java - homeandlearn數據庫教程 - rs.Next空指針異常

我正在通過以下教程。我無法從數據庫調用下一個項目。java - homeandlearn數據庫教程 - rs.Next空指針異常

http://www.homeandlearn.co.uk/java/database_scrolling_buttons.html

砍下我的課儘量使問題更容易閱讀..

package Employees; 

import java.sql.Connection; 
imports ... etc

進口在頂部，然後調用構造函數

public class Workers extends javax.swing.JFrame { 

    Connection con; 
    Statement stmt; 
    ResultSet rs; 

    /** 
    * Creates new form Workers 
    */ 
    public Workers() { 
     initComponents(); 
     DoConnect(); 
    } 

    cont.. 
}

這裏是DoConnect方法 - 這可以正常工作，因爲它設置變量，並且您可以在窗體上看到它們。

public void DoConnect(){ 

     try { 
      String host = "jdbc:derby://localhost:1527/Employees"; 
      String uName = "robert"; 
      String uPass = "robert"; 

      Connection con = DriverManager.getConnection(host, uName, uPass); 

      String SQL = "SELECT * FROM APP.WORKERS"; 

      Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE ); 
      ResultSet rs = stmt.executeQuery(SQL); 

      rs.next(); 

      int id_col  = rs.getInt("ID"); 
      String first_name = rs.getString("First_Name"); 
      String last_name = rs.getString("Last_Name"); 
      String job  = rs.getString("Job_Title"); 


      // DISPLAY THE FIRST RECORD IN THE TEXT FIELDS 
      textID.setText(Integer.toString(id_col)); 
      textFirstName.setText(first_name); 
      // etc... this works ok 
     } 
     catch(SQLException err){ 
      JOptionPane.showMessageDialog(Workers.this, err.getMessage()); 
     } 
    }

但是，這是我得到我的錯誤的功能。當我點擊下一個按鈕時，它應該從數據庫中獲取下一個項目。不過，我不認爲我會用正確的方式 - 我跟着教程？我不認爲它喜歡rs.Next（）方法？

這裏是下一個按鈕

private void buttonNextActionPerformed(java.awt.event.ActionEvent evt) {           

    try 
    { 
     if (rs.next()) { // this is the line where I get the error 

     } 
     else 
     { 
      rs.previous(); 
      JOptionPane.showMessageDialog(Workers.this, "End of File"); 
     } 
    } 
    catch (SQLException err) { 
     JOptionPane.showMessageDialog(Workers.this, err.getMessage()); 
    }   
}

代碼當我按下旁邊我得到以下錯誤。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
     at Employees.Workers.buttonNextActionPerformed(Workers.java:164) 
     at Employees.Workers.access$200(Workers.java:18) 
      ..

不知道我需要做什麼來解決它。非常感謝。

下面是完整的代碼的要求...... Workers.java

/* 
* To change this template, choose Tools | Templates 
* and open the template in the editor. 
*/ 
package Employees; 

import java.sql.Connection; 
import java.sql.DriverManager; 
import java.sql.SQLException; 
import java.sql.Statement; 
import java.sql.ResultSet; 
import javax.swing.JOptionPane; 

/** 
* 
* @author davidsonr 
*/ 
public class Workers extends javax.swing.JFrame { 

    Connection con; 
    Statement stmt; 
    ResultSet rs; 

    /** 
    * Creates new form Workers 
    */ 
    public Workers() { 
     initComponents(); 
     DoConnect(); 
    } 

