In my study of Scala I ran into the following snippet:
val r = Random
Now from my understanding from Alvin Alexander's excellent post (How to create Scala object instances without using the “new” keyword (apply) you can create a new Scala object instance without using the new keyword as follows:
Create a companion object for your class, and define an apply method
in the companion object with the desired constructor signature.
Define your class as a “case class.”
However, when I look at the source code for the Random class (Scala 2.12.8) I see that it is not a case class nor does the companion object have the apply method in its companion object.
Could someone please explain to me why it's valid syntax to create a Random as posted above even though it doesn't meet the requirements as described in Alvin Alexander's post?
Thanks!
In this context Random is a singleton instance of class Random
object Random extends Random
where
class Random(val self: java.util.Random) extends ... {
...
def this() = this(new java.util.Random())
...
}
hence it is already a value. For example consider the following
class Foo(val v: Int)
object Foo extends Foo(42)
Foo.v
which outputs
res0: Int = 42
Note how Foo in Foo.v is already referencing an instance singleton value of class Foo. Hence r in
val r = Random
is simply referencing existing singleton value. It is akin to
val a = 11
val r = a
Simply put: you are not creating a new instance of the Random class, you are assigning the existing instance of the Random class which also happens to be called Random to the field r.
It might sound confusing that there is both a class and an object named Random but there actually cannot possibly be any confusion, since in Scala, types and terms live in different "worlds" and their names can never mix.
Note that it is in fact quite common to have a class or trait and an object with the same name, for the following reason:
if an object has the same name as a class or trait, and
that object is defined in the same scope as the class or trait with the same name, and
that object is defined in the same compilation unit as the class or trait with the same name, then
that object is called a companion module and has privileged access to the internals of the class or trait.
Loading a ficus configuration like
loadConfiguration[T <: Product](): T = {
import net.ceedubs.ficus.readers.ArbitraryTypeReader._
import net.ceedubs.ficus.Ficus._
val config: Config = ConfigFactory.load()
config.as[T]
fails with:
Cannot generate a config value reader for type T, because it has no apply method in a companion object that returns type T, and it doesn't have a primary constructor
when instead directly specifying a case class instead of T i.e. SomeClass it works just fine. What am I missing here?
Ficus uses the type class pattern, which allows you to constrain generic types by specifying operations that must be available for them. Ficus also provides type class instance "derivation", which in this case is powered by a macro that can inspect the structure of a specific case class-like type and automatically create a type class instance.
The problem in this case is that T isn't a specific case class-like type—it's any old type that extends Product, which could be something nice like this:
case class EasyToDecode(a: String, b: String, c: String)
But it could also be:
trait X extends Product {
val youWillNeverDecodeMe: String
}
The macro you've imported from ArbitraryTypeReader has no idea at this point, since T is generic here. So you'll need a different approach.
The relevant type class here is ValueReader, and you could minimally change your code to something like the following to make sure T has a ValueReader instance (note that the T: ValueReader syntax here is what's called a "context bound"):
import net.ceedubs.ficus.Ficus._
import net.ceedubs.ficus.readers.ValueReader
import com.typesafe.config.{ Config, ConfigFactory }
def loadConfiguration[T: ValueReader]: T = {
val config: Config = ConfigFactory.load()
config.as[T]
}
This specifies that T must have a ValueReader instance (which allows us to use .as[T]) but says nothing else about T, or about where its ValueReader instance needs to come from.
The person calling this method with a concrete type MyType then has several options. Ficus provides instances that are automatically available everywhere for many standard library types, so if MyType is e.g. Int, they're all set:
scala> ValueReader[Int]
res0: net.ceedubs.ficus.readers.ValueReader[Int] = net.ceedubs.ficus.readers.AnyValReaders$$anon$2#6fb00268
If MyType is a custom type, then either they can manually define their own ValueReader[MyType] instance, or they can import one that someone else has defined, or they can use generic derivation (which is what ArbitraryTypeReader does).
The key point here is that the type class pattern allows you as the author of a generic method to specify the operations you need, without saying anything about how those operations will be defined for a concrete type. You just write T: ValueReader, and your caller imports ArbitraryTypeReader as needed.
I am trying to define an instance of Show (from cats 0.9) that can be used for all members of an ADT as follows:
import $ivy.`org.typelevel::cats:0.9.0`, cats.Show
sealed abstract class Colour(val name: String)
implicit val ColourShow = new Show[Colour] {
def show(c: Colour) = c.name
}
object Colour {
object Red extends Colour("Red")
object Blue extends Colour("Blue")
}
import Show._
println(Colour.Red.show)
An applicable instance cannot be found for Red, however:
Compiling /Users/Rich/Projects/worksheets/fp-patterns/Colours.sc
/Users/Rich/Projects/worksheets/fp-patterns/Colours.sc:16: value show is not a member of object ammonite.$file.Colours.Colour.Red
val res_5 = println(Colour.Red.show)
^
Compilation Failed
Is it possible to use typeclasses in this way? I am trying to avoid having to define a separate instance for each concrete instanct of Colour.
