If I write a class in Scala, does it automatically extends AnyVal, AnyRef or both?
If I write a class in Scala, does it automatically extends AnyVal, AnyRef or both?
It should be obvious that it cannot automatically extend both because a Scala class can only extend one class, not two.
It should also be obvious that it cannot automatically extend AnyVal because classes that extend AnyVal must obey some specific restrictions which cannot, in general, be satisfied by any random class.
Therefore, the only sensible way it could work is that a class automatically extends AnyRef.
However, we don't have to use common sense, we don't have to use our brains at all, we can just look into the Scala Language Specification [bold emphasis mine]:
5.3 Class Definitions
The extends clause extends sc with mt_1 with … with mt_m can be omitted, in which case extends scala.AnyRef is assumed.
Inside the Scala standard library, I noticed this:
package scala
package collection
package mutable
import generic._
...
/** Explicit instantiation of the `Seq` trait to reduce class file size in subclasses. */
abstract class AbstractSeq[A] extends scala.collection.AbstractSeq[A] with Seq[A]
AbstractSeq[A] extends AbstractSeq[A] with Seq[A]?
What is going on here?
It is extending scala.collection.AbstractSeq, while the definition is of scala.collection.mutable.AbstractSeq, so those are different classes. (Note the different package names: scala.collection vs. scala.collection.mutable.)
They are different classes. The one you are looking at is scala.collection.mutable.AbstractSeq, the one it is extending is scala.collection.AbstractSeq.
Package is a kind of namespace. Different classes can have the same name as long as they are in different packages.
So I'm building a library, and the problem I have is as follows:
I have a trait, such as
package my.library
trait Animal {
def randomFunctions
}
What I need to know is all the classes the consumer code has, that extend/implement said trait, such as
package code.consumer
case class Cat extends Animal
case class Dog extends Animal
So in summary: inside my library (which has the trait) I need to find out all classes (in consumer code) that extend/implement the trait.
I finally solved this by using reflections (https://github.com/ronmamo/reflections) with the following little snippet:
val reflection = new Reflections()
reflection.getSubTypesOf(classOf[Animal])
An option would be to use a sealed trait. This forces all implementations of the trait to reside in the same file as the trait was defined.
This would break your separation of consumer and library code but you would be sure to get all implementations.
The only other option I can think of is to use an IDE, like IntelliJ which has an option to find all implementation based on given trait.
So, I was trying to make a finagle server, talk to sentry (not important), and stumbled upon a case, where I needed to inherit from two classes (not traits) at the same time, let's call them class SentryHandler extends Handler and class TwitterHandler extends Handler, and assume, that I need to create MyHandler, that inherits from both of them.
After a moment of stupidity, when I thought it was impossible without using a dreaded "delegation pattern", I found a solution:
trait SentryTrait extends SentryHandler
class MyHandler extends TwitterHandler with SentryTrait
Now, this got me thinking: what is the purpose of having the notion of "trait" to being with? If the idea was to enforce that you can inherit from multiple traits but only a single class, it seems awfully easy to get around. It kinda sounds like class is supposed to be the "main" line of inheritance (that you "extend a class with traits", but that isn't true either: you can extend a trait with (or without) a bunch of other traits, and no class at all.
You cannot instantiate a trait, but the same holds for an abstract class ...
The only real difference I can think of is that a trait cannot have constructor parameters. But what is the significance of that?
I mean, why not? What would the problem with something like this?
class Foo(bar: String, baz: String) extends Bar(bar) with Baz(baz)
Your solution (if I understood correctly) - doesn't work. You cannot multiinherit classes in scala:
scala> class Handler
defined class Handler
scala> class SentryHandler extends Handler
defined class SentryHandler
scala> class TwitterHandler extends Handler
defined class TwitterHandler
scala> trait SentryTrait extends SentryHandler
defined trait SentryTrait
scala> class MyHandler extends TwitterHandler with SentryTrait
<console>:11: error: illegal inheritance; superclass TwitterHandler
is not a subclass of the superclass SentryHandler
of the mixin trait SentryTrait
class MyHandler extends TwitterHandler with SentryTrait
As for the question - why traits, as I see it, this is because traits are stackable in order to solve the famous diamond problem
trait Base { def x: Unit = () }
trait A extends Base { override def x: Unit = { println("A"); super.x}}
trait B extends Base { override def x: Unit = { println("B"); super.x}}
class T1 extends A with B {}
class T2 extends B with A {}
(new T1).x // Outputs B then A
(new T2).x // Outputs A then B
Even though trait A super is Base (for T1) it calls B implementation rather then Base. This is due to trait linearization
So for classes if you extend something - you can be sure that this base will be called next. But this is not true for traits. And that's probably why you do not have trait constructor parameters
The question should rather be: why do we need classes in Scala? Martin Odersky has said that Scala could get by with just traits. We would need to add constructors to traits, so that instances of traits can be constructed. That's okay, Odersky has said that he has worked out a linearization algorithm for trait constructors.
The real purpose is platform interoperability.
