为什么准备好的语句不能在java中工作？

Question

我试图通过从db中提供卷号和介质来获取学生信息。 swing应用程序执行时没有任何错误，但是当我输入roll no和medium时，它会转到“Student not Found”else循环。

我想用GET字符串或准备statements.kindly帮我找出这个问题的问题。

import javax.swing.*; 
import java.awt.event.*; 
import java.awt.*; 
import java.sql.*; 

public class Searchdb extends JFrame implements ActionListener { 

//Initializing Components 
    JLabel lb,lbd,lb1, lb2, lb3, lb5; 
    JTextField tf1, tf2,tf3,tf5,tfd; 
    JButton btn; 

    //Creating Constructor for initializing JFrame components 
    Searchdb() { 
     //Providing Title 
     super("Fetching Roll Information"); 
     lb5 = new JLabel("Roll Number:"); 
     lb5.setBounds(20, 20, 100, 20); 
     tf5 = new JTextField(20); 
     tf5.setBounds(130, 20, 200, 20); 

     lbd = new JLabel("Date:"); 
     lbd.setBounds(20, 50, 100, 20); 
     tfd = new JTextField(20); 
     tfd.setBounds(130, 50, 200, 20); 


     btn = new JButton("Submit"); 
     btn.setBounds(50, 50, 100, 20); 
     btn.addActionListener(this); 

     lb = new JLabel("Fetching Student Information From Database"); 
     lb.setBounds(30, 80, 450, 30); 
     lb.setForeground(Color.black); 
     lb.setFont(new Font("Serif", Font.PLAIN, 12)); 
     setVisible(true); 
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     setSize(500, 500); 

     lb1 = new JLabel("Name:"); 
     lb1.setBounds(20, 120, 100, 20); 
     tf1 = new JTextField(50); 
     tf1.setBounds(130, 120, 200, 20); 
     lb2 = new JLabel("Fathername:"); 
     lb2.setBounds(20, 150, 100, 20); 
     tf2 = new JTextField(100); 
     tf2.setBounds(130, 150, 200, 20); 
     lb3 = new JLabel("State:"); 
     lb3.setBounds(20, 180, 100, 20); 
     tf3 = new JTextField(50); 
     tf3.setBounds(130, 180, 200, 20); 

     setLayout(null); 

     //Add components to the JFrame 
     add(lb5); 
     add(tf5); 
     add(lbd); 
     add(tfd); 
     add(btn); 

     add(lb); 
     add(lb1); 
     add(tf1); 
     add(lb2); 
     add(tf2); 
     add(lb3); 
     add(tf3); 


     //Set TextField Editable False 
     tf1.setEditable(false); 
     tf2.setEditable(false); 
     tf3.setEditable(false); 

    } 

    public void actionPerformed(ActionEvent e) { 
     //Create DataBase Coonection and Fetching Records 

     try { 
      String str = tf5.getText(); 

      Datestri = tfd.getText();//Getting the unable to convert String to Date error 

      System.out.println(str); 
      System.out.println(stri); 

      Class.forName("oracle.jdbc.driver.OracleDriver"); 
      Connection con = DriverManager.getConnection("jdbc:oracle:thin:@//host:port/servicename","username","password"); 
      PreparedStatement st = con.prepareStatement("select Name,Fathername,State from student_db where roll_number=? and medium=?"); 
      System.out.println(st); 
      st.setString(1, str); 
      st.setDate(2, stri); 



      //Excuting Query 
      ResultSet rs = st.executeQuery(); 
      System.out.println(rs); 

      if (rs.next()) { 
       String s = rs.getString(1); 
       String s1 = rs.getString(2); 
       String s2 = rs.getString(3); 


       //Sets Records in TextFields. 
       tf1.setText(s); 
       tf2.setText(s1); 
       tf3.setText(s2); 

      } else { 
       JOptionPane.showMessageDialog(null, "Student not Found"); 
      } 

      //Create Exception Handler 
     } catch (Exception ex) { 

      System.out.println(ex); 
     } 
    } 
//Running Constructor 

    public static void main(String args[]) { 
     new Searchdb(); 
    } 
} 

SQL查询：

select Name,Fathername,State from student_db where roll_number='1441' and medium='2016-12-18'; 

