Question

我知道有一个类似的帖子，他们已经回答了它但是当我在我的代码上尝试它仍然有一个错误，netbeans显示它是一个AWT-EventQueue-0 java.lang.NullPointerException错误和ithink是因为con变量。它在两个班级都宣布了。

这是我的Login_System.java中的DatabaseConnect（）代码：

public Connection DatabaseConnect(){
     try {

        String host = "jdbc:mysql://localhost:3306/CPE541Project";
        String userName = "root";
        String userPass = "";
        con = DriverManager.getConnection(host, userName, userPass);
        return con;

   } 
   catch (SQLException | HeadlessException err) {
        javax.swing.JOptionPane.showMessageDialog(this, "Error:\n" + err);            
   }
   return null;
}

当我运行Login_System.java时没有错误，但是当我尝试登录到错误开始时的UI_Staff.java时，这里是来自UI_Staff.java的代码片段，其中出现错误：

 private void ViewVotersTable(){
    try{

        Login_System database = new Login_System();
        database.DatabaseConnect();

        String sql = "SELECT StudentID, Course, VoteProgression FROM Voters";
        SQLStatement= con.prepareStatement(sql); //this is where the errors appears to according to netbeans
        queryResultSet = SQLStatement.executeQuery();       
        tbl_ElectionProgress.setModel(DbUtils.resultSetToTableModel(queryResultSet));

        sql ="SELECT COUNT(*) from Voters";
        SQLStatement= con.prepareStatement(sql);
        queryResultSet = SQLStatement.executeQuery();
        while(queryResultSet.next()){
            NumberOfVoters= queryResultSet.getString("COUNT(*)");
        }
        lbl_NumberOfVoters.setText(NumberOfVoters);

        sql ="SELECT COUNT(*) from Voters where VoteProgression='DONE'";
        SQLStatement= con.prepareStatement(sql);
        queryResultSet = SQLStatement.executeQuery();
        while(queryResultSet.next()){
            ProcessedVotes = queryResultSet.getString("COUNT(*)");
        }
        lbl_ProcessedVotes.setText(ProcessedVotes);

        sql ="SELECT COUNT(*) from Voters where VoteProgression='NOT DONE'";
        SQLStatement= con.prepareStatement(sql);
        queryResultSet = SQLStatement.executeQuery();
        while(queryResultSet.next()){
            UnprocessedVotes = queryResultSet.getString("COUNT(*)");
        }
        lbl_UnprocessedVotes.setText(UnprocessedVotes);

    }
    catch ( SQLException | HeadlessException err ) {
    javax.swing.JOptionPane.showMessageDialog(this,"Error:\n"+err);

    }
}

这是stacktrace中的第一行错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at cpe541project.UI_Staff.ViewVotersTable(UI_Staff.java:869)
at cpe541project.UI_Staff.formWindowActivated(UI_Staff.java:859)
at cpe541project.UI_Staff.access$000(UI_Staff.java:15)
at cpe541project.UI_Staff$1.windowActivated(UI_Staff.java:115)

请帮忙。我不知道错误是什么。

Answer 1

您缺少con = database.DatabaseConnect();，这会导致您使用的con为空。另一个已设置，但您不会将值复制到另一个。

从数据库返回的连接.DatabaseConnect（）;并且在访问之前不会保存到本地变量或实例级变量。所以它抛出一个NullPointerException

更改以下代码中的第二行

Login_System database = new Login_System();
database.DatabaseConnect();

String sql = "SELECT StudentID, Course, VoteProgression FROM Voters";
SQLStatement= con.prepareStatement(sql); //this is where the errors appears to according to netbeans

到

Login_System database = new Login_System();
con = database.DatabaseConnect(); // or Connection con = database.DatabaseConnect();

String sql = "SELECT StudentID, Course, VoteProgression FROM Voters";
SQLStatement= con.prepareStatement(sql); //this is where the errors appears to according to netbeans

从另一个jframe调用数据库连接对象

1 个答案: