使用jfile选择器时出现nullpointer异常

Question

我通过单击btn obt获取nullpointer异常，这是由jfilechooser抛出的nullpointerexception。不知道该怎么做这个问题＆＃34; re = jf.showOpenDialog（null）;＆＃34;  任何人都可以请这个错误吗？

public void actionPerformed(ActionEvent e)
{
    if (e.getSource()==obt) 
    { 

        try
        {
        int re = jf.showOpenDialog(null);   
            if(re==JFileChooser.APPROVE_OPTION)
            {
                File f=jf.getSelectedFile();
                t1.append("Opening:"+f.getName()+ ".");
            }
            else
            {
                t1.append("Open command cancelled by user." );
            }

            t1.setCaretPosition(t1.getDocument().getLength());
        }
        catch(Exception ee)
        {
            System.out.println(ee);
        }
    }
    else if(e.getSource()==ebt)
    {
        try
        {
            int re=jf2.showSaveDialog(null);
            if(re==JFileChooser.APPROVE_OPTION)
            {
                File f=jf2.getSelectedFile();
                t1.append("Extracting:");//+f.getName()+ ".");
            }
            else
            {
                t1.append("Extract command cancelled by user.");
            }

            t1.setCaretPosition(t1.getDocument().getLength());
            }
        catch(Exception ee)
        {
            System.out.println(ee);
        }
    }
}