I think you're mistaking what's happening here. The implicit you've defined does actually work for the instances.
eg.
ColourShow.show(Colour.Red)
If you want to be able to call show() on an instance of a Colour, without any arguments, you'll need to provide a definition of a trait that has a method show, which takes no arguments, and an implicit conversion from Colour to that trait.
Additionally to what people have pointed out, you'll need to import import cats.implicits._
see a working example in: https://scastie.scala-lang.org/d1egoaz/LVaJEccDSeas9VmzHqf1ug/1
You can also use the shorter version to create a Show instance for Colour:
implicit val colourShow: Show[Colour] = Show.show[Colour](_.name)
I have created a companion object for my Scala class with an apply method in it so that I can create an instance of my class without using 'new'.
object StanfordTokenizer{
def apply() = new StanfordTokenizer()
}
class StanfordTokenizer() extends Tokenizer{
def tokenizeFile(docFile: java.io.File) = new PTBTokenizer(new FileReader(docFile), new CoreLabelTokenFactory(), "").tokenize.map(x => x.word().toLowerCase).toList
def tokenizeString(str: String) = new PTBTokenizer(new StringReader(str), new CoreLabelTokenFactory(), "").tokenize.map(x => x.word.toLowerCase()).toList
}
However when I try to instantiate the StanfordTokenizer class without 'new' e.g. StandfordTokenizer.tokenizeString(str).
I get the error
value tokenizeString is not a member of object StanfordTokenizer
However, if I explicitly include the apply method like StandfordTokenizer.apply().tokenizeString(str) it does work.
I feel like I am missing something fundamental about companion objects. Can someone shed some light on this for me?
^
It's exactly as the compiler message says. tokenizeString is a member of the class StandfordTokenizer, but not its companion object. The companion object does not inherit any methods from the class. Therefore, in order to use tokenizeString, you need an instance of StandfordTokenizer in order to call it.
StandfordTokenizer.apply creates an instance of the class StandfordTokenizer, which has the method tokenizeString. It seems as though the class StandfordTokenizer holds no real information, and won't have more than one instance. If that is true, you should probably just make it an object, and you'll be able to acquire the behavior you're looking for.
object StanfordTokenizer extends Tokenizer {
def tokenizeFile(docFile: java.io.File) = ...
def tokenizeString(str: String) = ...
}
This should work as well (as a class):
StandfordTokenizer().tokenizeString(str)
StandfordTokenizer without parenthesis does not call apply, it references the object. StandfordTokenizer() does call apply, and creates a new instance of the class. This is probably the source of your confusion.
I'm just going over some Scala tutorials on the Internet and have noticed in some examples an object is declared at the start of the example.
What is the difference between class and object in Scala?
tl;dr
class C defines a class, just as in Java or C++.
object O creates a singleton object O as instance of some anonymous class; it can be used to hold static members that are not associated with instances of some class.
object O extends T makes the object O an instance of trait T; you can then pass O anywhere, a T is expected.
if there is a class C, then object C is the companion object of class C; note that the companion object is not automatically an instance of C.
Also see Scala documentation for object and class.
object as host of static members
Most often, you need an object to hold methods and values/variables that shall be available without having to first instantiate an instance of some class.
This use is closely related to static members in Java.
object A {
def twice(i: Int): Int = 2*i
}
You can then call above method using A.twice(2).
If twice were a member of some class A, then you would need to make an instance first:
class A() {
def twice(i: Int): Int = 2 * i
}
val a = new A()
a.twice(2)
You can see how redundant this is, as twice does not require any instance-specific data.
object as a special named instance
You can also use the object itself as some special instance of a class or trait.
When you do this, your object needs to extend some trait in order to become an instance of a subclass of it.
Consider the following code:
object A extends B with C {
...
}
This declaration first declares an anonymous (inaccessible) class that extends both B and C, and instantiates a single instance of this class named A.
This means A can be passed to functions expecting objects of type B or C, or B with C.
Additional Features of object
There also exist some special features of objects in Scala.
I recommend to read the official documentation.
def apply(...) enables the usual method name-less syntax of A(...)
def unapply(...) allows to create custom pattern matching extractors
if accompanying a class of the same name, the object assumes a special role when resolving implicit parameters
A class is a definition, a description. It defines a type in terms of methods and composition of other types.
An object is a singleton -- an instance of a class which is guaranteed to be unique. For every object in the code, an anonymous class is created, which inherits from whatever classes you declared object to implement. This class cannot be seen from Scala source code -- though you can get at it through reflection.
There is a relationship between object and class. An object is said to be the companion-object of a class if they share the same name. When this happens, each has access to methods of private visibility in the other. These methods are not automatically imported, though. You either have to import them explicitly, or prefix them with the class/object name.
For example:
class X {
// class X can see private members of object X
// Prefix to call
def m(x: Int) = X.f(x)
// Import and use
import X._
def n(x: Int) = f(x)
private def o = 2
}
object X {
private def f(x: Int) = x * x
// object X can see private members of class X
def g(x: X) = {
import x._
x.o * o // fully specified and imported
}
}
An object has exactly one instance (you can not call new MyObject). You can have multiple instances of a class.
