爲什麼setTableHeader（）方法拋出NullPointerException？

Question

我嘗試將工具提示添加到我的JTable標頭。在這種情況下，我使用擴展JTableHeader Java類的TTHeader類。一切似乎都很好，但是當我嘗試向我的JTable添加新的TTHeader頭時，我得到的是帶有未知源的NullPointerException。我不知道爲什麼。 TTHeader類似乎沒問題。問題在別的地方。

這是我的方法代碼。 對於JTable中填入：

private JPanel contentPane; 
private JScrollPane scrollPane; 
private JTable table; 
private String tooltipsSDB[] = {"SessionID", "UserID", "PatientID", "PluginID", "Date", "Time"}; 

Connection conn = null; 
ResultSet rs = null; 
PreparedStatement ps = null; 

Table() { 
     // Connection Component 
     conn = ConnectionJDBC.ConnectDB(); 

     setTitle("My sample table"); 
     setSize(new Dimension(400, 400)); 
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     setLocationRelativeTo(null); 
     setVisible(true); 

     contentPane = new JPanel(); 
     contentPane.setLayout(new BorderLayout(0, 0)); 
     setContentPane(contentPane); 

     scrollPane = new JScrollPane(); 
     contentPane.add(scrollPane, BorderLayout.CENTER); 

     table = new JTable() { 
      public boolean isCellEditable(int row, int column) { 
       return false; 
      }; 
     }; 
     table.setAutoCreateRowSorter(true); 
     // Populate JTable with data from SQL DB 
     populateTable(); 
     scrollPane.setViewportView(table); 
} 

public void populateTable() { 
     String sql = "SELECT * FROM " + ExampleDatabase + " ORDER BY " + ExampleData + " DESC"; 
     try { 
      // Make Connection With DB 
      ps = conn.prepareStatement(sql); 
      rs = ps.executeQuery(); 
      ResultSetMetaData rsmetadata = rs.getMetaData(); 

      // Populate JTable 
      int columns = rsmetadata.getColumnCount(); 
      DefaultTableModel dtm = new DefaultTableModel(); 
      Vector columns_name = new Vector(); 
      Vector data_rows = new Vector(); 
      for (int i = 1; i <= columns; i++) { 
       columns_name.addElement(rsmetadata.getColumnName(i)); 
      } 
      dtm.setColumnIdentifiers(columns_name); 
      while (rs.next()) { 
       data_rows = new Vector(); 
       for (int j = 1; j <= columns; j++) { 
        data_rows.addElement(rs.getString(j)); 
       } 
       dtm.addRow(data_rows); 
      } 
      table.setModel(dtm); 

      // Create Header For JTable 
      TTHeader tth = new TTHeader(table.getColumnModel()); 
      tth.setToolTipStrings(tooltipsSDB); 
      table.setTableHeader(tth); // On This Line I Get NullPointerException With Uknown Source 

      table.repaint(); 
     } catch (SQLException e) { 
      JOptionPane.showMessageDialog(null, "Populate table error! \n" + e); 

     } 
} 

public static void main(String args[]) { 
     EventQueue.invokeLater(new Runnable() { 
      public void run() { 
       try { 
        new Table(); 
       } catch (Exception e) { 
        JOptionPane.showMessageDialog(null, e); 
       } 
      } 
     }); 
} 

而且TTHeader類：

String[] toolTips; 

public TTHeader(TableColumnModel model) { 
    super(model); 
} 

public String getToolTipText(MouseEvent e) { 
    int col = columnAtPoint(e.getPoint()); 
    int modelCol = getTable().convertColumnIndexToModel(col); 
    String retStr; 
    try { 
     retStr = toolTips[modelCol]; 
    } catch (NullPointerException ex) { 
     retStr = ""; 
    } catch (ArrayIndexOutOfBoundsException ex) { 
     retStr = ""; 
    } 
    if (retStr.length() < 1) { 
     retStr = super.getToolTipText(e); 
    } 
    return retStr; 
} 

public void setToolTipStrings(String[] toolTips) { 
    this.toolTips = toolTips; 
} 

當我選擇從JComboBox中第二個數據庫NPE出現。在JComboBox監聽器中，我調用populateTable（）方法。這裏是一個更具體的堆棧跟蹤：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
at mypackage.TTHeader.getToolTipText(TTHeader.java:19) 
at javax.swing.ToolTipManager$insideTimerAction.actionPerformed(Unknown Source) 
at javax.swing.Timer.fireActionPerformed(Unknown Source) 
at javax.swing.Timer$DoPostEvent.run(Unknown Source) 
at java.awt.event.InvocationEvent.dispatch(Unknown Source) 
at java.awt.EventQueue.dispatchEventImpl(Unknown Source) 
at java.awt.EventQueue.access$200(Unknown Source) 
at java.awt.EventQueue$3.run(Unknown Source) 
at java.awt.EventQueue$3.run(Unknown Source) 
at java.security.AccessController.doPrivileged(Native Method) 
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
at java.awt.EventQueue.dispatchEvent(Unknown Source) 
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
at java.awt.EventDispatchThread.run(Unknown Source) 

系NR 19：

int modelCol = getTable().convertColumnIndexToModel(col); 

Answer 1

當你得到一個NPE，並確定了行了，你檢查的第一件事是對其操作的所有出現（.） 。此操作員左側（！）的值爲null。 （如果你有一個在線多. s，則其中的一個。所以，如果您的線路是foo.getBar().doSomething()，要麼foo是null或foo.getBar()回報null。但是doSomething()回報也無所謂。）

在這種情況下，將可能是getTable()方法返回null。

由於Java 5出現了兩個新的，而更可能偷偷摸摸的原因 NPE的：

1. 在for (Foo foo : fooCollection)隱解引用，在這裏你會得到一個NPE如果fooCollection爲null。
2. 號的自動拆箱：Integer i = null; int j = i + 1; //NPE here

這就是所有你需要知道的關於一般的NPE的調試，有當然的更一般的建議，您應該登錄變量值或使用調試器一步一步地執行您的代碼。