How does the order of instructions works inside Scala classes when you override a method

user_11187781 0
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I am having my first experience with Scala, experimenting with basic concepts. I made a very simple class, doing shenanigans with overriding toString for printing the class instance upon creation:

class Person(var firstName: String, var lastName: String) {
  println(this)
  override def toString: String = "Overriden toString"
  println(this)
}
val p = new Person("John", "Smith")

I wondered what would happen if I run this code. First println before overriding toString is supposed to be

PlatformName$Person@address

if I understand it correctly, which is what you supposed to get when printing a class instance without overriding toString. But instead, both prints on construction printed

Overriden toString.

Why it is so? Does override takes the first priority over any other code, ignoring the order, or how does it work?

overriding compilation scala class
2 Answers

When you're defining such a class, you're basically writing the same thing as:

class Person {

  var firstName: String
  var lastName: String

  // Constructor
  def this(firstName: String, lastName: String) = {
    // Regular constructor member definition
    this.firstName = firstName
    this.lastName = lastName
    // The code you wrote in the class body
    println(this)
    println(this)
  }
  
  override def toString: String = "Overriden toString"
}

The code written in the class that is not part of a def will be executed when an instance of the class is created (in the constructor at run-time).

The def of the class are all defined (at compile-time) before you instantiate a class, no matter the order or the override.

user_53891270

There are 2 points of view (from my opinion): 1st is that toString method is evaluated in compile time, but println is when object is created and is done at runtime. 2nd is that when you want to print something, you are implicitly calling .toString() method (not only in Scala, same in Java and ...), so the method needs to be evaluated first, and when you call it (implicitly) you happen to see overriden string whenever you println that object.

user_146723830
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