Java 尝试连接到MySQL时出现NullPointerException

请帮助我确定为什么我总是出现此错误：

 java.lang.NullPointerException
    at TaskDAO.TaskDAO.main(TaskDAO.java:34)

注意：DatebaseHelper是一个使用JDBC连接到MySQL的类

import java.sql.SQLException;
import java.text.SimpleDateFormat;
import TaskManagement.*;

public class TaskDAO {

    DatabaseHelper help;

    public static void main(String[] args) throws InstantiationException,
            IllegalAccessException, ClassNotFoundException, SQLException {

        TaskDAO taskDAO = new TaskDAO();

        int taskID = 6;
        String subject = "lau cua";
        Status status = Status.Completeted;
        Priority priority = Priority.High;
        Float percentage = 4f;
        String tableName = "tasks";
        String startDate = "2012/11/11";
        String dueDate = "2012/11/12";

        String query = "INSERT INTO "
                + tableName
                + "(taskID,subject,status,priority,percentage,startDate,dueDate)"
                + " VALUES ('" + taskID + "'," + "'" + subject + "'," + "'"
                + status + "'," + "'" + priority + "'," + "'" + percentage
                + "'," + "'" + startDate + "'," + "'" + dueDate + "')";

        taskDAO.help.executeQuery(query);
    }

}

DatabaseHelper帮助是一个成员变量，因此初始化为null。在main中，您创建了一个TaskDAO，并立即使用它的帮助成员（为null）。您必须使用一些有效的DatabaseHelper初始化帮助。

DatabaseHelper帮助是一个成员变量，因此初始化为null。在main中，您创建了一个TaskDAO，并立即使用它的帮助成员（为null）。您必须使用一些有效的DatabaseHelper初始化帮助。

帮助为空，因此NPE忘记初始化它

DatabaseHelper help = new DatabaseHelper();

帮助为空，因此NPE忘记初始化它

DatabaseHelper help = new DatabaseHelper();

您正在调用taskDAO.help.executeQueryquery； 但直到有了数据库帮助；是null，因为引用变量在java中默认为null。 所以首先定义DatabaseHelper帮助

然后打电话

taskDAO.help.executeQuery(query);

您正在调用taskDAO.help.executeQueryquery； 但直到有了数据库帮助；是null，因为引用变量在java中默认为null。 所以首先定义DatabaseHelper帮助

然后打电话

taskDAO.help.executeQuery(query);

---

下面给出了一个从MySQL数据库读取的简单代码

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;




public class NewClass {

public static void main(String[] args) {


    Connection conn = null;

    String url = "jdbc:mysql://192.168.100.100:3306/";
    String dbName = "databaseName";
    Statement stmt = null;
    ResultSet result = null;
    String driver = "com.mysql.jdbc.Driver";
    String databaseUserName = "admin";
    String databasePassword = "root";
    try {
        Class.forName(driver).newInstance();
        conn = DriverManager.getConnection(url + dbName, databaseUserName, databasePassword);
        stmt = conn.createStatement();
        result = null;
        String password,username;
        result = stmt.executeQuery("select * from userTable where username ='user1' ");
        if(!result.isBeforeFirst()){
            System.out.println("resultset contin no rows");
        }
        while (result.next()) {

            username=result.getString("username");
            password = result.getString("password");
            System.out.println(username+"  "+password);

        }
        conn.close();
    } catch (Exception e) {      
        e.printStackTrace();
    }
}
}

---

下面给出了一个从MySQL数据库读取的简单代码

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;




public class NewClass {

public static void main(String[] args) {


    Connection conn = null;

    String url = "jdbc:mysql://192.168.100.100:3306/";
    String dbName = "databaseName";
    Statement stmt = null;
    ResultSet result = null;
    String driver = "com.mysql.jdbc.Driver";
    String databaseUserName = "admin";
    String databasePassword = "root";
    try {
        Class.forName(driver).newInstance();
        conn = DriverManager.getConnection(url + dbName, databaseUserName, databasePassword);
        stmt = conn.createStatement();
        result = null;
        String password,username;
        result = stmt.executeQuery("select * from userTable where username ='user1' ");
        if(!result.isBeforeFirst()){
            System.out.println("resultset contin no rows");
        }
        while (result.next()) {

            username=result.getString("username");
            password = result.getString("password");
            System.out.println(username+"  "+password);

        }
        conn.close();
    } catch (Exception e) {      
        e.printStackTrace();
    }
}
}

---

谢谢你的快速回答，我正在看，也许会请你再帮我一个忙。再次感谢你。谢谢你的快速回答，我正在看，也许还需要你帮忙。再次感谢你。