Basic Scala: Methods holding parameters?

I am reading the book Programming in Scala and in chapter 10 I had to write:

abstract class Element {
  def contents: Array[String]
  def height: Int = contents.length
  def width: Int = if (height == 0) 0 else contents(0).length
}

class ArrayElement(conts: Array[String]) extends Element {
  def contents: Array[String] = conts
}

but the concept I don't catch here is how can I define a method that is holding a variable? As far as I know, methods return a value, it can be a computed value or an instance variable directly, but they can't hold a value. Am I forgetting a basic concept about the programming language that applies here too?

Answers


Try this out:

abstract class Foo { def bar: Int }
class Baz(val bar: Int) extends Foo

In Scala, you can implement methods by creating member variables with same name and same type. The compiler then adds a corresponding getter-method automatically.

The member variable and the getter-method are still two different things, but you don't see much difference syntactically: when you try to access it, its both just foo.bar, regardless of whether bar is a method or a member variable.

In your case

def contents: Array[String] = conts

is just an ordinary method that returns an array, an equivalent way to write the same thing would be

def contents: Array[String] = {
  return conts
}

Since the array is mutable, you can in principle use this method to modify entries of your array, but the method itself is really just a normal method, it doesn't "hold" anything, it just returns reference to your array.

Edit: I've just noticed that conts is actually not a member variable. However, its still captured in the definition of the contents method by the closure-mechanism.


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