Question

我试图修复我很久以前为Craftbukkit写过的插件，但我在一节中难以理解。我很幸运地搜索过Google，而且我已经让其他Java开发人员听到我不应该使用for循环，因为它相当基本，或者我是＆＃39 ; m在错误的位置使用布尔表达式。没人会告诉我如何我可以解决这个问题，所以我知道将来的参考资料 - 下面是抛出错误的类：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.util.Hashtable;
import java.util.logging.Logger;
import org.bukkit.configuration.file.FileConfiguration;

public class Database
{
  private static String host = null;
  private static String port = null;
  private static String database = null;
  private static String table = null;
  private static String username = null;
  private static String password = null;
  private static String colUsername = null;
  private static Logger logger = null;

  public static void init(FileConfiguration config, Logger log)
  {
    logger = log;

    host = config.getString("DBHost");
    port = config.getString("DBPort");
    database = config.getString("DBName");
    table = config.getString("DBTable");
    username = config.getString("DBUser");
    password = config.getString("DBPass");
    colUsername = config.getString("ColUsername");
  }

  public static Hashtable<String, Object> getUserInfo(String user)
  {
    String url = "jdbc:mysql://" + host + ":" + port + "/" + database;
    String query = "SELECT * FROM " + table + " WHERE " + colUsername + " = ?";

    Connection connect = null;
    PreparedStatement stmt = null;
    ResultSet result = null;
    Hashtable<String, Object> userInfo = new Hashtable<String, Object>();
    try
    {
      Class.forName("com.mysql.jdbc.Driver");
      connect = DriverManager.getConnection(url, username, password);
      stmt = connect.prepareStatement(query);
      stmt.setString(1, user);
      result = stmt.executeQuery();
      ResultSetMetaData rsmd;
      int i;
      for (; result.next(); i <= rsmd.getColumnCount())
      {
        rsmd = result.getMetaData();
        i = 1; continue;
        userInfo.put(rsmd.getColumnName(i), result.getObject(i));i++;
      }
      return userInfo;
    }
    catch (ClassNotFoundException e)
    {
      logger.warning("Unable to load driver. Using default behaviour.");
      e.printStackTrace();
    }
    catch (SQLException e)
    {
      logger.warning("Database error. Using default behaviour.");
      e.printStackTrace();
    }
    finally
    {
      if (result != null) {
        try
        {
          result.close();
        }
        catch (SQLException e)
        {
          e.printStackTrace();
        }
      }
      if (stmt != null) {
        try
        {
          stmt.close();
        }
        catch (SQLException e)
        {
          e.printStackTrace();
        }
      }
      if (connect != null) {
        try
        {
          connect.close();
        }
        catch (SQLException e)
        {
          e.printStackTrace();
        }
      }
    }
    return null;
  }
}

我遇到的错误出现在代码的这一部分：

  for (; result.next(); i <= rsmd.getColumnCount())
  {
    rsmd = result.getMetaData();
    i = 1; continue;
    userInfo.put(rsmd.getColumnName(i), result.getObject(i));i++;
  }

我收到错误＆＃34;令牌上的语法错误＆＃34;＆lt; =＆＃34;，无效的AssignmentOperator＆＃34;

我该如何解决这个问题，如何改进呢？

编辑＃1：

这是我更新的代码，根据Jon的回答：

try
{
  Class.forName("com.mysql.jdbc.Driver");
  connect = DriverManager.getConnection(url, username, password);
  stmt = connect.prepareStatement(query);
  stmt.setString(1, user);
  result = stmt.executeQuery();
  ResultSetMetaData rsmd = result.getMetaData();
  while (result.next()) {
      for (int i = 1; i <= rsmd.getColumnCount(); i++) {
          rsmd = result.getMetaData();
          userInfo.put(rsmd.getColumnName(i), result.getObject(i));i++;
      }
  }
  return userInfo;
}

Answer 1

基本上这是错误的方式：

for (; result.next(); i <= rsmd.getColumnCount())

可能：

for (; i <= rsmd.getColumnCount(); result.next())

虽然更有可能，但实际上你想要：

while (result.next()) {
    // This outer loop is executed once per row        

    for (int i = 1; i <= rsmd.getColumnCount(); i++) {
        // This inner loop is executed once per column (per row, as it's
        // within the outer loop)
    }
}

话虽如此，你甚至没有初始化rsmd，这没有用。我怀疑你可能想打电话给ResultSet.getMetadata()，例如

rsmd = result.getMetadata();

作为参考，for语句声明的三个部分如下：

第一部分（在您的情况下为空）执行一次，作为初始化
第二部分是检查每次迭代的条件;当条件评估为false
第三部分是声明是每次迭代结束时采取的步骤

有关详细信息，请参阅section 14.14.1 of the JLS或for statement part of the Java tutorial。

Answer 2

这不适用于for循环的最后一部分：

i <= rsmd.getColumnCount()

也许你是这个意思？

for (; i <= rsmd.getColumnCount(); result.next())

Answer 3

for循环的基本语法：

  for (any; boolean; any)

你的是

  for (any; any; boolean)

只需更改订单。

令牌“＆lt; =”上的语法错误，AssignmentOperator无效

3 个答案: