当我在JTable中设置超过2列时，有谁知道为什么我的JTable抛出NullPointerException？

Question

我做了一些调查，问题似乎位于DefaultTableModel。具体来说，在getColumnClass方法中。当我在columnNames String数组中仅使用2列时：

(String[] columnNames = {"Icon", "Filename"}) 

一切正常。但是当我添加第三列时：

(String[] columnNames = {"Icon", "Filename", "Size"}): 

我开始收到NullPointerException错误。

我做错了什么？

package jtablefun;

import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.io.File;
import javax.swing.Icon;
import javax.swing.JFrame;
import javax.swing.JTable;
import javax.swing.filechooser.FileSystemView;
import javax.swing.table.DefaultTableModel;


public class JtableFun extends JFrame{

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    // TODO code application logic here
   new JtableFun().show();
}

public JtableFun(){
    getContentPane().setLayout(new GridBagLayout());

    //String s[] = f.list();




    buildTable();

    pack();

    addWindowListener(new WindowAdapter(){

        public void windowClosing(WindowEvent e){
            System.exit(0);
        }

    });

}



public void buildTable(){
             File f = new File("/");
             File[] fl = f.listFiles();

             String fileNames[] = f.list();
             Icon iconArray[] = new Icon[fileNames.length];
            //Icon icon = FileSystemView.getFileSystemView().getSystemIcon(f);
            for(int i=0; i<fileNames.length; i++){

                iconArray[i] = FileSystemView.getFileSystemView().getSystemIcon(fl[i]);

            }


    String[] columnNames = {"Icon", "Filename", "Size"};


    Object[][] data = new Object[fl.length][(fl.length * 2)];

    for(int row=0; row<fl.length; row++){

         for(int column=0; column<2; column++){
             if(column == 0){
             data[row][column]= iconArray[row];
                     }else if(column == 1){
                        data[row][column]= fileNames[row]; 
                     }else if(column == 3){
                        //data[row][column]= fl[row].length(); 
                     }
         }

    }


            DefaultTableModel model = new DefaultTableModel(data, columnNames)
    {
        //  Returning the Class of each column will allow different
        //  renderers to be used based on Class
        public Class getColumnClass(int column)
        {
            return getValueAt(0, column).getClass();
        } 
    }; 
                    JTable table = new JTable( model );
                            table.setPreferredScrollableViewportSize(table.getPreferredSize());
    GridBagConstraints g = new GridBagConstraints();
    g.gridx =0;
    g.gridy =0;
    getContentPane().add(table,g);
}
}