编写一个java小程序，实现五子棋游戏功能。

好的，我可以为您解答，以下是一个简单的五子棋游戏的Java程序：

import java.util.Scanner;

public class GomokuGame {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        char[][] board = new char[15][15]; // 棋盘大小为15*15
        initBoard(board); // 初始化棋盘
        printBoard(board); // 打印棋盘

        int x, y;
        char player = 'X'; // X先手，O后手
        while (true) {
            System.out.print("请输入" + player + "的落子位置(x,y): ");
            x = scanner.nextInt();
            y = scanner.nextInt();
            if (isValid(board, x, y)) {
                board[x][y] = player; // 落子
                printBoard(board); // 打印棋盘
                if (isWin(board, x, y)) {
                    System.out.println(player + "获胜！");
                    break;
                }
                player = (player == 'X') ? 'O' : 'X'; // 交换玩家
            } else {
                System.out.println("无效位置，请重新输入！");
            }
        }
    }

    // 初始化棋盘
    public static void initBoard(char[][] board) {
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[i].length; j++) {
                board[i][j] = '+';
            }
        }
    }

    // 打印棋盘
    public static void printBoard(char[][] board) {
        System.out.print("  ");
        for (int i = 0; i < board.length; i++) {
            System.out.print(i + " ");
        }
        System.out.println();
        for (int i = 0; i < board.length; i++) {
            System.out.print(i + " ");
            for (int j = 0; j < board[i].length; j++) {
                System.out.print(board[i][j] + " ");
            }
            System.out.println();
        }
    }

    // 判断位置是否有效
    public static boolean isValid(char[][] board, int x, int y) {
        if (x < 0 || x >= board.length || y < 0 || y >= board[0].length) {
            return false;
        }
        return board[x][y] == '+';
    }

    // 判断是否获胜
    public static boolean isWin(char[][] board, int x, int y) {
        int count = 0;
        char player = board[x][y];
        // 水平方向
        for (int i = y - 4; i <= y + 4; i++) {
            if (i < 0 || i >= board.length) {
                continue;
            }
            if (board[x][i] == player) {
                count++;
                if (count == 5) {
                    return true;
                }
            } else {
                count = 0;
            }
        }
        count = 0;
        // 垂直方向
        for (int i = x - 4; i <= x + 4; i++) {
            if (i < 0 || i >= board[0].length) {
                continue;
            }
            if (board[i][y] == player) {
                count++;
                if (count == 5) {
                    return true;
                }
            } else {
                count = 0;
            }
        }
        count = 0;
        // 左上到右下方向
        for (int i = x - 4, j = y - 4; i <= x + 4 && j <= y + 4; i++, j++) {
            if (i < 0 || i >= board.length || j < 0 || j >= board[0].length) {
                continue;
            }
            if (board[i][j] == player) {
                count++;
                if (count == 5) {
                    return true;
                }
            } else {
                count = 0;
            }
        }
        count = 0;
        // 右上到左下方向
        for (int i = x - 4, j = y + 4; i <= x + 4 && j >= y - 4; i++, j--) {
            if (i < 0 || i >= board.length || j < 0 || j >= board[0].length) {
                continue;
            }
            if (board[i][j] == player) {
                count++;
                if (count == 5) {
                    return true;
                }
            } else {
                count = 0;
            }
        }
        return false;
    }
}

这个程序使用二维数组来表示棋盘，每个位置用字符表示落子情况（‘+’表示空位，‘X’表示X方落子，‘O’表示O方落子）。程序实现了以下功能：

1. 初始化棋盘
2. 打印棋盘
3. 输入玩家落子位置，落子，并判断是否有效和是否获胜
4. 交换玩家

希望能够帮到您！