Tree parsing exercise java

Tree.java

package scratch.example;

import java.util.List;

public class Tree {

    int value;
    List<Tree> children;

    public Tree(int value, List<Tree> children) {
        this.value = value;
        this.children = children;
    }
}

TreeFormatter.java

package scratch.example;

import java.util.Optional;

public interface TreeFormatter {
    String format(Tree tree);
    Optional<Tree> parse(String in);
}