java - 尝试连接到MySQL时出现NullPointerException

Question

请帮助我确定为什么总是出现此错误：

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

注意：DatebaseHelper是使用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);
    }

}

Answer 1

DatabaseHelper help是成员变量，因此被初始化为null。在main中，创建一个TaskDAO，并立即使用其help成员-null。您必须使用一些有效的help初始化DatabaseHelper。