hello World"
str.let { println("$it!!") }
OR
var str = "Hello World"
println("$str!!")
What is the Actual use of let?.Is that more memory efficient or more readable?
let is one of Kotlin's Scope functions which allow you to execute a code block within the context of an object. In this case the context object is str. There are five of them: let, run, with, apply, and also. Their usages range from but are not exclusive to initialization and mapping.
They are all very similar but they differ in terms of how the context object is referenced and the value that is returned. In the case of let the context object is referenced by the it keyword as opposed to the this keyword. The return value is whatever is returned from the lambda code block. Other scope functions like apply will return the context object instead.
Because let returns whatever the lambda block evaluates to, it is most suited to performing a mapping of some kind:
var upperStr = str.let { it.toUpperCase()}
apply is a more suited function for what you are doing.
To answer your question as to which code is more preferable, it really depends on what you are using the scope function for. In the above case there is no reason to use let. If you are using IntelliJ it will give a warning saying the call to let is redundant. Readability here is a matter of preference, and may be preferred.
The let function is useful when you wish to perform a null safe operation on an Object by using the the safe call operator ?. When doing this the let code block will only be executed if the object is not null. Another reason to use let is if you need to introduce new variables for the operation but you want to confine them to the scope of the let block. This is true for all scope functions, so I reiterate that let is best used for a mapping operation.
Edit: The let function should incur no additional cost. Normally we would expect the lambda/Code-block to be compiled to a Function object but this is not the case for an inline function in Kotlin for which the compiler will emit code not dissimilar to the second code example you have given. See the documentation for more information.
One of usages you can check nullable types
var str: String? = null
str?.let { println("$it!!") }
it's equal
if (str != null) {
System.out.println(str);
}
in Java, but shorter and more useful
let takes the object it is invoked upon as the parameter and returns the result of the lambda expression.
Kotlin let is a scoping function wherein the variables declared inside the expression cannot be used outside.
One of the examples would be here :
fun main(args: Array<String>) {
var str = "Hello World"
str.let { println("$it!!") }
println(str)
}
You can find more information on Kotlin let function here
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.)
How can i make an arraylist of functions, and call each function easily? I have already tried making an ArrayList<Function<Unit>>, but when i tried to do this:
functionList.forEach { it }
and this:
for(i in 0 until functionList.size) functionList[i]
When i tried doing this: it() and this: functionList[i](), but it wouldn't even compile in intellij. How can i do this in kotlin? Also, does the "Unit" in ArrayList<Function<Unit>> mean return value or parameters?
Just like this:
val funs:List<() -> Unit> = listOf({}, { println("fun")})
funs.forEach { it() }
The compiler can successfully infer the type of funs here which is List<() -> Unit>. Note that () -> Unit is a function type in Kotlin which represents a function that does not take any argument and returns Unit.
I think there are two problems with the use of the Function interface here.
The first problem is that it doesn't mean what you might think. As I understand it, it's a very general interface, implemented by all functions, however many parameters they take (or none). So it doesn't have any invoke() method. That's what the compiler is complaining about.
Function has several sub-interfaces, one for each 'arity' (i.e. one for each number of parameters): Function0 for functions that take no parameters, Function1 for functions taking one parameter, and so on. These have the appropriate invoke() methods. So you could probably fix this by replacing Function by Function0.
But that leads me on to the second problem, which is that the Function interfaces aren't supposed to be used this way. I think they're mainly for Java compatibility and/or for internal use by the compiler.
It's usually much better to use the Kotlin syntax for function types: (P1, P2...) -> R. This is much easier to read, and avoids these sorts of problems.
So the real answer is probably to replace Function<Unit> by () -> Unit.
Also, in case it's not clear, Kotlin doesn't have a void type. Instead, it has a type called Unit, which has exactly one value. This might seem strange, but makes better sense in the type system, as it lets the compiler distinguish functions that return without an explicit value, from those which don't return. (The latter might always throw an exception or exit the process. They can be defined to return Nothing -- a type with no values at all.)
I'm taking my first steps with kotlin.
I am migrating some my existing java code to kotlin.
I have the folllowing line:
storyDate.ifPresent(article::setPublishDate);
Where storyDate is an Optional and article has a method setPublishDate(Date) method.
How would I migrate this line to kotlin?
The auto migrator at https://try.kotlinlang.org is
storyDate.ifPresent(Consumer<Date>({ article.setPublishDate() }))
But this line doesn't compile with the kotlin compiler.