Several of the platforms Scala intends to integrate with (currently Java, formerly .NET, maybe in the future Cocoa/Core Foundation/Swift/Objective-C) have a distinct notion of classes, and it is not always easy to have a 1:1 mapping between Scala traits and platform classes. This is different, for example, from interfaces: there is a trivial mapping between platform interfaces and Scala traits – a trait with only abstract members is isomorphic to an interface.
Classes, packages, and null are some examples of Scala features whose main purpose is platform integration.
The Scala designers try very hard to keep the language small, simple, and orthogonal. But Scala is also explicitly intended to integrate well with existing platforms. In fact, even though Scala is a fine language in itself, it was specifically designed as a replacement for the major platform languages (Java on the Java platform, C# on the .NET platform). And in order to do that, some compromises have to be made:
Scala has classes, even though they are redundant with traits (assuming we add constructors to traits), because it's easy to map Scala classes to platform classes and almost impossible to map traits to platform classes. Just look at the hoops Scala has to jump through to compile traits to efficient JVM bytecode. (For every trait there is an interface which contains the API and a static class which contains the methods. For every class the trait is mixed into, a forwarder class is generated that forwards the method calls to trait methods to the static class belonging to that trait.)
Scala has packages, even though they are redundant with objects. Scala packages can be trivially mapped to Java packages and .NET namespaces. Objects can't.
Package Objects are a way to overcome some of the limitations of packages, if we didn't have packages, we wouldn't need package objects.
Type Erasure. It is perfectly possible to keep generic types around when compiling to the JVM, e.g. you could store them in annotations. But third-party Java libraries will have their types erased anyway, and other languages won't understand the annotations and treat Scala types as erased, too, so you have to deal with Type Erasure anyway, and if you have to do it anyway, then why do both?
null, of course. It is just not possible to automatically map between null and Option in any sane way, when interoperating with real-world Java code. You have to have null in Scala, even though we rather wished it weren't there.
The problem with having constructors and state in a trait (which then makes it a class) is with multiple inheritance. While this is technically possible in a hypothetical language, it is terrible for language definition and for understanding the program code. The diamond problem, mentioned in other responses to this question), causes the highest level base class constructor to be called twice (the constructor of A in the example below).
Consider this code in a Scala-like language that allows multiple inheritance:
Class A(val x: Int)
class B extends A(1)
class C extends A(2)
class D extends B, C
If state is included, then you have to have two copies of the value x in class A. So you have two copies of class A (or one copy and the diamond problem - so called due to the diamond shape of the UML inheritance diagram).
Diamond Multiple Inheritance
The early versions of the C++ compiler (called C-Front) had lots of bugs with this and the compiler or the compiled code often crashed handling them. Issues include if you have a reference to B or C, how do you (the compiler, actually) determine the start of the object? The compiler needs to know that in order to cast the object from the Base type (in the image below, or A in the image above) to the Descendant type (D in the image above).
Multiple Inheritance Memory Layout
But, does this apply to traits? The way I understand it, Traits are an easy way to implement composition using the Delegation Pattern (I assume you all know the GoF patterns). When we implement Delegation in any other language (Java, C++, C#), we keep a reference to the other object and delegate a message to it by calling the method in its class. If traits are implemented in Scala internally by simply keeping a reference and calling its method, then traits do exactly the same thing as Delegation. So, why can't it have a constructor? I think it should be able to have one without violating its intent.
The only real difference I can think of is that a trait cannot have constructor parameters. But what is the significance of that? I mean, why not?
Consider
trait A(val x: Int)
trait B extends A(1)
trait C extends A(2)
class D extends B with C
What should (new D {}).x be? Note: there are plans to add trait parameters in Scala 3, but still with restrictions, so that the above is not allowed.
I notice that there were several discussions about how to choose between abstract classes and traits, but it seems that none of them focused on the following point. One reason that made me use abstract classes is, they can have constructor parameters, while traits cannot. But why not the following
trait X {
def haha: Int
}
class Y(val haha: Int) extends X
and early definition is even not necessary to get everything work properly (which I worried about). The abstract class version is
abstract class X(haha: Int)
class Y(val haha: Int) extends X(haha)
and I don't like the abstract class version because, when you extend several times, these constructor parameters appear everywhere (maybe someone tells me how to avoid this?).
I am aware that abstract classes interpolate with Java better, and match the "is-a" concept more. Despite these, is there any reason that I should use abstract classes somewhere? Thanks!
The class parameter does not have to be a member (field or def).
abstract class X(haha: Int) {
val hoho = 2 * haha // compile-time constant
}
Similarly, trait initialization order depends on linearization (mix-in order), which is why trait members should be defs and not vals. (And you can always override the def with a val.) With an abstract class, you know who your supers are, and you're defining extension points for subclasses.
But note your abstract class has the val in the wrong place:
abstract class X(val haha: Int)
class Y(haha: Int) extends X(haha)
That is, you would expect X to decide if the param is a val (and it doesn't have to be). Usage of the param in either X or Y could turn it into a field.
Your observation about value params for classes also applies to type params: What a nuisance to pass Foo[A] up the hierarchy. So in Scala, we can have a member type A instead that can remain abstract until defined in a leaf. But this doesn't actually bear on whether to define a trait or a class.
But trait parameters are coming to Scala. (See the Scala bugs for early definitions which are low-priority for this reason.)