classes.md

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Classes in Scala are blueprints for creating objects. They can contain methods, values, variables, types, objects, traits, and classes which are collectively called members. Types, objects, and traits will be covered later in the tour.

Defining a class

A minimal class definition is simply the keyword class and an identifier. Class names should be capitalized.

class User

val user1 = new User

The keyword new is used to create an instance of the class. User has a default constructor which takes no arguments because no constructor was defined. However, you'll often want a constructor and class body. Here is an example class definition for a 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 =
    s"($x, $y)"
}

val point1 = new Point(2, 3)
point1.x  // 2
println(point1)  // prints (2, 3)

This Point class has four members: the variables x and y and the methods move and toString. Unlike many other languages, the primary constructor is in the class signature (var x: Int, var y: Int). The move method takes two integer arguments and returns the Unit value (), which carries no information. This corresponds roughly with void in Java-like languages. toString, on the other hand, does not take any arguments but returns a String value. Since toString overrides toString from AnyRef, it is tagged with the override keyword.

Constructors

Constructors can have optional parameters by providing a default value like so:

class Point(var x: Int = 0, var y: Int = 0)

val origin = new Point  // x and y are both set to 0
val point1 = new Point(1)
println(point1.x)  // prints 1

In this version of the Point class, x and y have the default value 0 so no arguments are required. However, because the constructor reads arguments left to right, if you just wanted to pass in a y value, you would need to name the parameter.

class Point(var x: Int = 0, var y: Int = 0)
val point2 = new Point(y=2)
println(point2.y)  // prints 2

This is also a good practice to enhance clarity.

Private Members and Getter/Setter Syntax

Members are public by default. Use the private access modifier to hide them from outside of the class.

class Point {
  private var _x = 0
  private var _y = 0
  private val bound = 100

  def x = _x
  def x_= (newValue: Int): Unit = {
    if (newValue < bound) _x = newValue else printWarning
  }

  def y = _y
  def y_= (newValue: Int): Unit = {
    if (newValue < bound) _y = newValue else printWarning
  }

  private def printWarning = println("WARNING: Out of bounds")
}

val point1 = new Point
point1.x = 99
point1.y = 101 // prints the warning

In this version of the Point class, the data is stored in private variables _x and _y. There are methods def x and def y for accessing the private data. def x_= and def y_= are for validating and setting the value of _x and _y. Notice the special syntax for the setters: the method has _= appended to the identifier of the getter and the parameters come after.

Primary constructor parameters with val and var are public. However, because vals are immutable, you can't write the following.

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

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

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