如何解決這個運行時異常？

Question

如何解決此代碼的運行時異常？
當點擊按鈕，文件選擇，並從中添加文件的面板顏色消失（錯誤的事情在面板發生）

這是我的代碼：

import javax.swing.JPanel; 
import javax.swing.JButton; 
import javax.swing.JLabel; 
import java.awt.Color; 
import java.awt.Component; 
import java.awt.Desktop; 
import java.awt.datatransfer.DataFlavor; 
import java.awt.datatransfer.Transferable; 
import java.awt.datatransfer.UnsupportedFlavorException; 
import java.awt.dnd.DropTarget; 
import java.awt.dnd.DropTargetDragEvent; 
import java.awt.dnd.DropTargetDropEvent; 
import java.awt.dnd.DropTargetEvent; 
import java.awt.dnd.DropTargetListener; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 
import java.awt.event.MouseEvent; 
import java.awt.event.MouseListener; 
import java.io.File; 
import java.io.IOException; 
import java.util.List; 
import javax.swing.DefaultListCellRenderer; 
import javax.swing.DefaultListModel; 
import javax.swing.JList; 
import javax.swing.JScrollPane; 
import javax.swing.UIManager; 
import javax.swing.UnsupportedLookAndFeelException; 
import javax.swing.filechooser.FileSystemView; 
import javax.swing.border.BevelBorder; 
import javax.swing.JFileChooser; 
import javax.swing.plaf.FileChooserUI; 

public class pan extends JPanel implements DropTargetListener { 

private DefaultListModel listModel = new DefaultListModel(); 
private DropTarget dropTarget; 
private JScrollPane droparea; 
private JList list; 
private JButton addbutton; 


// Create the panel. 
//and add to it component 

public pan() { 
    setLayout(null); 
    addbutton = new JButton("New button"); 
    addbutton.setBounds(10, 10, 90, 100); 
    addbutton.addActionListener(new Action()); 
    add(addbutton); 

    list = new JList(); 
    dropTarget = new DropTarget(list, this); 
    list.setModel(listModel); 
    list.setDragEnabled(true); 
    FileListCellRenderer renderer = new FileListCellRenderer(); 
    list.setCellRenderer(renderer); 
    list.addMouseListener(new mouselistner()); 
    list.clearSelection(); 
    list.setFixedCellHeight(40); 
    droparea = new JScrollPane(); 
    droparea.setViewportView(list); 
    droparea.setBounds(10, 150, 635, 330); 
    add(droparea); 

}// </editor-fold> 
    // Variables declaration - do not modify 

// End of variables declaration 

public void dragEnter(DropTargetDragEvent arg0) { 
    // nothing 
} 

public void dragOver(DropTargetDragEvent arg0) { 
    // nothing 
} 

public void dropActionChanged(DropTargetDragEvent arg0) { 
    // nothing 
} 

public void dragExit(DropTargetEvent arg0) { 
    // nothing 
} 

public void drop(DropTargetDropEvent evt) { 
    int action = evt.getDropAction(); 
    evt.acceptDrop(action); 
    try { 
     Transferable data = evt.getTransferable(); 
     if (data.isDataFlavorSupported(DataFlavor.javaFileListFlavor)) { 
     List<File> files = (List<File>) data 
       .getTransferData(DataFlavor.javaFileListFlavor); 
      for (File file : files) { 
       listModel.addElement(file); 
      } 
     } 
    } catch (UnsupportedFlavorException e) { 
     e.printStackTrace(); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } finally { 
     evt.dropComplete(true); 
    } 
} 

/** A FileListCellRenderer for a File. */ 
class FileListCellRenderer extends DefaultListCellRenderer { 

    private static final long serialVersionUID = -7799441088157759804L; 
    private FileSystemView fileSystemView; 
    private JLabel label; 
    private Color textSelectionColor = Color.BLACK; 
    private Color backgroundSelectionColor = Color.CYAN; 
    private Color textNonSelectionColor = Color.BLACK; 
    private Color backgroundNonSelectionColor = Color.WHITE; 

    FileListCellRenderer() { 
     label = new JLabel(); 
     label.setOpaque(true); 
     fileSystemView = FileSystemView.getFileSystemView(); 
    } 

    @Override 
    public Component getListCellRendererComponent(JList list, Object value, 
      int index, boolean selected, boolean expanded) { 
     label.setBorder(new BevelBorder(BevelBorder.RAISED, 
       Color.LIGHT_GRAY, Color.GRAY, null, null)); 
     File file = (File) value; 
     label.setIcon(fileSystemView.getSystemIcon(file)); 
     label.setText(fileSystemView.getSystemDisplayName(file)); 
     label.setToolTipText(file.getPath()); 
     if (selected) { 
      label.setBackground(Color.blue); 
      label.setForeground(textSelectionColor); 
     } else { 
      label.setBackground(backgroundNonSelectionColor); 
      label.setForeground(textNonSelectionColor); 
     } 

     return label; 
    } 
} 

class Action implements ActionListener { 

    @Override 
    public void actionPerformed(ActionEvent e) { 
     if(e.getSource()==addbutton){ 
     // FileSystemView fsv=FileSystemView.getFileSystemView(); 

      JFileChooser filechooser=new JFileChooser(); 
      filechooser.setMultiSelectionEnabled(true); 
filechooser.setFileSelectionMode (JFileChooser.FILES_AND_DIRECTORIES); 
      File files=filechooser.getSelectedFile(); 
     filechooser.showDialog(null, "add"); //here panel problem i think 
     listModel.addElement(files); //and here run time error genrate 

     } 

    } 
} 
} 

這兩個問題在最後2行

Answer 1

問題是你在之前得到的選定文件你顯示你的JFileChooser對話框，所以選中的文件將是null。 SWAP這兩條線獲得：

int returnValue = filechooser.showDialog(null, "add"); 
if (returnValue == JFileChooser.APPROVE_OPTION) { 
    File files = filechooser.getSelectedFile(); 
    listModel.addElement("fooooo"); 
} 

而且更好的是，取消按鈕被點擊的情況下使用了JFileChooser返回值。