Tic-Tac-Toe AI minimax function in Java

Question

I have here my code for the minimax algorithm. It seems to be making smart decisions, but not always, which shouldn't be happening and I need help because I can't seem to find where I went wrong.

private char level2Move() {
  try {
    NBoard game_copy = (NBoard) game.clone();

    int bestScore = Integer.MIN_VALUE;
    char bestMove = '-';

    for (char move: game_copy.getValidMoves()) {
      game_copy.enterMove(move);
      int score = minimax(game_copy, true);
      game_copy.undoMove();
      if (score > bestScore) {
        bestScore = score;
        bestMove = move;
      }
    }

    return bestMove;
  } catch(CloneNotSupportedException e) {
    e.printStackTrace();
  }

  return '-';
}

public int minimax(NBoard game, boolean isMaximizing) {
  int bestScore;

  switch (game.getGameState()) {
  case NBoard.X_WIN:
    return - 10;
  case NBoard.O_WIN:
    return 10;
  case NBoard.DRAW:
    return 0;
  }

  if (isMaximizing) {
    bestScore = Integer.MIN_VALUE;
    for (char move: game.getValidMoves()) {
      if (game.getGameState() == NBoard.ONGOING) {
        game.enterMove(move);
        int score = minimax(game, false);
        game.undoMove();
        bestScore = Math.max(score, bestScore);
      }
    }
  } else {
    bestScore = Integer.MAX_VALUE;
    for (char move: game.getValidMoves()) {
      if (game.getGameState() == NBoard.ONGOING) {
        game.enterMove(move);
        int score = minimax(game, true);
        game.undoMove();
        bestScore = Math.min(score, bestScore);
      }
    }
  }
  return bestScore;
}

Board input (move) are letters A-I, like so...

A|B|C
D|E|F
G|H|I

I seem to have not used "score" yet and am thinking of adding depth, but I can't think of how that would help.

Answer 1

I fixed it. Simply changed the boolean values in the minimax function.

private char level2Move() {
  try {
    NBoard game_copy = (NBoard) game.clone();

    int bestScore = Integer.MIN_VALUE;
    char bestMove = '-';

    for (char move: game_copy.getValidMoves()) {
      game_copy.enterMove(move);
      int score = minimax(game_copy, false);       // here
      game_copy.undoMove();
      if (score > bestScore) {
        bestScore = score;
        bestMove = move;
      }
    }

    return bestMove;
  } catch(CloneNotSupportedException e) {
    e.printStackTrace();
  }

  return '-';
}

public int minimax(NBoard game, boolean isMaximizing) {
  int bestScore;

  switch (game.getGameState()) {
  case NBoard.X_WIN:
    return - 10;
  case NBoard.O_WIN:
    return 10;
  case NBoard.DRAW:
    return 0;
  }

  if (isMaximizing) {
    bestScore = Integer.MIN_VALUE;
    for (char move: game.getValidMoves()) {
      if (game.getGameState() == NBoard.ONGOING) {
        game.enterMove(move);
        int score = minimax(game, false);          // here
        game.undoMove();
        bestScore = Math.max(score, bestScore);
      }
    }
  } else {
    bestScore = Integer.MAX_VALUE;
    for (char move: game.getValidMoves()) {
      if (game.getGameState() == NBoard.ONGOING) {
        game.enterMove(move);
        int score = minimax(game, true);           // here
        game.undoMove();
        bestScore = Math.min(score, bestScore);
      }
    }
  }
  return bestScore;
}