syntactic-sugars

apply , unapply and update

Create instance without new

In scala, we can create an instance of a class without using keyword new. When we do this, scala does something behind the scenes. It creates a companion object of the class and creates an apply method which returns the instance of the class.

class A(var value: Int)
 
val a = A(1)
 
// companion object
 
object A {
    def apply(value: Int): A = new A(value)
}

Calling the instance

class A(var value: Int):
    def apply: Int = value
 
val a = A(1)
 
println(a())  // prints 1

Defining apply in the class itself makes the instance callable like a method.

update

class A(var value: Int):
    def update(newVal: Int): Unit = value = newVal
 
 
val a = A(1)
 
a() = 2
 
a.value // 2

It is a syntactic sugar for updating a value in an instance of a class. Scala compiler converts a() = 2 to a.update(2).