结果：

Name Fathername State 
SA  TH  YA 

假设，如果我没有通过 “S饰” 变量在查询中我得到的结果。

Answer 1

我在下面更新的代码和它工作正常，当我检查的数据库类型为中列名是VARCHAR2（40），所以我决定只使用getString。

掌握这个代码适当响应，

import javax.swing.*; 
import java.awt.event.*; 
import java.awt.*; 
import java.sql.*; 

public class Searchdb extends JFrame implements ActionListener { 

//Initializing Components 
    JLabel lb,lbd,lb1, lb2, lb3, lb5; 
    JTextField tf1, tf2,tf3,tf5,tfd; 
    JButton btn; 

    //Creating Constructor for initializing JFrame components 
    Searchdb() { 
     //Providing Title 
     super("Fetching Roll Information"); 
     lb5 = new JLabel("Roll Number:"); 
     lb5.setBounds(20, 20, 100, 20); 
     tf5 = new JTextField(20); 
     tf5.setBounds(130, 20, 200, 20); 

     lbd = new JLabel("Date:"); 
     lbd.setBounds(20, 50, 100, 20); 
     tfd = new JTextField(20); 
     tfd.setBounds(130, 50, 200, 20); 


     btn = new JButton("Submit"); 
     btn.setBounds(50, 50, 100, 20); 
     btn.addActionListener(this); 

     lb = new JLabel("Fetching Student Information From Database"); 
     lb.setBounds(30, 80, 450, 30); 
     lb.setForeground(Color.black); 
     lb.setFont(new Font("Serif", Font.PLAIN, 12)); 
     setVisible(true); 
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     setSize(500, 500); 

     lb1 = new JLabel("Name:"); 
     lb1.setBounds(20, 120, 100, 20); 
     tf1 = new JTextField(50); 
     tf1.setBounds(130, 120, 200, 20); 
     lb2 = new JLabel("Fathername:"); 
     lb2.setBounds(20, 150, 100, 20); 
     tf2 = new JTextField(100); 
     tf2.setBounds(130, 150, 200, 20); 
     lb3 = new JLabel("State:"); 
     lb3.setBounds(20, 180, 100, 20); 
     tf3 = new JTextField(50); 
     tf3.setBounds(130, 180, 200, 20); 

     setLayout(null); 

     //Add components to the JFrame 
     add(lb5); 
     add(tf5); 
     add(lbd); 
     add(tfd); 
     add(btn); 

     add(lb); 
     add(lb1); 
     add(tf1); 
     add(lb2); 
     add(tf2); 
     add(lb3); 
     add(tf3); 


     //Set TextField Editable False 
     tf1.setEditable(false); 
     tf2.setEditable(false); 
     tf3.setEditable(false); 

    } 

    public void actionPerformed(ActionEvent e) { 
     //Create DataBase Coonection and Fetching Records 

     try { 
      String str = tf5.getText(); 

      String stri = tfd.getText(); 

      System.out.println(str); 
      System.out.println(stri); 

      Class.forName("oracle.jdbc.driver.OracleDriver"); 
      Connection con = DriverManager.getConnection("jdbc:oracle:thin:@//host:port/servicename","username","password"); 

      String str1 ="select Name,Fathername,State from student_db where roll_number='"+str+"' and medium='"+stri+"'"; 

      PreparedStatement st = con.prepareStatement(str1); 
      System.out.println(st); 
      st.setString(1, str); 
      st.setString(2, stri); 



      //Excuting Query 
      ResultSet rs = st.executeQuery(); 
      System.out.println(rs); 

      if (rs.next()) { 
       String s = rs.getString(1); 
       String s1 = rs.getString(2); 
       String s2 = rs.getString(3); 


       //Sets Records in TextFields. 
       tf1.setText(s); 
       tf2.setText(s1); 
       tf3.setText(s2); 

      } else { 
       JOptionPane.showMessageDialog(null, "Student not Found"); 
      } 

      //Create Exception Handler 
     } catch (Exception ex) { 

      System.out.println(ex); 
     } 
    } 
//Running Constructor 

    public static void main(String args[]) { 
     new Searchdb(); 
    } 
}