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scalasingletoncompanion-object
Vinod Chandak
Vinod Chandak

Reputation: 363

Scala companion objects are not singleton

I have following two classes.

class A (name: String) {
}

object A {
}

According to definition of Singleton, we can have only one object of that type. However I am able to create two different objects of type A using following piece of code.

object B {
   def main(args: Array[String]): Unit = {
     val a = new A("Vinod")
     println(a)

     val b = new A("XYZ")
     println(b)
  }
}

can someone please explain me, where my understanding is not correct?

Upvotes: 2

Views: 799

Answers (4)

francoisr
francoisr

Reputation: 4595

An object by itself is a singleton. It has its own class and no other instance of the same class exist at runtime.

However, the pattern you describe here is different: object A is not an instance of class A unless you make it so using object A extends A. You could make it the only instance of class A by making class A a sealed class, but this is unnecessary in almost all cases.

If you really want the singleton pattern, drop the class and use only object A, all of its members will be "static" in the sense of Java.

Note that the actual type of object A can be referred to as A.type, which by default is completely unrelated to type A if class A exists. Again, A.type could be a subtype of A if you explicitly make it so.

Upvotes: 4

jwvh
jwvh

Reputation: 51271

The companion object is not an instance of the companion class. They're not even the same type.

class A
object A {
  var state = 0
  def update() :Unit = state = state + 1
}

val abc :A = new A   //instance of class A
val xyz :A.type = A  //2nd reference to object A

// both reference the same singleton object
xyz.update()  //res0: Unit = ()
A.state       //res1: Int = 1
abc.state     //Error: value state is not a member of A$A2521.this.A

Upvotes: 4

Alexey Romanov
Alexey Romanov

Reputation: 170919

new A refers to class A (which is not a singleton), not to object A. You can easily check it: if you remove class A, the new A lines will no longer compile.

Also note that objects aren't necessarily singletons: they can be nested inside classes or traits, in this case there is one for each instance of the outer type.

Upvotes: 0

Dominic Egger
Dominic Egger

Reputation: 1016

the companion object can be thought of as the static space of a class. if you want to make A a singleton you can make it an object rather than a class

Upvotes: 0

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