    /** 
    * This method is called from within the constructor to initialize the form. 
    * WARNING: Do NOT modify this code. The content of this method is always 
    * regenerated by the Form Editor. 
    */ 
    @SuppressWarnings("unchecked") 
    // <editor-fold defaultstate="collapsed" desc="Generated Code">       
    private void initComponents() { 

     jPanel1 = new javax.swing.JPanel(); 
     textID = new javax.swing.JTextField(); 
     textFirstName = new javax.swing.JTextField(); 
     textLastName = new javax.swing.JTextField(); 
     jLabel1 = new javax.swing.JLabel(); 
     textJobTitle = new javax.swing.JTextField(); 
     buttonFirst = new javax.swing.JButton(); 
     buttonPrevious = new javax.swing.JButton(); 
     buttonNext = new javax.swing.JButton(); 
     buttonLast = new javax.swing.JButton(); 

     setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE); 

     textID.addActionListener(new java.awt.event.ActionListener() { 
      public void actionPerformed(java.awt.event.ActionEvent evt) { 
       textIDActionPerformed(evt); 
      } 
     }); 

     jLabel1.setText("Job Title"); 

     javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1); 
     jPanel1.setLayout(jPanel1Layout); 
     jPanel1Layout.setHorizontalGroup(
      jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) 
      .addGroup(jPanel1Layout.createSequentialGroup() 
       .addContainerGap() 
       .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false) 
        .addComponent(jLabel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE) 
        .addComponent(textID)) 
       .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED) 
       .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) 
        .addGroup(jPanel1Layout.createSequentialGroup() 
         .addComponent(textFirstName, javax.swing.GroupLayout.PREFERRED_SIZE, 48, javax.swing.GroupLayout.PREFERRED_SIZE) 
         .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED) 
         .addComponent(textLastName, javax.swing.GroupLayout.DEFAULT_SIZE, 232, Short.MAX_VALUE)) 
        .addComponent(textJobTitle)) 
       .addContainerGap()) 
     ); 
     jPanel1Layout.setVerticalGroup(
      jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) 
      .addGroup(jPanel1Layout.createSequentialGroup() 
       .addContainerGap() 
       .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE) 
        .addComponent(textID, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE) 
        .addComponent(textFirstName, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE) 
        .addComponent(textLastName, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)) 
       .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED) 
       .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE) 
        .addComponent(textJobTitle, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE) 
        .addComponent(jLabel1)) 
       .addContainerGap(38, Short.MAX_VALUE)) 
     ); 

     buttonFirst.setText("First"); 
     buttonFirst.addActionListener(new java.awt.event.ActionListener() { 
      public void actionPerformed(java.awt.event.ActionEvent evt) { 
       buttonFirstActionPerformed(evt); 
      } 
     }); 

     buttonPrevious.setText("Previous"); 

     buttonNext.setText("Next"); 
     buttonNext.addActionListener(new java.awt.event.ActionListener() { 
      public void actionPerformed(java.awt.event.ActionEvent evt) { 
       buttonNextActionPerformed(evt); 
      } 
     }); 

     buttonLast.setText("Last"); 

     javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane()); 
     getContentPane().setLayout(layout); 
     layout.setHorizontalGroup(
      layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) 
      .addGroup(layout.createSequentialGroup() 
       .addContainerGap() 
       .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) 
        .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE) 
        .addGroup(layout.createSequentialGroup() 
         .addComponent(buttonFirst) 
         .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED) 
         .addComponent(buttonPrevious) 
         .addGap(104, 104, 104) 
         .addComponent(buttonNext) 
         .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED) 
         .addComponent(buttonLast))) 
       .addContainerGap(30, Short.MAX_VALUE)) 
     ); 
     layout.setVerticalGroup(
      layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) 
      .addGroup(layout.createSequentialGroup() 
       .addContainerGap() 
       .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE) 
       .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED) 
       .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE) 
        .addComponent(buttonFirst) 
        .addComponent(buttonPrevious) 
        .addComponent(buttonNext) 
        .addComponent(buttonLast)) 
       .addContainerGap(160, Short.MAX_VALUE)) 
     ); 

     pack(); 
    }// </editor-fold>       

    private void textIDActionPerformed(java.awt.event.ActionEvent evt) {          
     // TODO add your handling code here: 
    }          

    private void buttonFirstActionPerformed(java.awt.event.ActionEvent evt) {            
     // TODO add your handling code here: 
    }           



