TwentQuestions that reads questions and guesses from file

TwentyQuestions.java

import java.util.*;
import java.io.*;
import java.nio.file.*;

/**
 * Twenty questions that parses from a file that looks like:
 *
 *  Is it a mammal? {
 *    Does it live in the water? {
 *      Does it get caught in tuna nets? {
 *        a dolphin;
 *        a whale;
 *      }
 *      Is it a pet? {
 *        a dog;
 *        an elephant;
 *      }
 *    }
 *    Does it live in the water? {
 *      Does it have claws? {
 *        a lobster;
 *        a fish;
 *      }
 *      Can it lick it's own eyeball? {
 *        a gecko;
 *        a snake;
 *      }
 *    }
 *  }
 *
 */
public class TwentyQuestions {

  private static final Scanner scanner = new Scanner(System.in);

  /*
   * Common interface for nodes and leaves of our tree of questions.
   */
  private static interface Tree {
    public void play();
  }

  /*
   * A yes/no question that leads to one of two paths deeper in the tree.
   */
  private static Tree question(String question, Tree ifYes, Tree ifNo) {
    return () -> {
      System.out.print(question + " ");
      if (scanner.nextLine().startsWith("y")) {
        ifYes.play();
      } else {
        ifNo.play();
      }
    };
  }

  /*
   * A final guess.
   */
  private static Tree guess(String guess) {
    return () -> {
      System.out.print("I guess " + guess + ". Is that right? ");
      if (scanner.nextLine().startsWith("y")) {
        System.out.println("Yay! I win.");
      } else {
        System.out.println("Darn! I was sure that was it.");
      }
    };
  }

  /*
   * Read a tree of questions from a given file.
   */
  private static Tree read(Path file) throws IOException {
    var i = Files.lines(file).map(String::trim).iterator();
    var tree = parse(i);
    if (!i.hasNext()) {
      return tree;
    } else {
      throw new RuntimeException("Syntax error. Extra lines");
    }
  }

  /*
   * Recursive parser that consumes an iterator of the lines in the file.
   */
  private static Tree parse(Iterator<String> lines) {
    if (lines.hasNext()){
      var line = lines.next();
      if (line.endsWith(" {")) {
        var q = upTo(line, " {");
        var y = parse(lines);
        var n = parse(lines);
        if (lines.hasNext() && lines.next().endsWith("}")) {
          return question(q, y, n);
        } else {
          throw new RuntimeException("Syntax error in queston '" + q + "'. Expected }");
        }
      } else if (line.endsWith(";")) {
        return guess(upTo(line, ";"));
      } else {
        throw new RuntimeException("Syntax error. Malformed line: '" + line + "'. Expected ;");
      }
    } else {
      throw new RuntimeException("Syntax error. Ran out of lines");
    }
  }

  /*
   * Helper method to extract text up to ending puncuatin.
   */
  private static String upTo(String s, String end) {
    return s.substring(0, s.indexOf(end));
  }

  /*
   * Run game based on contents of file named in the first argument.
   */
  public static void main(String[] args) throws IOException {
    read(Path.of(args[0])).play();
  }
}