CODEWITHSUNDEEP
classscalaimmutability
Marco Prins
Marco Prins

Reputation: 7419

Declaring an instance of a class with val

In Scala, the val keyword declares an immutable variable. So if you declare val x = 3, it stays 3. But if you use var, it can change.

But what happens if you declare an instance of a class with val? For example, take this class Point

   class Point(_x: Int, _y: Int) {
       var x = _x
       var y = _y

       def doubleValues = {
           x *= 2
           y *= 2
       }
   }

First, I declare var location = new Point(2,2) Then run

  println(location.x)
  println(location.y)

  location.doubleValues

  println(location.x)
  println(location.y)

The output, as expected, is

2
2
4
4

But if I rerun and use the val keyword to declare location, the result is the same. So even though location was declared immutable, the mutable variables inside it are still mutable.

So then what does actually happen when you declare an instance of a class with val ? What exactly is the difference between using var and val?

Upvotes: 1

Views: 114

Answers (2)

lmm
lmm

Reputation: 17431

With a var you can change which Point it is; with a val you can't:

var x = new Point(2, 2)
x = new Point(3, 2)
val y = new Point(2, 2)
y = new Point(3, 2) // won't compile

Conversely if I make an object containing a val, I can't change that value "inside" the object, even though I can change the object itself:

class Q(i: Int) {val x: Int = i}
var q = new Q(4)
q = new Q(6)
q.x = 2 // won't compile

To make an object truly immutable you need to use val "all the way down". A common pattern is to have all your "inner" objects immutable, and then have some "top-level" object which is mutable, either explicitly (with var) or by using something like the State monad. There's relatively little value in having an "immutable" object with mutable members.

Upvotes: 3

user1804599
user1804599

Reputation:

The pointer to the object is immutable, not the object itself.

Upvotes: 0

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