java - 各位大神，QuizCardPlayer 程序，编译没有问题；但begin时，showNextCard ( )就会出现错误 。

Question

编译没有问题，请大神帮忙运行一下

made a card

    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at QuizCardPlayer.showNextCard(QuizCardPlayer.java:172)
    at QuizCardPlayer$NextCardListener.actionPerformed(QuizCardPlayer.java:106

---

import javax . swing . *;        
import java . awt . *;
import java . awt . event . * ;
import java . io . * ; 
import java . util . * ;
 
public class QuizCardPlayer
{
    JTextArea display ;                        
    JTextArea answer ;
    ArrayList<QuizCard> cardList  ;
    JFrame frame;   
    boolean isShowAnswer ;
    int currentCardIndex ;
    QuizCard currentCard;
    JButton nextButton;
                                                         
    public static void main (String[] args)
    {
        QuizCardPlayer qcp = new QuizCardPlayer ( );
        qcp . go ( );
    }
     
    public void go ( )
    {
        frame = new JFrame ("Quiz Card Player");
        frame . setDefaultCloseOperation (JFrame . EXIT_ON_CLOSE);
        Font bigFont = new Font ("微软雅黑" , Font . PLAIN , 15 ) ;
         
        JPanel mainPanel = new JPanel ( ) ;                                        
                 
        JTextArea display = new JTextArea ( 10 , 10) ;   
        display . setLineWrap (true) ;
        display . setWrapStyleWord (true) ;
        display . setFont (bigFont) ;   
        JScrollPane qScroller = new JScrollPane ( display ) ;
        qScroller . setVerticalScrollBarPolicy (ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS) ;        
        qScroller . setHorizontalScrollBarPolicy (ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER) ; 
                 
        JButton nextButton = new JButton ("Begin") ; 
        nextButton . addActionListener ( new NextCardListener ( ) );
         
        JMenuBar menuBar = new JMenuBar ( ) ;    
        JMenu fileMenu = new JMenu ("file");       
         
        JMenuItem loadMenuItem = new JMenuItem ("Load card set");
        loadMenuItem . addActionListener ( new LoadMenuListener ( ) );
         
        fileMenu . add (loadMenuItem);
        menuBar . add (fileMenu);
         
        mainPanel . setLayout ( new BoxLayout (mainPanel , BoxLayout . Y_AXIS)) ;
         
        mainPanel. add (menuBar) ; 
        mainPanel. add (qScroller);
        mainPanel. add (nextButton);
         
        frame. getContentPane ( ) . add (BorderLayout . CENTER, mainPanel) ;
        frame . setSize (500, 600) ; 
        frame . setVisible (true);
         
 
    }
     
    public class LoadMenuListener implements ActionListener       
    {
        public void actionPerformed (ActionEvent ev)     
                                                 
        {                                            
            JFileChooser fileOpen = new JFileChooser ( );
            fileOpen . showOpenDialog (frame)  ;            
            loadFile (fileOpen. getSelectedFile ( ));
        }
    }
    public class NextCardListener implements ActionListener 
    {
        public void actionPerformed (ActionEvent ev)
        {
            if (isShowAnswer)
            {
                display . setText (currentCard . getAnswer ( ) );
                nextButton . setText ("Next Card");
                isShowAnswer = false;
            }
            else
            {
                if (currentCardIndex < cardList . size ( ))
                {
                    showNextCard ( );
                }
                else
                {
                    display . setText ("That's the last card ");
                    nextButton .setEnabled (false);
                }
            }   
             
             
        }   
    }
     
    void loadFile (File file)   
    {
        cardList = new ArrayList <QuizCard> ( );
        try
        {
            BufferedReader reader = new BufferedReader (new FileReader (file)) ;
            String line = null ;
            while ( (line = reader . readLine ( )) != null)
            {
                makeCard (line) ;
            }
            reader . close ( ) ;
        }
        catch (Exception ex)
        {
            System. out . println ("Could't make a card") ;
            ex. printStackTrace ( ) ;
        }
    }
     
    void makeCard ( String lineToParse )    
    {
        String [ ] result = lineToParse . split ("/") ; 
        /*display = new JTextArea ( ) ;                       
        QuizCard card = new QuizCard (result [0] , result [1] ) ;      
        cardList . add (car
        System .out. println ("made a card") ;           
    }
    void showNextCard ( )
    {
        currentCard = cardList . get (currentCardIndex);
        currentCardIndex ++;
        display . setText (currentCard . getQuestion ( ));
        nextButton . setText ("Show Answer");
        isShowAnswer = true;
    }
}
 
class QuizCard
{
    String question;
    String answer;
     
    public QuizCard (String q , String a) 
    {
        question = q;
        answer = a;
    }
     
    public String getQuestion ( )
    {
        return question;
    }
     
    public String getAnswer ( )
    {
        return answer ; 
    }
}