Mixin Class Composition

As opposed to languages that only support single inheritance, Scala has a more general notion of class reuse. Scala makes it possible to reuse the new member definitions of a class (i.e. the delta in relationship to the superclass) in the definition of a new class. This is expressed as a mixin-class composition. Consider the following abstraction for iterators.

abstract class AbsIterator {
  type T
  def hasNext: Boolean
  def next: T

Next, consider a mixin class which extends AbsIterator with a method foreach which applies a given function to every element returned by the iterator. To define a class that can be used as a mixin we use the keyword trait.

trait RichIterator extends AbsIterator {
  def foreach(f: T => Unit) { while (hasNext) f(next) }

Here is a concrete iterator class, which returns successive characters of a given string:

class StringIterator(s: String) extends AbsIterator {
  type T = Char
  private var i = 0
  def hasNext = i < s.length()
  def next = { val ch = s charAt i; i += 1; ch }

We would like to combine the functionality of StringIterator and RichIterator into a single class. With single inheritance and interfaces alone this is impossible, as both classes contain member impementations with code. Scala comes to help with its mixin-class composition. It allows the programmers to reuse the delta of a class definition, i.e., all new definitions that are not inherited. This mechanism makes it possible to combine StringIterator with RichIterator, as is done in the following test program which prints a column of all the characters of a given string.

object StringIteratorTest {
  def main(args: Array[String]) {
    class Iter extends StringIterator(args(0)) with RichIterator
    val iter = new Iter
    iter foreach println

The Iter class in function main is constructed from a mixin composition of the parents StringIterator and RichIterator with the keyword with. The first parent is called the superclass of Iter, whereas the second (and every other, if present) parent is called a mixin.

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