Added example program "Rational".

src/main/example/Rational.scala

@@ -0,0 +1,26 @@
+package example
+
+// Primary constructor takes two arguments.
+class Rational(n: Int, d: Int) {
+  // an auxiliary constructor: must call the primary constructor first.
+  def this(n: Int) = this(n, 1)
+  // A requirement: an exception is thrown upon construction if the condition is false.
+  require(d > 0, "denominator must be greater than zero")
+  
+  // A grossly inefficient greatest common divisor, for illustrative purposes only.
+  private def gcd(n: Int, d: Int) = (
+    n max d to 2 by -1 find (g => n % g == 0 && d % g == 0) getOrElse 1
+  )
+  // Using the gcd to calculate reduced numerator and denominator.
+  private val g = gcd(n, d)
+  // Public, immutable values.
+  val numerator   = n / g
+  val denominator = d / g
+  
+  // toString overrides a method in AnyRef so "override" is required.
+  override def toString = n + "/" + d + (
+    // The result of the if/else is a String:
+    if (numerator == n) ""                          // the empty string if it is irreducible
+    else " (" + numerator + "/" + denominator + ")" // the reduced form otherwise
+  )
+}