How to count the number of class instances

Question
1

I tried the following code to count the number of class instances. But it returns 1 instead of the correct value 3.


package com.alain

class MyClass(var x:Int) {

}

object MyClass {
  var counter : Int = 0
  counter = counter +1
}


object Main {
  def main(args: Array[String]): Unit =
    println("Start")
    var c:MyClass = MyClass(1)
    var d:MyClass = MyClass(2)
    var e:MyClass = MyClass(3)
    println(MyClass.counter)
    println("Stop")

}
2

I found the solution :


package com.alain

object MyClass {
  var counter: Int = 0
}

class MyClass(var x: Int) {
  MyClass.counter = MyClass.counter + 1

}


object Main {
    var c: MyClass = MyClass(1)
    var d: MyClass = MyClass(2)
    var e: MyClass = MyClass(3)

  def main(args: Array[String]): Unit = println("Start")
    println(MyClass.counter)
    println("Stop")

}
3 Likes
3

Also make counter’s type to be AtomicInteger to prevent race conditions in case object creations are across threads.

1 Like