ResultSetがNullPointersをスローしていますが、next（）がtrueを返します

Question

JDBCプロジェクトを少しセットアップしようとしていますが、私は混乱しているエラーに遭遇しています。

これは語彙と複数の選択肢のトレーナーです。 私はMultiple Choiceをうまく走らせていますが、ほぼそこからコピーしたものを私の語彙の部分ではうまくいきません。ここで

それは次のとおりです。私は、エラー、次のreceieveん

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); 
} 

：

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(); //--> 

と初期化（）：ここで

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 
    }} 

が前に起こる一部です

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) 

あまりにも良いですが、混乱しているのは次のとおりです。 最初に、なぜwhile（rs.next（））ループに入るのですか？ rs.next（）はNullPointerを実行する前にfalseを返すべきではありませんか？ そして：rs.getMetaData（）。getColumnCount（）で列の数を取得できます。 したがって、データベースとResultSetの間には何らかの通信が必要です。

私がQuerysを手動で実行すると、私はIDのリストを取得します。

ここでエラーが発生する可能性がありますか？

詳細が必要な場合は、お気軽にお問い合わせください。

Answer 1

idはnullです。私はこれを監督する可能性がどのように

使用

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