I strongly prefer using extension functions and extension fields, so I've written smth like
val <T> Optional<T>.value: T?
get() = orElse(null)
And then you can use it anywhere (after import) like
myOptional.value?.let {
// handle here
}
It’s rather uncommon to use Optional in Kotlin. If you can make storyDate work as an ordinary unwrapped type, such constructs can often be expressed with a simple let call:
storyDate?.let {
article.setPublishDate(it)
//probably property access works as well:
article.publishDate = it
}
How it works: The safe call ?. will invoke let only if storyDate is not null, otherwise the whole expression evaluates to, again, null. When the variable is not null, let is called with a simple lambda where storyDate is accessible by it (or you can rename it to whatever you like).
Side note:
If storyDate really must be Optional, you can still use the depicted construct by unwrapping it like this:
storyDate.orElse(null)?.let {}
storyDate.ifPresent { Article.setPublishDate(it) }
or
storyDate.ifPresent(Article::setPublishDate)
will work.
In the first example, it denotes the value in the optional instance, which is the Date in the optional storyDate.
I assumed that Article is a class, which has the setPublishDate(Date) static method, because class names are always capitalized.
But if article is an instance, not a class, and it has non-static method, then the following will work.
// article = Article(/* some Article-typed object */)
storyDate.ifPresent { article.setPublishDate(it) }
it has the same meaning as the above one, i.e., the actual Date value in Optional.
In the 14th Kotlin Koan on operator overloading, I was suprised when after solving I viewed the answer and saw that the operator modifier was not required on the compareTo method:
data class MyDate(val year: Int, val month: Int, val dayOfMonth: Int) : Comparable<MyDate> {
override fun compareTo(other: MyDate) = when {
year != other.year -> year - other.year
month != other.month -> month - other.month
else -> dayOfMonth - other.dayOfMonth
}
}
The operator overloading docs linked to from the exercise explicitly says:
Functions that overload operators need to be marked with the operator
modifier.
So what's going on here? Why does this code compile? When exactly is operator required?
Why does this code compile?
This compiles because the overridden interface method, Comparable<T>.compareTo, is itself an operator fun.
/**
* Compares this object with the specified object for order. Returns zero if this object is equal
* to the specified [other] object, a negative number if it's less than [other], or a positive number
* if it's greater than [other].
*/
public operator fun compareTo(other: T): Int
As the function overrides this, it is also an operator function.
When exactly is operator required?
operator in general is required whenever you wish to be able to use a function as if it were an operator, since operator usages are simply compiled to function calls (except on primitive types, etc.)
That is, foo += bar, for example, is equivalent to foo.plusAssign(bar), foo[bar] = baz is equivalent to foo.set(bar, baz), etc.
Personally I prefer specifying operator wherever possible even if it is not required, for readability reasons.
If MyDate were not a Comparable, and you omitted the operator modifier, comparing two dates via <, >, <=, or >= would not compile.
I couldn't find anything in the specification on this, though. However in a polymorphic sense it makes sense - why should you be able to write a < b where the type of a and b are Comparables, but not when they are a MyDate? Since you wouldn't be able to remove the "operator-ness" of this function, it makes sense that operator should be inheritable from the superclass method.
Kotlin has many features that are enabled via particular conventions. Those can be identified by the use of an operator keyword. Examples are ranges, operator overloads, index operators, destructuring declarations and more.
If we want to compare two objects in Java, for sorting e.g., we implement the Comparable interface with its compareTo method. This is also done in Kotlin, but with much better support and a shorthand syntax. If you implement this method in a class, you can use all the nice operators like <, <=, >, >= with that class out of the box. These operators are translated to appropriate calls of compareTo by the compiler:
obj1 > obj2 ⇒ obj1.compareTo(obj2) > 0
The interface method compareTo in Comparable already defines the operator keyword, which makes it redundant to add the keyword in your own implementation.
In your example, the operator keyword is not mandatory since the overridden method already defines it.
In Java, operators are tied to specific Java types. For example, String and numeric types in Java can use the + operator for concatenation and addition, respectively. No other Java type can reuse this operator for its own benefit. Kotlin, on the contrary, provides a set of conventions to support limited Operator Overloading.
Let’s start with a simple data class:
data class Point(val x: Int, val y: Int)
We’re going to enhance this data class with a few operators.
In order to turn a Kotlin function with a pre-defined name into an operator, we should mark the function with the operator modifier. For example, we can overload the “+” operator:
operator fun Point.plus(other: Point) = Point(x + other.x, y + other.y)
This way we can add two Points with “+”:
val p1 = Point(0, 1)
val p2 = Point(1, 2)
println(p1 + p2)
Point(x=1, y=3)