我正在尝试创建一个简单的Java应用程序，它允许我从Jtable标头（TableColumn）拖放并在JList中删除列的名称，这是代码：
import java.awt.BorderLayout;
import java.awt.Cursor;
import java.awt.EventQueue;
import javax.swing.DefaultListModel;
import javax.swing.DropMode;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.ListSelectionModel;
import javax.swing.TransferHandler;
import javax.swing.border.EmptyBorder;
import javax.swing.JScrollPane;
import java.awt.Dimension;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.awt.dnd.DnDConstants;
import java.awt.dnd.DragGestureEvent;
import java.awt.dnd.DragGestureListener;
import java.awt.dnd.DragSource;
import javax.swing.JList;
import javax.swing.JTable;
import javax.swing.table.DefaultTableModel;
import javax.swing.table.JTableHeader;
import javax.swing.table.TableColumn;
import java.io.IOException;
import java.util.Enumeration;
public class DnDTableToList extends JFrame implements DragGestureListener, Transferable{
private JPanel contentPane;
private JTable table;
private JList list;
DefaultListModel model;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
DnDTableToList frame = new DnDTableToList();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the frame.
*/
public DnDTableToList() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 672, 430);
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
setContentPane(contentPane);
contentPane.setLayout(new BorderLayout(0, 0));
JScrollPane scrollPane = new JScrollPane();
scrollPane.setPreferredSize(new Dimension(150, 2));
contentPane.add(scrollPane, BorderLayout.WEST);
model = new DefaultListModel();
list = new JList(model);
scrollPane.setViewportView(list);
JScrollPane scrollPane_1 = new JScrollPane();
contentPane.add(scrollPane_1, BorderLayout.CENTER);
table = new JTable();
table.getTableHeader().setReorderingAllowed(false);
table.setModel(new DefaultTableModel(
new Object[][] {
{"00001", "Mark", "Developper", "700"},
{"00002", "Anna", "Developper", "700"},
{"00003", "Jack", "Leader", "1200"},
},
new String[] {
"Code", "Name", "Post", "Salary"
}
));
scrollPane_1.setViewportView(table);
//---set drop mode for JList--------------------------
list.setDropMode(DropMode.INSERT);
list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
list.setTransferHandler(new ListHandler(model));
//----create drag source for JTableHeader-----------
DragSource ds = new DragSource();
ds.createDefaultDragGestureRecognizer(table.getTableHeader(), DnDConstants.ACTION_COPY, this);
}
@Override
public Object getTransferData(DataFlavor e) throws UnsupportedFlavorException, IOException {
return null;
}
@Override
public DataFlavor[] getTransferDataFlavors() {
System.out.println("getTransferDataFlavors");
return new DataFlavor[0];
}
@Override
public boolean isDataFlavorSupported(DataFlavor e) {
System.out.println("isDataFlavorSupported");
return false;
}
@Override
public void dragGestureRecognized(DragGestureEvent event) {
System.out.println("grag gesture");
Cursor cursor = null;
try {
JTableHeader th = (JTableHeader) event.getComponent();
Enumeration<?> e = th.getColumnModel().getColumns();
while (e.hasMoreElements()) {
TableColumn tc = (TableColumn) e.nextElement();
System.out.println(tc.getHeaderValue());
}
TableColumn tb = th.getDraggedColumn();
System.out.println("dragged column: "+tb.getHeaderValue());//---This is where the exception happens
if (event.getDragAction() == DnDConstants.ACTION_COPY) {
cursor = DragSource.DefaultCopyDrop;
}
event.startDrag(cursor, new TransferableTableColumn(tb));
} catch (Exception e) {
e.printStackTrace();
}
}
}
/**
* Custom TransferHandler for JList
*
*/
class ListHandler extends TransferHandler {
private DefaultListModel model;
public ListHandler(DefaultListModel model) {
this.model = model;
}
public boolean canImport(TransferSupport support) {
if (!support.isDrop()) {
return false;
}
return support.isDataFlavorSupported(TransferableTableColumn.tableColumnFlavor);
}
public boolean importData(TransferSupport support) {
if (!canImport(support)) {
System.out.println("!canImport() -> return false");
return false;
}
TableColumn tb;
String line;
try {
Transferable transferable = support.getTransferable();
tb = (TableColumn) transferable.getTransferData(TransferableTableColumn.tableColumnFlavor);
line = (String) tb.getHeaderValue();
JOptionPane.showMessageDialog(null, line);
} catch (Exception e) {
e.printStackTrace();
return false;
}
JList.DropLocation dl = (JList.DropLocation) support.getDropLocation();
int index = dl.getIndex();
model.add(index++, line);
JOptionPane.showMessageDialog(null, line);
return true;
}
}
/**
* Custom Transferable with TableColumn DataFlavor
*
*/
class TransferableTableColumn implements Transferable {
protected static DataFlavor tableColumnFlavor = new DataFlavor(TableColumn.class, "A TableColumn Object");
protected static DataFlavor[] supportedFlavors = {
tableColumnFlavor,
DataFlavor.stringFlavor,
};
TableColumn tabColumn;
public TransferableTableColumn(TableColumn tabColumn) { this.tabColumn = tabColumn; }
public DataFlavor[] getTransferDataFlavors() { return supportedFlavors; }
public boolean isDataFlavorSupported(DataFlavor flavor) {
if (flavor.equals(tableColumnFlavor) ||
flavor.equals(DataFlavor.stringFlavor)) return true;
return false;
}
public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException{
System.out.println("getTransferData");
if (flavor.equals(tableColumnFlavor))
return tabColumn;
else if (flavor.equals(DataFlavor.stringFlavor))
return tabColumn.getHeaderValue();
else
throw new UnsupportedFlavorException(flavor);
}
}
但是当我尝试获取拖动列的名称时，我得到NullPointerException
异常堆栈跟踪：
java.lang.NullPointerException
at pck.DnDTableToList.dragGestureRecognized(DnDTableToList.java:137)
at java.awt.dnd.DragGestureRecognizer.fireDragGestureRecognized(Unknown Source)
at sun.awt.windows.WMouseDragGestureRecognizer.mouseDragged(Unknown Source)
at java.awt.AWTEventMulticaster.mouseDragged(Unknown Source)
at java.awt.AWTEventMulticaster.mouseDragged(Unknown Source)
at java.awt.Component.processMouseMotionEvent(Unknown Source)
at javax.swing.JComponent.processMouseMotionEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$400(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.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.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)
我错过了什么？
答案 0 :(得分：0)
快速查看JavaDoc：http://docs.oracle.com/javase/7/docs/api/java/awt/dnd/DragGestureEvent.html
它表明该事件有一个方法startDrag()。并在：
http://docs.oracle.com/javase/7/docs/api/javax/swing/table/JTableHeader.html#getDraggedColumn()它说：
当且仅当拖动正在进行时，返回拖动的列，否则返回null。
所以，我猜您需要先致电event.startDrag()才能从getDraggedColumn()获取任何内容。
请查看：http://www.programcreek.com/java-api-examples/index.php?api=java.awt.dnd.DragGestureEvent（并搜索其他教程或代码示例）。