TicTacToeGame.java

import java.util.Scanner;

/**
 * Driver for playing a game of Tic Tac Toe
 */
public class TicTacToeGame {
  public static enum Player {
    X("X"), O("O"), Nobody(" ");

    private final String playerName;

    Player(String name) {
      this.playerName = name;
    }

    public String toString() {
      return this.playerName;
    }
  }

  private static final String WELCOME_MESSAGE = "\nWelcome to Tic Tac Toe!\n";
  private static final String MOVE_INSTRUCTION_MESSAGE = "The game board looks"
          + " like this:\n\n" + TicTacToeBoard.LABELED_BOARD + "\nOn your turn, "
          + "enter the number of the square you want to make your move on.\n";
  private static final String MOVE_PROMPT = "Please enter a move";
  private static final String VALID_MOVE_PROMPT = "Please enter a move between "
          + TicTacToeBoard.MIN_MOVE + " and " + TicTacToeBoard.MAX_MOVE;
  private static final String REPEAT_MOVE = "That move has already been done.";
  private static final String COMPUTER_TURN = "Computer's turn!\n";
  private static final String YOU_WON = "You won!";
  private static final String COMP_WON = "The computer won!";
  private static final String TIE = "It was a tie!";

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

  /**
   * Write a line to the human player.
   * @param toWrite the message to write.
   */
  private static void say(String toWrite) {
    System.out.println(toWrite);
  }

  /**
   * Write a message to the human player, asking for input.
   * @param toPrompt the message to prompt with.
   */
  private static void prompt(String toPrompt) {
    System.out.print(toPrompt + ": ");
  }

  /**
   * Prompt the user for a move, and return it.
   * @param board the board the game is being played on.
   * @return a valid move entered by the user.
   */
  private static int getMove(TicTacToeBoard board) {
    prompt(MOVE_PROMPT);
    return getValidMove(board);
  }

  /**
   * Get a move from the user. If it is invalid, display an error and prompt the
   * user again.
   * @param board the board the game is being played on.
   * @return the valid move the user made.
   */
  private static int getValidMove(TicTacToeBoard board) {
    String playerInput = scanner.nextLine();
    try {
      int move = Integer.parseInt(playerInput);
      if (move < TicTacToeBoard.MIN_MOVE || move > TicTacToeBoard.MAX_MOVE) {
        prompt(VALID_MOVE_PROMPT);
      } else {
        int[] possibleMoves = board.getPossibleMoves();
        for (int possibleMove: possibleMoves) {
          if (move == possibleMove) {
            return move;
          }
        }
        // If the move was valid and was not returned, it was already made
        say(REPEAT_MOVE);
        prompt(MOVE_PROMPT);
      }
    } catch (NumberFormatException e) {
      prompt(VALID_MOVE_PROMPT);
    }
    return getValidMove(board);
  }

  /**
   * Have the computer play a randomly chosen valid move.
   */
  private static void playComputerMove(TicTacToeBoard board) {
    int[] possibleMoves = board.getPossibleMoves();
    int move = possibleMoves[(int)(Math.random() * (possibleMoves.length - 1))];
    board.playMove(move, Player.O);
  }

  /**
   * Play a round of Tic Tac Toe with the user. A round consists of a move
   * by the user and one by the computer.
   * @param board the board to play on.
   * @return true if the game has ended.
   */
  private static boolean play(TicTacToeBoard board) {
    int move = getMove(board);
    board.playMove(move, Player.X);
    System.out.println();
    System.out.println(board);
    if (board.isDone()) {
      return true;
    }
    playComputerMove(board);
    System.out.println(COMPUTER_TURN);
    System.out.println(board);
    return board.isDone();
  }

  /**
   * Plays a game of Tic Tac Toe on the command line with a randomized
   * computer opponent.
   */
  public static void main(String[] args) {
    TicTacToeBoard board = new TicTacToeBoard();
    say(WELCOME_MESSAGE);
    say(MOVE_INSTRUCTION_MESSAGE);
    while(!play(board)); // Will loop until the board game is over.
    if (board.whoWon() == Player.X) {
      say(YOU_WON);
    } else if (board.whoWon() == Player.O) {
      say(COMP_WON);
    } else {
      say(TIE);
    }
  }
}