點擊滾動瀏覽數據庫--java Derby

我正在嘗試創建一個Java Derby數據庫，它必須在點擊一個按鈕時顯示下一條記錄。我下面的教程：
homeandlearn.co.uk/java/database_scrolling_buttons.html
在按鈕我有下面的代碼的方法：點擊滾動瀏覽數據庫--java Derby

String host = "jdbc:derby://localhost:1527/Employees"; 
    String uName = "bjorn"; 
    String uPass= ""; 
    con = DriverManager.getConnection(host, uName, uPass); 
    PreparedStatement pstmt = con.prepareStatement("SELECT * FROM App.Workers",ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_UPDATABLE); 

    ResultSet rs = pstmt.executeQuery(); 
    rs.next(); 
if(rs.next()){ 

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

    textID.setText(id); 
    textFirstName.setText(first_name); 
    textLastName.setText(last_name); 
    textJobTitle.setText(job); 

}

當我運行應用程序，並單擊按鈕一次，它顯示下一個記錄。但是，如果我再次點擊，應用程序崩潰。我想多次單擊按鈕時滾動瀏覽整個數據庫。我是否必須在此方法中再次聲明連接con和ResultSet rs？當我不這樣做的代碼變成：

if(rs.next()){ 

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

    textID.setText(id); 
    textFirstName.setText(first_name); 
    textLastName.setText(last_name); 
    textJobTitle.setText(job); 

}

現在我得到的錯誤信息：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
    at Employees.Workers.btnNextActionPerformed(Workers.java:255) 
    at Employees.Workers.access$200(Workers.java:19) 
    at Employees.Workers$3.actionPerformed(Workers.java:145) 
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018) 
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341) 
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402) 
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259) 
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252) 
    at java.awt.Component.processMouseEvent(Component.java:6505) 
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321) 
    at java.awt.Component.processEvent(Component.java:6270) 
    at java.awt.Container.processEvent(Container.java:2229) 
    at java.awt.Component.dispatchEventImpl(Component.java:4861) 
    at java.awt.Container.dispatchEventImpl(Container.java:2287) 
    at java.awt.Component.dispatchEvent(Component.java:4687) 
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832) 
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492) 
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422) 
    at java.awt.Container.dispatchEventImpl(Container.java:2273) 
    at java.awt.Window.dispatchEventImpl(Window.java:2719) 
    at java.awt.Component.dispatchEvent(Component.java:4687) 
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723) 
    at java.awt.EventQueue.access$200(EventQueue.java:103) 
    at java.awt.EventQueue$3.run(EventQueue.java:682) 
    at java.awt.EventQueue$3.run(EventQueue.java:680) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87) 
    at java.awt.EventQueue$4.run(EventQueue.java:696) 
    at java.awt.EventQueue$4.run(EventQueue.java:694) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76) 
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:693) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163) 
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151) 
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147) 
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139) 
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

線255分if(rs.next()){和錯誤顯示，因爲rs指向空對象。我如何讓rs指向數據庫中的上一條記錄，以便rs.next（）指向下一條記錄？

來源

2013-04-23 user1189952

如果應用程序崩潰，你需要幫助，你*必須*顯示完整的錯誤信息，並指出哪些行導致異常發生。否則，你是在要求我們猜測和閱讀頭腦，這是我們不擅長的事情。 – 2013-04-23 18:08:50

當我省略上面的代碼行if（rs.next（））{它顯示以下錯誤消息：線程「AWT-EventQueue-0」中的異常java.lang.NullPointerException \t at Employees.Workers.btnNextActionPerformed（Workers .java：255） \t at Employees.Workers.access $ 200（Workers.java:19） \t at Employees.Workers $ 3.actionPerformed（Workers.java:145）第255行指向行if（rs.next（））{因爲rs指向一個空對象。我該如何指向數據庫的以前記錄？ – user1189952 2013-04-23 18:23:50

所以你正試圖調用一個null ResultSet變量的方法。解決方案：不要這樣做 - 不要在空變量上調用方法。更大的問題，爲什麼變量爲空，可能不是我們可以幫助的東西，除非你顯示更多的代碼。你需要回溯你的代碼，看看你在哪裏*不*初始化這個變量。 – 2013-04-23 18:37:30

你知道，當你聲明，在這裏變量初始化RS：

String host = "jdbc:derby://localhost:1527/Employees"; 
String uName = "bjorn"; 
String uPass= ""; 
con = DriverManager.getConnection(host, uName, uPass); 
PreparedStatement pstmt = con.prepareStatement("SELECT * FROM App.Workers",ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_UPDATABLE); 

ResultSet rs = pstmt.executeQuery(); // ****** here 
rs.next();

你的RS變量是本地變量，所以你只初始化一個局部變量，不是一類領域。這個局部變量很可能會影響類字段，這就是爲什麼你會遇到你的問題。

如果是這樣，一個解決方案是不是要重新申報RS變量而是初始化RS類字段，不是一個局部變量：

// ResultSet rs = pstmt.executeQuery(); // ****** here 
rs = pstmt.executeQuery(); // ****** Note the difference

來源

2013-04-23 18:38:27

我如何讓程序知道我想指向類變量？ – user1189952 2013-04-23 18:40:52

@ user1189952：正如我上面所說的，不要*在本地重新聲明該字段。 – 2013-04-23 18:42:09

@ user1189952通過改變'ResultSet rs = pstmt.executeQuery（）;'到'rs = pstmt.executeQuery（）;'注意區別。 – 2013-04-23 18:48:12

點擊滾動瀏覽數據庫--java Derby

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