Swap order of class implementation (#105)

define `NonEmpty` class first, so it can be used in `Empty` class

Co-authored-by: James Santucci <[email protected]>

Author: xiaowei-zhang

src/main/scala/scalatutorial/sections/ObjectOrientedProgramming.scala

@@ -457,11 +457,6 @@ object ObjectOrientedProgramming extends ScalaTutorialSection {
    * Here are their implementations:
    *
    * {{{
-   *   class Empty extends IntSet {
-   *     def contains(x: Int): Boolean = false
-   *     def incl(x: Int): IntSet = new NonEmpty(x, new Empty, new Empty)
-   *   }
-   *
    *   class NonEmpty(elem: Int, left: IntSet, right: IntSet) extends IntSet {
    *
    *     def contains(x: Int): Boolean =
@@ -474,6 +469,11 @@ object ObjectOrientedProgramming extends ScalaTutorialSection {
    *       else if (x > elem) new NonEmpty(elem, left, right incl x)
    *       else this
    *   }
+   *
+   *   class Empty extends IntSet {
+   *     def contains(x: Int): Boolean = false
+   *     def incl(x: Int): IntSet = new NonEmpty(x, new Empty, new Empty)
+   *   }
    * }}}
    *
    * `Empty` and `NonEmpty` both ''extend'' the class `IntSet`.