In the Scala documentation at https://docs.scala-lang.org/tour/classes.html, the following example shows a minimal class definition that includes parameters:

class Point(var x: Int, var y: Int) {
    ...
}

Both of the following code snippets work correctly for me:

• Snippet 1: class Train(number: Int)
• Snippet 2: class Train(val number: Int)

What would be the benefit of using val in snippet 2? From what I understand, by default val is implicit. Would code readability by being explicit to say that number is a val and not a var be the only benefit? Thank you.

CodePudding user response：

From the documentation you linked:

Primary constructor parameters with val and var are public.

E.g. (as in the example):

class Point(val x: Int, val y: Int)
val point = new Point(1, 2)
point.x = 3  // <-- does not compile (because vals are immutable)

In this definition you can access x and y, as they are public vals

point.x // 1
point.y // 2

The next paragraph explains:

Parameters without val or var are private values, visible only within the class.

E.g. (as in the example):

class Point(x: Int, y: Int)
val point = new Point(1, 2)
point.x  // <-- does not compile

In this definition, x and y are private vals

Thus your snippet 1 has a private val called number And your snippet 2 has a public val called number

(Just to note) A small caveat to this is case classes, where all arguments of the constructor are public, regardless of the val or var keyword. If omitted, the variable/value will be public.

CodePudding user response：

In Snippet 1, number is a construction parameter and is not a member of the class. It can only be used inside the constructor (the body of the class).

In Snippet 2, number is a public read-only member of the class. It can be used by any code that has an instance of the class.

From what I understand, by default val is implicit.

This is not true for class parameters but is true for case class parameters, which can cause confusion when learning Scala.