java - ResultSet#isFirst() 并不表示存在一行

Question

我刚刚在 java 程序中从 ms access 获取数据时遇到了问题，我正在使用 ucanaccess 驱动程序，以下是我的代码

public void loginUser(){
  // variables
    Connection connection = null;
    Statement statement = null;
    ResultSet resultSet = null;

    // Step 1: Loading or 
    // registering Oracle JDBC driver class
    try {

        Class.forName("net.ucanaccess.jdbc.UcanaccessDriver");
    }
    catch(ClassNotFoundException cnfex) {

        System.out.println("Problem in loading or "
                + "registering MS Access JDBC driver");
        cnfex.printStackTrace();
    }

    // Step 2: Opening database connection
    try {

        String msAccDB; 
                msAccDB = "C:/Users/Hidayat Ullah/Documents/Record.accdb";
        String dbURL = "jdbc:ucanaccess://"
                + msAccDB; 

        // Step 2.A: Create and 
        // get connection using DriverManager class
        connection = DriverManager.getConnection(dbURL); 

        // Step 2.B: Creating JDBC Statement 
  String  sql= "SELECT * FROM Student WHERE studentid = ? AND password = ?";
 PreparedStatement pstmt = connection.prepareStatement(sql);
  //printing here loginId.getText() and loginPass.getText() always has value 
  //i entered in the gui
 pstmt.setString(1, loginId.getText()); 
 pstmt.setString(2,loginPass.getText());
 ResultSet row = pstmt.executeQuery();

if(row.isFirst()){
  JOptionPane.showMessageDialog(null, "Student ID 
:"+row.getString("studentid"));
 System.exit(0);
  }else{
   JOptionPane.showMessageDialog(null, "No such record exists in database 
 against this ID, either ID or Password is incorrect.");
 System.exit(0);
 }

问题是，当我输入数据库中的用户 ID 和密码时，它总是不返回任何记录。我在这里做错了什么？

Answer 1

当 ResultSet 对象最初创建时，它位于第一行之前，因此 .isFirst() 将返回 false 直到调用 .next() 将 ResultSet 定位在第一行(如果存在)上。您的代码可以简单地调用 .next() 而不是调用 .isFirst() ，如果它返回 true 则至少找到一个匹配行.