[英]Null Pointer Exception while printing the array entries
我做了一個Test
類,它計算與數據庫中用戶ID對應的條目數(將每個條目稱為電子郵件)。 我使用11120059
作為id,數據庫中與此對應的條目數是countMail
函數的輸出運行正常,但是由於我從getMail()
函數返回一個數組並將其放入新數組,因此向我顯示了null指針異常。 請幫助我陷入其中。 代碼是:
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());
}
}
}
輸出為:
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)
當您編寫Email[] mail = new Email[length];
您創建一個給定長度的數組,其中包含空引用。 您不能通過此命令自動創建該數組的所有對象。 添加mail[i] = new Email()
語句:
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++;
}
還要注意,當創建一個dummyMail
數組時,它也是一個具有單個null
dummyMail
數組,因此也不會在其中創建Email
對象。 可能您需要:
Email[] dummyMail = new Email[] {new Email()};
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