    private void buttonNextActionPerformed(java.awt.event.ActionEvent evt) {           

     try 
     { 
      if (rs.next()) { // this is the line where I get the error 

      } 
      else 
      { 
       rs.previous(); 
       JOptionPane.showMessageDialog(Workers.this, "End of File"); 
      } 
     } 
     catch (SQLException err) { 
      JOptionPane.showMessageDialog(Workers.this, err.getMessage()); 
     }   
    }           


    public void DoConnect(){ 


     try { 

      String host = "jdbc:derby://localhost:1527/Employees"; 
      String uName = "robert"; 
      String uPass = "robert"; 

      Connection con = DriverManager.getConnection(host, uName, uPass); 

      String SQL = "SELECT * FROM APP.WORKERS"; 

      Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE ); 
      ResultSet rs = stmt.executeQuery(SQL); 

      rs.next(); 

      int id_col  = rs.getInt("ID"); 
      String first_name = rs.getString("First_Name"); 
      String last_name = rs.getString("Last_Name"); 
      String job  = rs.getString("Job_Title"); 

      // DISPLAY THE FIRST RECORD IN THE TEXT FIELDS 
      textID.setText(Integer.toString(id_col)); 
      textFirstName.setText(first_name); 
      textLastName.setText(last_name); 
      textJobTitle.setText(job); 


     } 
     catch(SQLException err){ 
      JOptionPane.showMessageDialog(Workers.this, err.getMessage()); 
     } 



    } 

    /** 
    * @param args the command line arguments 
    */ 
    public static void main(String args[]) { 
     /* Set the Nimbus look and feel */ 
     //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) "> 
     /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel. 
     * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
     */ 
     try { 
      for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) { 
       if ("Nimbus".equals(info.getName())) { 
        javax.swing.UIManager.setLookAndFeel(info.getClassName()); 
        break; 
       } 
      } 
     } catch (ClassNotFoundException ex) { 
      java.util.logging.Logger.getLogger(Workers.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); 
     } catch (InstantiationException ex) { 
      java.util.logging.Logger.getLogger(Workers.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); 
     } catch (IllegalAccessException ex) { 
      java.util.logging.Logger.getLogger(Workers.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); 
     } catch (javax.swing.UnsupportedLookAndFeelException ex) { 
      java.util.logging.Logger.getLogger(Workers.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); 
     } 
     //</editor-fold> 

     /* Create and display the form */ 
     java.awt.EventQueue.invokeLater(new Runnable() { 
      public void run() { 
       new Workers().setVisible(true); 
      } 
     }); 
    } 
    // Variables declaration - do not modify      
    private javax.swing.JButton buttonFirst; 
    private javax.swing.JButton buttonLast; 
    private javax.swing.JButton buttonNext; 
    private javax.swing.JButton buttonPrevious; 
    private javax.swing.JLabel jLabel1; 
    private javax.swing.JPanel jPanel1; 
    private javax.swing.JTextField textFirstName; 
    private javax.swing.JTextField textID; 
    private javax.swing.JTextField textJobTitle; 
    private javax.swing.JTextField textLastName; 
    // End of variables declaration     
}

來源

2013-10-11 Robbo_UK

當你調用'buttonNextActionPerformed'方法時，是否已經初始化了'ResultSet'？ –

不知道？我推測它在頂部宣佈？ –

它被聲明瞭，但它被初始化了嗎？可能不會。考慮你的程序流程。 –

在DoConnect（）中，你會隱藏你的'rs'變量。改爲寫：

rs = stmt.executeQuery(SQL); //without 'ResultSet' 
rs.next();

來源

2013-10-14 09:17:56 wrm

確保執行buttonNextActionPerformed(Workers.java:164)之前，無論如何rs必須得到初始化。您按照執行步驟和代碼進行確認。

來源

2013-10-14 09:10:58

java - homeandlearn數據庫教程 - rs.Next空指針異常

回答

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