Classes

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(xc: Int, yc: Int) {
  var x: Int = xc
  var y: Int = yc
  def move(dx: Int, dy: Int) {
    x = x + dx
    y = y + dy
  }
  override def toString(): String = "(" + x + ", " + y + ")";
}

The class defines two variables x and y and two methods: move and toString. move takes two integer arguments but does not return a value (the implicit 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 has to be tagged with the override flag.

Classes in Scala are parameterized with constructor arguments. The code above defines two constructor arguments, xc and yc; they are both visible in the whole body of the class. In our example they are used to initialize the variables x and y.

Classes are instantiated with the new primitive, as the following example will show:

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)

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