java ResultSet和语句问题

Question

我不明白为什么我的变量状态无法解析。

我在一个Java Mysql项目中。

这是Commands类代码：

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;


public class Commands {

    public Commands() throws SQLException{
        Connection conn = DbConn.getInstance();
        Statement state = conn.createStatement();
    }

    public String getList(){
        System.out.println("Here is a List of our Products:");
        // Get list from db
        ResultSet result = state.executeQuery("SELECT * FROM products");
        ResultSetMetaData resultMeta = result.getMetaData();
        // Display the List

        System.out.println("List displayed");
        return null;
    }
}

这是DbConn类代码：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;


public class DbConn {
      private static String url = "jdbc:mysql://localhost:3306/myDB";
      private static String user = "root";
      private static String passwd = "";
      private static Connection connect;

      // create new instance if not exists
       public static Connection getInstance(){
           if(connect == null){
                try {
                    connect = DriverManager.getConnection(url, user, passwd);
                } catch (SQLException e) {
                    e.printStackTrace();
                }
           }      
           return connect;  
      }   
}

我的代码尚未完成，但是消息出现在这一行：

ResultSet result = state.executeQuery("SELECT * FROM products");

我的Eclipse编辑器说此消息状态无法解决

任何想法？

Answer 1

这是范围问题。 您在此处定义变量

public Commands() throws SQLException{
    Connection conn = DbConn.getInstance();
    Statement state = conn.createStatement();
}

那是变量可见的唯一位置-在构造函数中。 在类中定义它，并在构造函数中对其进行初始化：

private Connection conn = null;
private Statement state = null;

public Commands() throws SQLException{
    conn = DbConn.getInstance();
    state = conn.createStatement();
}

Answer 2

在该构造函数之外声明“状态”。

Connection conn = null;
Statement state = null;

public Commands() throws SQLException{
    conn = DbConn.getInstance();
    state = conn.createStatement();
}