ResultSet 抛出空指针，但 next() returns 为真

Question

我正在尝试建立一个小 JDBC 项目，但运行遇到了一个让我感到困惑的错误。

这是一个词汇和选择题训练器。我的多项选择题运行很好，但是我几乎从中复制的一个片段对我的词汇部分不起作用。

这里是：

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
    }}

这是之前发生的部分：

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(); //-->

并初始化():

private void initialize() {
    try {
        createIDList();
    } catch (ClassNotFoundException ex) {
        Logger.getLogger(Session.class.getName()).log(Level.SEVERE, null, ex);
    } catch (SQLException ex) {
        Logger.getLogger(Session.class.getName()).log(Level.SEVERE, null, ex);
    }
    Collections.shuffle(id);
}

我收到以下错误：

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.access0(DB_Selector.java:18)
at DB_Selector.actionPerformed(DB_Selector.java:83)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6533)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6298)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access0(EventQueue.java:97)
at java.awt.EventQueue.run(EventQueue.java:709)
at java.awt.EventQueue.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
at java.awt.EventQueue.run(EventQueue.java:731)
at java.awt.EventQueue.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$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)

非常好，但让我困惑的是：第一：为什么会进入while(rs.next())循环。 rs.next() return false 在运行进入 NullPointer 之前不应该吗？并且：我可以使用 rs.getMetaData().getColumnCount() 获取列数。所以数据库和结果集之间必须有某种通信。

当我手动执行查询时，我确实得到了一个 ID 列表。

这里可能哪里出错了？

如果您确实需要更多信息，请随时询问。

Answer 1

你的id是null。

使用

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

ResultSet 抛出空指针，但 next() returns 为真

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