How to copy an object in Scala?

Question

I have this tree structure

sealed trait Tree
case class Node(var left: Tree, var right: Tree, var value: String) extends Tree
case object EmptyNode extends Tree

and a Tree object called myTree. I want to make another tree with the exact same structure and values called otherTree, but myTree.clone() does not work. What else can I do?

Answer 1

Something like this will copy your Tree:

  def copyTree(t:Tree):Tree = {
    t match {
      case EmptyNode => EmptyNode
      case Node(left, right, value) => Node(copyTree(left), copyTree(right), value)
    }
  }

Answer 2

Just for completeness - as the question asks for 'How to copy...' and you mentioned clone(): You can copy case classes with the copy() method. Yet it won't help with a tree structure that is mutable. But with an immutable structure (as Gregor suggests above), e.g. like this

sealed trait Tree
case class Node(left: Tree, right: Tree, value: String) extends Tree
case object EmptyNode extends Tree { override def toString = " _ " }

and some data

scala> val leaf1 = Node(EmptyNode, EmptyNode, "leaf1")
scala> val leaf2 = Node(EmptyNode, EmptyNode, "leaf2")
scala> val tree1 = Node(leaf1, leaf2, "tree1")
scala> tree1
res0: Node = Node(Node( _ , _ ,leaf1),Node( _ , _ ,leaf2),tree1)

you can copy your tree like this

scala> val tree2 = tree1.copy(value = "tree2")
tree2: Node = Node(Node( _ , _ ,leaf1),Node( _ , _ ,leaf2),tree2)

or

scala> val tree3 = Node(tree1.left, tree1.right, "tree3")
tree3: Node = Node(Node( _ , _ ,leaf1),Node( _ , _ ,leaf2),tree3)

As usual with immutable instances, for updates you would need to create new tree structures for example using recursive operations as in Noah's answer.