I need to get and set a property of another class from a method and therefore need to pass in either the property reference of lambdas for the getter and the setter:
Passing in the property reference
otherInstance::property
Passing in a lambda for the getter and one for the setter:
{otherInstance.property} // getter
{value -> otherInstance.property = value} // setter
I like the first one, because for me the code is easier to read and shorter, but my alarm bells ring when I read about it on the official documentation, because of the term "reflection". My knowledge from Java is that reflection generally isn't a good thing. Is that also valid with Kotlin? Is it valid with this case? Is one of both ways (property reference or lambdas) more performant or more safe?
By using KMutableProperty0 you would technically be exposing an object that can be used for reflection. If you want to be strict about avoiding reflection, you could use the separate function references for the getter and setter. Note that it's not necessary to pass a lambda as a function reference to a higher-order function. The compiler can interpret property references as functions if the effective signature matches. This would unfortunately mean having to pass the property reference twice. Unfortunately, the setter has to be retrieved via what is technically reflection in this case:
class Test (var x: Int)
fun foo(getter: () -> Int, setter: (Int) -> Unit) {
//...
}
val test = Test(1)
foo(test::x, test::x.setter)
// Zero reflection call:
foo(test::x) { test.x = it }
At some point you have to question how badly you want to avoid reflection, because the above code looks very messy to me. If your class takes a KMutableProperty0 reference, it is much simpler to use. As long as your receiving function isn't using the reference to introspect the code, and only calls get() or set() on it, you are not really using reflection in the ways that are suggested should be avoided.
fun foo(property: KMutableProperty0<Int>) {
//...
}
val test = Test(1)
foo(test::x)
The documentation is about Member references and reflection,
If you are referring to Property references which isn't using reflection itself,
Reflection is only referred in different section Obtaining member references from a class reference
dynamically inspect an object to see e.g. what properties and functions it contains and which annotations exist on them. This is called reflection, and it's not very performant, so avoid it unless you really need it.
Kotlin has got its own reflection library (kotlin-reflect.jar must be included in your build). When targeting the JVM, you can also use the Java reflection facilities. Note that the Kotlin reflection isn't quite feature-complete yet - in particular, you can't use it to inspect built-in classes like String.
Related
Using getters and setters is a very well known practice in object oriented languages. This is done in order to have a greater control on the variables. To achieve this, we make the variables private in java and hence we need both getters and setters there.
But in kotlin this is not the case. Here even public variables are accessed through getters and setters by default. Though setters can be used to validate an assignment to a variable, getters just return the variable as it is stored (and I think this is it for them). Hence custom getters are not required at all.
I have also seen some wrong usage of this feature where instead of writing a zero argument function, they use a val and do the computation in the getter. This creates an illusion that the thing is just a val but in reality it does not store anything and instead it performs a computation every time.
So is there a real need to have a custom getter?
getters just return the variable as it is stored (and I think this is it for them). Hence custom getters are not required at all.
If that was really the case, why have getters at all in Java? One of the goals of encapsulation is to make sure a change in the class doesn't change it's API. It's the same in Kotlin.
I have also seen some wrong usage of this feature where instead of writing a zero argument function, they use a val and do the computation in the getter. This creates an illusion that the thing is just a val but in reality it does not store anything and instead it performs a computation every time.
This is a perfectly valid use case for a custom getter. In Kotlin, one must not assume that using a property is entirely free of overhead. There are many questions to ask yourself when choosing between a property with a getter or a zero-arg function:
Does it describe behavior? Use a function (walk(), build(), etc)
Does it describe state? Use a property (firstName, lastIndex, etc)
Additionally, a property getter should not throw an exception, should be either cheap to calculate or cached on first access, and should return the same result for multiple consecutive executions. Here's examples from the standard library:
ArrayDeque.first() is a function, it throws if deque is empty.
List.lastIndex is a property, it's cheap to calculate.
Lazy<T>.value is a property, the value is computed and cached on first access.
Most delegated properties make use of custom getters.
More reading:
Why use getters and setters/accessors?
Kotlin: should I define Function or Property?