Object serves the same (and some additional) purposes as the static methods and fields in Java.
As has been explained by many, object defines a singleton instance. The one thing in the answers here that I believe is left out is that object serves several purposes.
It can be the companion object to a class/trait, containing what might be considered static methods or convenience methods.
It can act much like a module, containing related/subsidiary types and definitions, etc.
It can implement an interface by extending a class or one or more traits.
It can represent a case of a sealed trait that contains no data. In this respect, it's often considered more correct than a case class with no parameters. The special case of a sealed trait with only case object implementors is more or less the Scala version of an enum.
It can act as evidence for implicit-driven logic.
It introduces a singleton type.
It's a very powerful and general construct. What can be very confusing to Scala beginners is that the same construct can have vastly different uses. And an object can serve many of these different uses all at once, which can be even more confusing.
Defining an object in Scala is like defining a class in Java that has only static methods. However, in Scala an object can extend another superclass, implement interfaces, and be passed around as though it were an instance of a class. (So it's like the static methods on a class but better).
The formal difference -
you can not provide constructor parameters for Objects
Object is not a type - you may not create an instance with new operator. But it can have fields, methods, extend a superclass and mix in traits.
The difference in usage:
Scala doesn't have static methods or fields. Instead you should use object. You can use it with or without related class. In 1st case it's called a companion object. You have to:
use the same name for both class and object
put them in the same source file.
To create a program you should use main method in object, not in class.
object Hello {
def main(args: Array[String]) {
println("Hello, World!")
}
}
You also may use it as you use singleton object in java.
In scala, there is no static concept. So scala creates a singleton object to provide entry point for your program execution.
If you don't create singleton object, your code will compile successfully but will not produce any output. Methods declared inside Singleton Object are accessible globally. A singleton object can extend classes and traits.
Scala Singleton Object Example
object Singleton{
def main(args:Array[String]){
SingletonObject.hello() // No need to create object.
}
}
object SingletonObject{
def hello(){
println("Hello, This is Singleton Object")
}
}
Output:
Hello, This is Singleton Object
In scala, when you have a class with same name as singleton object, it is called companion class and the singleton object is called companion object.
The companion class and its companion object both must be defined in the same source file.
Scala Companion Object Example
class ComapanionClass{
def hello(){
println("Hello, this is Companion Class.")
}
}
object CompanoinObject{
def main(args:Array[String]){
new ComapanionClass().hello()
println("And this is Companion Object.")
}
}
Output:
Hello, this is Companion Class.
And this is Companion Object.
In scala, a class can contain:
1. Data member
2. Member method
3. Constructor Block
4. Nested class
5. Super class information etc.
You must initialize all instance variables in the class. There is no default scope. If you don't specify access scope, it is public. There must be an object in which main method is defined. It provides starting point for your program. Here, we have created an example of class.
Scala Sample Example of Class
class Student{
var id:Int = 0; // All fields must be initialized
var name:String = null;
}
object MainObject{
def main(args:Array[String]){
var s = new Student() // Creating an object
println(s.id+" "+s.name);
}
}
I am sorry, I am too late but I hope it will help you.
The object keyword creates a new singleton type, which is like a class that only has a single named instance. If you’re familiar with Java, declaring an object in Scala is a lot like creating a new instance of an anonymous class.
Scala has no equivalent to Java’s static keyword, and an object is often used in Scala where you might use a class with static members in Java.
Object is a class but it already has(is) an instance, so you can not call new ObjectName. On the other hand, Class is just type and it can be an instance by calling new ClassName().
A class is just like any other class in other languages. You define class just like any other language with some syntax difference.
class Person(val name: String)
val me = new Person("My name")
However, object is a class with single object only. This makes it interesting as it can be used to create static members of a class using companion object. This companion object has access to private members of the class definition and it has the same name as the class you're defining.
class Person(var name: String) {
import Person._
def hi(): String = sayHello(name)
}
object Person {
private def sayHello(name: String): String = "Hello " + name
}
val me = new Person("My name")
me.hi()
Also, noteworthy point is that object class is lazily created which is another important point. So, these are not instantiated unless they are needed in our code.
If you're defining connection creation for JDBC, you can create them inside object to avoid duplication just like we do in Java with singleton objects.
Scala class same as Java Class but scala not gives you any entry method in class, like main method in java. The main method associated with object keyword. You can think of the object keyword as creating a singleton object of a class that is defined implicitly.
more information check this article
class and object keyword in scala programming
The object is similar to the static class in Java to some extend, the static characteristic means the static class need not to create an object when putting to the JVM, it can be used by it's class name directly and the same instance(same data state) is shared wherever it is used.
If you are coming from java background the concept of class in scala is kind of similar to Java, but class in scala cant contain static members.
Objects in scala are singleton type you call methods inside it using object name, in scala object is a keyword and in java object is a instance of class