Sealed classes

A sealed class in Kotlin is an abstract class, which can be extended by subclasses defined as nested classes within the sealed class itself. In a way, this is a rather more powerful enumeration option. Just like Enum, a sealed class hierarchy contains a fixed set of possible choices. However, unlike Enum, where each option is represented by one instance, the derived classes of a sealed class can have many instances. Sealed classes are ideal for defining algebraic data types. Imagine you want to model a binary tree structure; you would do the following:

    sealed class IntBinaryTree {
      class EmptyNode : IntBinaryTree()
      class IntBinaryTreeNode(val left: IntBinaryTree, val value: Int, val right: IntBinaryTree) : IntBinaryTree()
    }

    …

    val tree = IntBinaryTree.IntBinaryTreeNode(
    IntBinaryTree.IntBinaryTreeNode(
      IntBinaryTree.EmptyNode(),
      1,
      IntBinaryTree.EmptyNode()),
     ...