不管是否为空，但执行以下操作总是错误:

Empty or not, but doing the following is always faulty:

resultSet = statement.executeQuery(sql);
string = resultSet.getString(1); // Epic fail. The cursor isn't set yet.

这不是错误.这是记录在案的行为.每个 体面的 JDBC 教程 提到了它.您需要使用 next() 设置 ResultSet 的光标，然后才能访问任何数据.

This is not a bug. This is documented behaviour. Every decent JDBC tutorial mentions it. You need to set the ResultSet's cursor using next() before being able to access any data.

如果您真的对所谓的唯一行 存在 是否真的感兴趣，那么只需检查 next() 的结果.例如在一个虚构的 UserDAO 类中:

If you're actually interested whether the supposedly unique row exist or not, then just check the outcome of next(). For example in a fictive UserDAO class:

public boolean exist(String username, String password) throws SQLException {
    boolean exist = false;

    try (
        Connection connection = database.getConnection();
        PreparedStatement statement = connection.prepareStatement("SELECT id FROM user WHERE username = ? AND password = MD5(?)");
    ) {
        statement.setString(1, username);
        statement.setString(2, password);

        try (ResultSet resultSet = statement.executeQuery()) {
            exist = resultSet.next();
        }
    }

    return exist;
}

如果您实际上只期望 零 或 一个 行，那么只需执行以下操作:

If you actually expect only zero or one row, then just do something like:

public User find(String username, String password) throws SQLException {
    User user = null;

    try (
        Connection connection = database.getConnection();
        PreparedStatement statement = connection.prepareStatement("SELECT id, username, email, birthdate FROM user WHERE username = ? AND password = MD5(?)");
    ) {
        statement.setString(1, username);
        statement.setString(2, password);

        try (resultSet = statement.executeQuery()) {
            if (resultSet.next()) {
                user = new User(
                    resultSet.getLong("id"),
                    resultSet.getString("username"),
                    resultSet.getString("email"),
                    resultSet.getDate("birthdate")); 
            }
        }
    }

    return user;
}

然后在业务/域对象中相应地处理它，例如

and then just handle it accordingly in the business/domain object, e.g.

User user = userDAO.find(username, password);

if (user != null) {
    // Login?
}
else {
    // Show error?
}

如果您实际上只期望 零 或 许多 行，那么只需执行以下操作:

If you actually expect only zero or many rows, then just do something like:

public List<User> list() throws SQLException {
    List<User> users = new ArrayList<User>();

    try (
        Connection connection = database.getConnection();
        PreparedStatement statement = connection.prepareStatement("SELECT id, username, email, birthdate FROM user");
        ResultSet resultSet = statement.executeQuery();
    ) {
        while (resultSet.next()) {
            users.add(new User(
                resultSet.getLong("id"),
                resultSet.getString("username"),
                resultSet.getString("email"),
                resultSet.getDate("birthdate")));
        }
    }

    return users;
}

然后在业务/域对象中相应地处理它，例如

and then just handle it accordingly in the business/domain object, e.g.

List<User> users = userDAO.list();

if (!users.isEmpty()) {
    int count = users.size();
    // ...
}
else {
    // Help, no users?
}