Lab 5 最佳实践与思考题

[geeeeeeeeek]

基本功能

使用我们已有的知识，最优的代码应该在 20 - 30 行左右。大家可以对比一下自己的代码和下面的示例代码，看看你有哪些冗余的代码是可以精简的。

字符串实现

字符串实现完全使用了我们已学的知识，示例如下：

import java.util.Scanner;

public class TicTacToe {

    public static void main(String[] args) {
        String board = ".........";
        boolean player = true;
        Scanner scanner = new Scanner(System.in);
        while (true) {
            System.out.println(board.substring(0, 3));
            System.out.println(board.substring(3, 6));
            System.out.println(board.substring(6, 9));
            System.out.print("Next move: ");
            int nextPosition = scanner.nextInt();
            board = board.substring(0, nextPosition)
                    + ((player) ? 'O' : 'X')
                    + board.substring(nextPosition + 1, board.length());
            player = !player;
        }
    }
}

有以下地方需要同学们注意：

• 使用 while (true) 循环：基本功能不要求实现终局判断，因此我们只需要一个死循环来实现用户的交替输入。for 循环的等价实现是 for (;;) ，显然没有 while (true) 来得直观。
• 使用一个 boolean 变量来记录当前玩家：游戏中只有两个玩家，而布尔值恰好只提供两个值（true 和 false），因此我们可以用 true 来代表玩家 X ，而 false 代表玩家 O 。
• while () {}循环的意思是当()中的表达式值为true时会执行{}中的语句。while(true)因为括号中的布尔表达式恒为true，所以会无限循环下去。请明确布尔变量player的意义，这个布尔变量与循环控制条件没有任何关系。
• 更新棋盘状态：假设我们要更新第 nextPosition 个位置，其实是把前几位、更新的这一位及后几位拼起来，再重新赋值给 board 。这里还用到了三目运算符，你也可以用 if...else... 替换 。

一维数组实现

上次 lab 最后让大家自学了一维数组。如果使用这种方法，下面的实现应该是最优的：

import java.util.Scanner;

public class TicTacToe {

    public static void main(String[] args) {
        char[] board = {'.', '.', '.', '.', '.', '.', '.', '.', '.'};
        boolean player = true;
        Scanner scanner = new Scanner(System.in);
        while (true) {
            for (int i = 0; i < 3; i++) {
                String row = "" + board[i * 3] + board[i * 3 + 1] + board[i * 3 + 2];
                System.out.println(row);
            }
            System.out.print("Next move: ");
            int nextPosition = scanner.nextInt();
            board[nextPosition] = (player) ? 'O' : 'X';
            player = !player;
        }
    }
}

有以下地方需要同学们注意：

• 数组的输出需要借助循环来完成。思考里面的 for 循环，它为什么能输出整个棋盘？
• 思考 row 的赋值语句中，为什么要在最左边加上 "" ？

二维数组实现

二维数组还没有学到，仅供学有余力的同学了解。

import java.util.Scanner;

public class TicTacToe {

    public static void main(String[] args) {
        char[][] board = {{'.', '.', '.'}, {'.', '.', '.'}, {'.', '.', '.'}};
        boolean player = true;
        Scanner scanner = new Scanner(System.in);
        while (true) {
            for (int i = 0; i < 3; i++) {
                for (int j = 0; j < 3; j++) {
                    System.out.print(board[i][j]);
                }
                System.out.println();
            }
            System.out.print("Next move: ");
            int nextPosition = scanner.nextInt();
            board[nextPosition / 3][nextPosition % 3] = (player) ? 'O' : 'X';
            player = !player;
        }
    }
}

