Sum: a Pattern Matcher example

How to Specify an algorithm on a predefined recursive type without change any classes that defined this type.

interface Tree {}

class Leaf implements Tree {
  Leaf(int value) { 
    this.value=value; 
  }
  int value; 
}

class Node implements Tree {
  Node(Tree left,Tree right) { 
    this.left=left; 
    this.right=right; 
  }
  Tree left,right;
}
  

public class Main extends PatternMatcher
{
  public int sum(Node node)
  { return sum(node.left)+sum(node.right); }
  public int sum(Leaf leaf)
  { return leaf.value; }

  public int sum(Tree tree)
  { return ((Integer)match("sum",tree)).intValue(); }

  public static void main(String[] args) {
    Tree tree=new Node(new Node(new Leaf(1),new Leaf(3)),new Leaf(7));
    System.out.println("sum "+new Main().sum(tree));
  }
}