pattern matching Java

pattern.java

public abstract class Tree {
  // Constructor private so the type is sealed.
  private Tree() {}

  public abstract <T> T match(Function<Empty, T> a,
                              Function<Leaf, T> b,
                              Function<Node, T> c);

  public static final class Empty extends Tree {
    public <T> T match(Function<Empty, T> a,
                       Funciton<Leaf, T> b,
                       Function<Node, T> c) {
      return a.apply(this);
    }

    public Empty() {}
  }

  public static final class Leaf extends Tree {
    public final int n;

    public <T> T match(Function<Empty, T> a,
                       Function<Leaf, T> b,
                       Function<Node, T> c) {
      return b.apply(this);
    }

    public Leaf(int n) { this.n = n; }
  }

  public static final class Node extends Tree {
    public final Tree left;
    public final Tree right;

    public <T> T match(Function<Empty, T> a,
                       Function<Leaf, T> b,
                       Function<Node, T> c) {
      return c.apply(this);
    }

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