java - 打印数组条目时出现空指针异常

Question

我制作了一个类 Test，它计算数据库中与用户 ID 对应的条目数(将每个条目称为电子邮件)。我使用 11120059 作为 id，数据库中与此对应的条目数为 2。countMail 函数的输出工作正常，但因为我从 返回一个数组>getMail() 函数并将其放入新数组，它向我显示空指针异常。请帮助我陷入困境。代码是:

package src.service;

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

import model.Email;
import model.User;

public class Test {

public int countMail(User user){

    try{

        Class.forName("com.mysql.jdbc.Driver").newInstance();

        // Get a connection to the database
        Connection myConn = DriverManager.getConnection("jdbc:mysql://localhost:3306/chillmaarodb", "root", "rsystems");

        PreparedStatement myStatement = myConn.prepareStatement("select * from complaints where RID=? ORDER BY date desc");


        myStatement.setString(1, user.getId());

        ResultSet rs = myStatement.executeQuery();

        int count=0;

        while(rs.next())
        {
        count++;
        }

        return count;
    }

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

    return 0;

}

public Email[] getMail(User user){

    try{

        Class.forName("com.mysql.jdbc.Driver").newInstance();

        // Get a connection to the database
        Connection myConn = DriverManager.getConnection("jdbc:mysql://localhost:3306/chillmaarodb", "root", "rsystems");

        PreparedStatement myStatement = myConn.prepareStatement("select * from complaints where RID=? ORDER BY date desc");


        myStatement.setString(1, user.getId());

        ResultSet rs = myStatement.executeQuery();

        Home home= new Home();

        int length = home.countMail(user);

        Email[] mail = new Email[length];

        int i=0;

        while(rs.next())
        {
        mail[i].setMessage((String) rs.getString(5));
        mail[i].setTitle((String) rs.getString(4)); 
        mail[i].setSender((String) rs.getString(2));
        mail[i].setReceiver((String) rs.getString(1));
        i++;
        }

        return mail;

        }
    catch (Exception e){

        e.printStackTrace();

    }

    Email[] dummyMail = new Email[1];

    return dummyMail;



}

public static void main(String[] args){

    Test test = new Test();

    User user = new User();

    user.setId("11120059");

    System.out.println(test.countMail(user));

    Email[] email = test.getMail(user);

    for (int i=0 ; i<test.countMail(user) ; i++){

        System.out.println(email[i].getSender());

    }


}

}

输出是:

2 

java.lang.NullPointerException
    at src.service.Test.getMail(Test.java:73)
    at src.service.Test.main(Test.java:107)
Exception in thread "main" java.lang.NullPointerException
    at src.service.Test.main(Test.java:111)

Answer 1

当您编写 Email[] mail = new Email[length]; 时，您创建了一个给定长度的数组，其中包含空引用。您不能通过此命令自动为该数组创建所有对象。添加mail[i] = new Email()语句:

    Email[] mail = new Email[length];

    int i=0;

    while(rs.next())
    {
    mail[i] = new Email(); // or use appropriate constructor parameters
    mail[i].setMessage((String) rs.getString(5));
    mail[i].setTitle((String) rs.getString(4)); 
    mail[i].setSender((String) rs.getString(2));
    mail[i].setReceiver((String) rs.getString(1));
    i++;
    }

另请注意，当您创建一个 dummyMail 数组时，它也是一个具有单个 null 引用的数组，不会创建 Email 对象那里也一样。可能你需要:

Email[] dummyMail = new Email[] {new Email()};