Java ResultSet正在抛出空指针，但next（）返回true

我试图建立一个小JDBC项目，但遇到了一个让我困惑的错误

这是一个词汇和多项选择训练器。 我的多项选择题运行得很好，但我几乎是从中抄来的一个片段对我的词汇部分不起作用

这是：

private void createIDList() throws ClassNotFoundException, SQLException {

    Class.forName("org.apache.derby.jdbc.ClientDriver");
    con = DriverManager.getConnection("jdbc:derby://localhost:1527/DataBase");
    stmt = con.createStatement();
    String s_s = "SUB LIKE '%";
    s_s = s_s + db_s.get(0) + "%' ";
    db_s.remove(0);
    while (!db_s.isEmpty()) {
        s_s = s_s + "OR SUB LIKE '%" + db_s.get(0) + "%' ";
        db_s.remove(0);
    }
    String s_t = "TOPIC LIKE '%";
    s_t = s_t + db_t.get(0) + "%' ";
    db_t.remove(0);
    while (!db_t.isEmpty()) {
        s_t = s_t + "OR TOPIC LIKE '%" + db_t.get(0) + "%' ";
        db_t.remove(0);            
    }
    rs = stmt.executeQuery("SELECT ID FROM APP.Voc WHERE (" + s_t + ") AND (" + s_s + ")");

    while (rs.next()) {
        id.add(rs.getInt("ID")); //Exception is here java.lang.NullPointerException
    }}

以下是之前发生的部分：

public class Session extends javax.swing.JFrame implements KeyListener {
int p_r = 0;
int p_w = 0;
int currentID = -1;
private List db_t, db_s;
private List<Integer> id;
Connection con;
Statement stmt;
ResultSet rs;
String from = "V_ORIG";
String to = "V_DEST";
String ans;

/**
 * Creates new form Session
 */
public Session(List p_db_t, List p_db_s) {
    db_t = p_db_t;
    db_s = p_db_s;
    initialize(); //-->

我确实收到以下错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Session.createIDList(Session.java:520)
at Session.initialize(Session.java:489)
at Session.<init>(Session.java:45)
at DB_Selector.bt_goActionPerformed(DB_Selector.java:193)
at DB_Selector.access$100(DB_Selector.java:18)
at DB_Selector$2.actionPerformed(DB_Selector.java:83)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
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:6533)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6298)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

线程“AWT-EventQueue-0”java.lang.NullPointerException中的异常 在Session.createIDList（Session.java:520）中 初始化（Session.java:489） 在会话中。（Session.java:45） 在DB_Selector.bt_执行的操作（DB_Selector.java:193） 在DB_Selector.access$100（DB_Selector.java:18） 数据库选择器$2.actionPerformed（数据库选择器.java:83） 在javax.swing.AbstractButton.fireActionPerformed（AbstractButton.java:2022） 位于javax.swing.AbstractButton$Handler.actionPerformed（AbstractButton.java:2348） 在javax.swing.DefaultButtonModel.fireActionPerformed上（DefaultButtonModel.java:402） 在javax.swing.DefaultButtonModel.setPressed中（DefaultButtonModel.java:259） 在javax.swing.plaf.basic.BasicButtonListener.MouseRelease（BasicButtonListener.java:252） 位于java.awt.Component.ProcessMouseeEvent（Component.java:6533） 位于javax.swing.JComponent.ProcessMouseeEvent（JComponent.java:3324） 位于java.awt.Component.processEvent（Component.java:6298） 位于java.awt.Container.processEvent（Container.java:2236） 位于java.awt.Component.dispatchEventImpl（Component.java:4889） 位于java.awt.Container.dispatchEventImpl（Container.java:2294） 位于java.awt.Component.dispatchEvent（Component.java:4711） 位于java.awt.LightweightDispatcher.RetargetMouseeEvent（Container.java:4888） 位于java.awt.LightweightDispatcher.ProcessMouseeEvent（Container.java:4525） 位于java.awt.LightweightDispatcher.dispatchEvent（Container.java:4466） 位于java.awt.Container.dispatchEventImpl（Container.java:2280） 位于java.awt.Window.dispatchEventImpl（Window.java:2746） 位于java.awt.Component.dispatchEvent（Component.java:4711） 位于java.awt.EventQueue.dispatchEventImpl（EventQueue.java:758） 在java.awt.EventQueue.access$500（EventQueue.java:97） 在java.awt.EventQueue$3.run（EventQueue.java:709） 在java.awt.EventQueue$3.run（EventQueue.java:703） 位于java.security.AccessController.doPrivileged（本机方法） 位于java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege（ProtectionDomain.java:80） 位于java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege（ProtectionDomain.java:90） 在java.awt.EventQueue$4.run（EventQueue.java:731） 在java.awt.EventQueue$4.run（EventQueue.java:729） 位于java.security.AccessController.doPrivileged（本机方法） 位于java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege（ProtectionDomain.java:80） 位于java.awt.EventQueue.dispatchEvent（EventQueue.java:728） 位于java.awt.EventDispatchThread.pumpOneEventForFilters（EventDispatchThread.java:201） 在java.awt.EventDispatchThread.pumpEventsForFilter（EventDispatchThread.java:116）中 位于java.awt.EventDispatchThread.PumpeEventsforHierarchy（EventDispatchThread.java:105） 位于java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:101） 位于java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:93） 在java.awt.EventDispatchThread.run（EventDispatchThread.java:82） 这么好，但让我困惑的是： 第一：为什么它会进入while（rs.next（））循环。rs.next（）在遇到null指针之前不应该返回false吗？ 并且：我可以使用rs.getMetaData（）.getColumnCount（）获得列数。 因此，数据库和结果集之间必须进行某种通信

当我手动执行查询时，我会得到一个ID列表

这里的错误在哪里

---

如果您确实需要更多信息，请随时询问。

您的

id

为

null

使用

id=newarraylist（）；
while（rs.next（））{
id.add（rs.getInt（“id”）；
}

您的

id

为

null

使用

id=newarraylist（）；
while（rs.next（））{
id.add（rs.getInt（“id”）；
}

天哪，这太简单了，我怎么能监督呢。多谢各位！我在这上面浪费了这么多时间。天哪，这太简单了，我怎么能监督这件事呢。多谢各位！我在这件事上浪费了太多时间。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Session.createIDList(Session.java:520)
at Session.initialize(Session.java:489)
at Session.<init>(Session.java:45)
at DB_Selector.bt_goActionPerformed(DB_Selector.java:193)
at DB_Selector.access$100(DB_Selector.java:18)
at DB_Selector$2.actionPerformed(DB_Selector.java:83)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
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:6533)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6298)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

id = new ArrayList<>();
while (rs.next()) {
    id.add(rs.getInt("ID"));
}