Just some more info. Other than readability, the possibility of defining a custom getter allows you to evolve a class without changing its public members, even if you started with a simple val with no custom getter.
In a language without properties like Java, if you define a public field:
public class Foo {
public final int value;
public Foo(int value) {
this.value = value;
}
}
And then later you want to modify the class to add a feature where it returns negated values if you flip a Boolean, there's no way to do it without breaking code that uses the original version of the class. So you should have used getters and setters to begin with.
But in Kotlin, you can't directly expose a backing field like this, so it's impossible to paint yourself in a corner like you could with a public field in Java. If your original class is like this:
class Foo(val value: Int)
You could modify it like this to add the feature and have no impact on code that already uses the class.
class Foo(private val originalValue: Int) {
var isNegated = false
val value: Int
get() = if (isNegated) -originalValue else originalValue
}
Is it possible to create an anonymous delegate in Kotlin for the purpose of passing to a function argument? I'm particularly interested in by lazy, but this question probably applies to all delegates. For example, say I have this function:
fun sayHello(name: String){
println("Hello $name")
}
this works just fine:
val name by lazy{ "Ralph" }
sayHello(name)
But none of the following are correct:
sayHello(lazy{"Ralph"})
sayHello(by lazy{"Ralph"})
sayHello({"Ralph") as lazy})
Is this possible somehow?
There's not a practical way to do this for any general delegate. Delegates are designed for use specifically with properties, so their getter implementation takes an object instance (the property owner) and a KProperty argument (see ReadOnlyProperty). They might specifically need these references for their functionality.
The Lazy interface happens to have a value property so you can use it like this, but this does not apply to all delegates:
sayHello( lazy{"Ralph"}.value )
I am aware that extension functions are used in Kotlin to extend the functionality of a class (for example, one from a library or API).
However, is there any advantage, in terms of code readability/structure, by using extension functions:
class Foo { ... }
fun Foo.bar() {
// Some stuff
}
As opposed to member functions:
class Foo {
...
fun bar() {
// Some stuff
}
}
?
Is there a recommended practice?
When to use member functions
You should use member functions if all of the following apply:
The code is written originally in Kotlin
You can modify the code
The method makes sense to be able to use from any other code
When to use extension functions
You should use extension functions if any of the following apply:
The code was originally written in Java and you want to add methods written in Kotlin
You cannot change the original code
You want a special function that only makes sense for a particular part of the code
Why?
Generally, member functions are easier to find than extension functions, as they are guaranteed to be in the class they are a member of (or a super class/interface).
They also do not need to be imported into all of the code that uses them.
From my point of view, there are two compelling reasons to use extension functions:
To "extend" the behaviour of a class you're not the author of / can't change (and where inheritance doesn't make sense or isn't possible).
To provide a scope for particular functionality. For example, an extension function may be declared as a freestanding function, in which case it's usable everywhere. Or you may choose to declare it as a (private) member function of another class, in which case it's only usable from inside that class.
It sounds like #1 isn't a concern in your case, so it's really more down to #2.
Extension functions are similar to those you create as a utility functions.
A basic example would be something like this:
// Strings.kt
fun String.isEmail() : Boolean {
// check for email pattern and return true/false
}
This code can be written as a utility function in Java like this:
class StringUtils {
public static boolean isEmail(String email) {
// check for email pattern and return true/false
}
}
So what it essentially does is, calling the same function with the object you call on will be passed as the first parameter to the argument. Like the same function I have given example of in Java.
If you want to call the extension function created in kotlin from java, you need to pass the caller as the first argument. Like,
StringsKt.isEmail("example#example.com")
As per the documentation,
Extensions do not actually modify classes they extend. By defining an extension, you do not insert new members into a class, but merely make new functions callable with the dot-notation on variables of this type.
They are simply static functions with the caller as the first argument and other parameters followed by it. It just extends the ability for us to write it that way.
When to create extension functions?
When you don't have access to that class. When that class belongs to some library you have not created.
For primitive types. Int, Float, String, etc.
