A version of Twenty Questions (Four Questions, actually)

TwentyQuestions.java

import java.util.Scanner;

public class TwentyQuestions  {

  public static final String[] GUESSES = {
    "a hamster",
    "a ferret",
    "a dog",
    "a cat",
    "an elephant",
    "a monkey",
    "a dolphin",
    "a whale",
    "an octopus",
    "a shark",
    "a crocodile",
    "a piranha",
    "a tortise",
    "a turtle",
    "a gecko",
    "a iguana",
  };

  public static final String[] QUESTIONS = {
    "Is it a mammal?",
    "Do people keep it as a pet?",
    "Does it usually live in a cage?",
    "Does it run in a wheel?",
    "Does it like to fetch a ball?",
    "Does it live on land?",
    "Does it have a trunk?",
    "Does it get caught in tuna nets?",
    "Does it live in water?",
    "Does it live in the sea?",
    "Does it have eight arms?",
    "Does it look like a submerged log?",
    "Does it have a hard shell?",
    "Are the Galapagos islands famous for these?",
    "Can it lick it's own eyeballs?",
  };

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

  private boolean ask(String question) {
    System.out.print(question + " ");
    return scanner.nextLine().startsWith("y");
  }

  private void makeFinalGuess(String guess) {
    System.out.print("\nOkay, time to guess. ");
    if (ask("Is it " + guess + "?")) {
      System.out.println("I win!");
    } else {
      System.out.println("Dang it; I was so sure that was it.");
    }
    System.out.println();
  }

  public void play(String topic, String[] questions, String[] guesses) {

    double levels = Math.log(guesses.length) / Math.log(2);
    assert levels % 1 == 0;

    System.out.println("\n\nLet's play " + (int) levels + " questions. Think of " + topic + " and I'll try to guess it.\n");

    int questionIndex = 0;
    int guessIndex = 0;

    for (int i = 0; i < levels; i++) {
      guessIndex <<= 1;
      if (ask(questions[questionIndex])) {
        questionIndex += 1;
      } else {
        questionIndex += (1 << (int)(levels - (i + 1)));
        guessIndex++;
      }
    }
    makeFinalGuess(guesses[guessIndex]);
  }

  public static void main(String[] args)  {
    new TwentyQuestions().play("an animal", QUESTIONS, GUESSES);
  }
}