FP Module 2 - 101 Scala

Classes in Scala

Classes in Scala are static templates that can be instantiated into many objects at runtime. Here is a class definition which defines a class Point:

class Point(var x: Int, var y: Int) {
  def move(dx: Int, dy: Int): Unit = {
    x = x + dx
    y = y + dy
  }
  override def toString: String =
    "(" + x + ", " + y + ")"
}

Classes in Scala are parameterized with constructor arguments. The code above defines two constructor arguments, x and y; they are both visible in the whole body of the class.

The class also includes two methods, move and toString. move takes two integer arguments but does not return a value (the return type Unit corresponds to void in Java-like languages). toString, on the other hand, does not take any parameters but returns a String value. Since toString overrides the pre-defined toString method, it is tagged with the override keyword.

Note that in Scala, it isn’t necessary to say return in order to return a value. The value returned from a method is simply the last value in the method body. In the case of the toString method above, the expression after the equals sign is evaluated and returned to the caller.

Classes are instantiated with the new primitive, as follows:

object Classes {
  def main(args: Array[String]) {
    val pt = new Point(1, 2)
    println(pt)
    pt.move(10, 10)
    println(pt)
  }
}

The program defines an executable application Classes in form of a top-level singleton object with a main method. The main method creates a new Point and stores it in value pt. Note that values defined with the val construct are different from variables defined with the var construct (see class Point above) in that they do not allow updates; i.e. the value is constant.

Here is the output of the program:

(1, 2)
(11, 12)

Controlling the Visibility of Constructor Fields

There are several ways to define a class.

Class with private immutable constructor arguments (<=> field without var/val)

class Point(x: Int, y: Int)

The code above will declare a class Point with two private immutable arguments, such as

> val p = Point(2,4)
> p.x
error: value x is not a member of Point

Class with public immutable constructor arguments (<=> val fields)

class Point(val x: Int,val y: Int)

The code above will declare a class Point with two public immutable arguments, such as

> val p = Point(2,4)
> p.x
2
> p.x = 3
error: reassignment to val

Class with public mutable constructor arguments (<=> var fields)

class Point(var x: Int, var y: Int)

The code above will declare a class Point with two public immutable arguments, such as

> val p = Point(2,4)
> p.x
2
> p.x = 3
p.x: Int = 3