Scala: How to modify some variable in class by method? [duplicate]

Question

Possible Duplicate:
Use of def, val, and var in scala

I'm learning now Scala and I can't modify variable in class.

class Person(name: String, var variable: Int) {
  def change() {
    variable = 42
  }
}
def person = new Person("name", 0)
println(person.variable)
person.change()
println(person.variable)

And the output is:

0
0

Why output contains 2 times 0?

How I can modify variable in this Code?

I have Scala version 2.9.1.

Answer 1

This took me a while to realize as well :)

You are not defining a val or var "person", but creating a new method person. Therefore, every time you you invoke person, it will create a new instance of Person, which will always have its variable set to 0. This is a side effect of the fact that in Scala you don't need the () to invoke parameterless methods.

def person = new Person("foo", 1)

is roughly equivalent to (in Java code)

public Person person() {
    return new Person("foo", 1);
}

whereas val person is what you want, i.e.

Person person = new Person("foo", 1)

(of course that is not a real val because Scala does not support those :))

and when you then use person, what Scala understands is person().

Just change def, which is used for defining methods, to val or var and you'll be fine :)

Answer 2

Wasn't easy to spot! Here:

def person = new Person("name", 0)

you define a method named person. To make it more clear, you do something like this:

def person(): Person = {
  return new Person("name", 0)
}

That means every time you say person you are not referring to a variable but calling a method and creating new instance over and over again.

Just say val person =... and you'll be fine.

Believe it or not, that's actually a feature of the language. E.g. you can create a field (val) in your class, then change your mind and turn it into a getter by simply changing to def. From the client code perspective nothing changes.