方法调用实现

一些同学使用了方法调用，因此这里也给出了一份在一维数组基础上的方法调用实现。当然，你也可以用其他方式组织你的方法。

import java.util.Scanner;

public class TicTacToe {

    public static void main(String[] args) {
        char[] board = {'.', '.', '.', '.', '.', '.', '.', '.', '.'};
        boolean player = false;
        Scanner scanner = new Scanner(System.in);
        while (true) {
            printBoard(board);
            board = setBoard(board, getNextPosition(scanner), player = !player);
        }
    }

    private static int getNextPosition(Scanner scanner) {
        System.out.print("Next move: ");
        return scanner.nextInt();
    }

    private static void printBoard(char[] board) {
        for (int i = 0; i < 3; i++) {
            String row = "" + board[i * 3] + board[i * 3 + 1] + board[i * 3 + 2];
            System.out.println(row);
        }
    }

    private static char[] setBoard(char[] board, int position, boolean player) {
        board[position] = (player) ? 'O' : 'X';
        return board;
    }
}

附加功能

提供一份在一维数组基础上实现的附加功能示例代码。

import java.util.Scanner;

public class TicTacToe {

    public static void main(String[] args) {
        char[] board = {'.', '.', '.', '.', '.', '.', '.', '.', '.'};
        int availablePositions = 9;
        Scanner scanner = new Scanner(System.in);
        printBoard(board);
        while (availablePositions > 0) {
            int nextPosition = getNextPosition(scanner, board);

            board[nextPosition] = (availablePositions % 2 == 1) ? 'O' : 'X';
            availablePositions -= 1;

            printBoard(board);
            if (checkGameOver(board, nextPosition, availablePositions)) return;
        }
    }

    /**
     * Read next position from stdin.
     */
    private static int getNextPosition(Scanner scanner, char[] board) {
        System.out.print("Next move: ");
        int nextPosition = scanner.nextInt();
        while (!checkIsValidInput(nextPosition, board)) {
            System.out.println("Invalid input. Try again with an available position by an integer between [0, 8].");
            nextPosition = scanner.nextInt();
        }
        return nextPosition;
    }

    /**
     * Check if the game has ended.
     */
    private static boolean checkGameOver(char[] board, int nextPosition, int availablePositions) {
        boolean hasWon = checkHasWon(board, nextPosition);
        // Check if the game is over
        if (hasWon) {
            System.out.println(((availablePositions % 2 == 0) ? 'O' : 'X') + " has won. Exit.");
            return true;
        } else if (availablePositions == 0) {
            System.out.println("Ended in a draw. Exit.");
            return true;
        }
        return false;
    }

    /**
     * Check if a player has won.
     */
    private static boolean checkHasWon(char[] board, int nextPosition) {
        int row = nextPosition / 3, col = nextPosition % 3;
        boolean hasWon = ((board[row * 3] == board[row * 3 + 1]) && (board[row * 3 + 1] == board[row * 3 + 2]))
                || ((board[col] == board[col + 3]) && (board[col + 3] == board[col + 6]))
                || ((row == col) && (board[0] == board[4]) && (board[4] == board[8]))
                || ((row == 2 - col) && (board[2] == board[4]) && (board[4] == board[6]));
        return hasWon;
    }

    /**
     * Check if input is valid.
     */
    private static boolean checkIsValidInput(int nextPosition, char[] board) {
        return nextPosition >= 0 && nextPosition <= 8 && board[nextPosition] == '.';
    }

    /**
     * Print board.
     */
    private static void printBoard(char[] board) {
        for (int i = 0; i < 3; i++) {
            String row = "" + board[i * 3] + board[i * 3 + 1] + board[i * 3 + 2];
            System.out.println(row);
        }
    }
}

有以下地方需要同学们注意：

• 思考终局判断中下列四行的作用分别是什么？

  boolean hasWon = ((board[row * 3] == board[row * 3 + 1]) && (board[row * 3 + 1] == board[row * 3 + 2]))
  || ((board[col] == board[col + 3]) && (board[col + 3] == board[col + 6]))
  || ((row == col) && (board[0] == board[4]) && (board[4] == board[8]))
  || ((row == 2 - col) && (board[2] == board[4]) && (board[4] == board[6]));

• 程序是如何判断能否落子的？