The another reason for using extension function is, you don't have to extend that class in order to use the methods, as if they belong to that class (but not actually part of that class).
Hope it makes a bit clear for you..
As mentioned in other answers, extension functions are primarily used in code that you can't change - maybe you want to change complex expression around some library object into easier and more readable expression.
My take would be to use extension functions for data classes. My reasoning is purely philosophical, data classes should be used only as data carriers, they shouldn't carry state and by themselves shouldn't do anything. That's why I think you should use extension function in case you need to write a function around data class.
What is the naming convention for boolean returning methods?
Using an 'is', 'has', 'should', 'can' in the front of method sound ok for some cases, but I'm not sure.
Is there a better way to name such methods?
for example: a function that checks card's validation. Should I call it isValidCard or cardValidation or another name?
(I didn't find it here: https://kotlinlang.org/docs/reference/coding-conventions.html)
Something about naming convention for properties in Kotlin, I know it's not for methods. But it's related:
From book Kotlin in Action (by Dmitry Jemerov & Svetlana Isakova) - section 2.2.1 Properties:
In Kotlin, properties are a first-class language feature, which entirely replaces fields and accessor methods.
Listing 2.5. Declaring a mutable property in a class:
class Person {
val name: String, // read only property: generates a field and a trivial getter
var isMarried: Boolean // writable property: a field, getter and a setter
}
Kotlin’s name property is exposed to Java as a getter method called
getName. The getter and setter naming rule has an exception: if the
property name starts with is, no additional prefix for the getter is
added and in the setter name, is is replaced with set. Thus, from
Java, you call isMarried().
For those using properties prefixed with can, should, etc. in mixed Kotlin/Java projects, you can also use #get:JvmName to make the generated Java method more idiomatic for Java clients.
For example, say you have a class like this:
class User(
#get:JvmName("canView")
val canView: Boolean
)
Without the annotation, Java clients would be forced to call user.getCanView(), but now they can call the more idiomatic user.canView().
Kotlin naming style assumes you use the Java naming conventions to the possible extend. I suggest you use this answer to the same question about Java.
UPDATE: they have released coding conventions
http://kotlinlang.org/docs/reference/coding-conventions.html
So I'm new to Scala (and have almost zero java experience). I thought I understood OOP, in abstract, but disregard that. My question -- in a similar vein to "method name qualification when using a companion object" -- is about when a Scala pro would think to implement a class - companion object pattern?
From the question referenced above, it's not clear that companion objects were intended to store methods for the class's "internal use" (e.g. the poster wanted to use ^, defined in the object, inside /, defined in the class). So, I don't want to think of companion objects as "containers" for methods the companion class can use, because that's clearly not true...
I'm sorry if this is a vague question: I just want to know the correct way to use these guys.
Companion objects are useful for what you would use static methods for in Java...
One very common use is to define an apply() method in the companion object, which gives users the ability to use MyObject(arg, arg) as shorthand for new MyObject(arg, arg).
Companion objects are also a good place to put things like implicit defs.
I recently have been using companion objects in my akka apps as places to put message case classes which are specific to a supervisor actor and its children, but that I don't necessarily want code outside that subsystem to use directly.
Here's a simple example:
class Complex(real:Double, imag:Double) {
def +(that:Complex):Complex = Complex(this.real + that.real, this.imag + that.imag)
// other useful methods
}
// the companion object
object Complex {
def apply(real:Double, imag:Double) = new Complex(real, imag)
val i = Complex(0, 1)
implicit def fromInt(i:Int) = Complex(i, 0)
}
The normal OOP way to instantiate a new Complex object would be new Complex(x, i). In my companion object, I defined the function apply, to give us a syntactic sugar that allows us to write Complex(x, i). apply is a special function name which is invoked whenever you call an object directly as if it were a function (i.e., Complex()).
I also have a value called i which evaluates to Complex(0, 1), which gives me a shorthand for using the common complex number i.
This could be accomplished in Java using a static method like:
public static Complex i() {
return new Complex(0, 1);
}
The companion object essentially gives you a namespace attached to your class name which is not specific to a particular instance of your class.