一段Swing程序无法实现触发按钮事件,该如何处理

import javax.swing.*;import java.awt.*;import java.awt.event.*;import javax.swing.border.*;import java.util.*;public class OOPPanel extends JPanel implements ActionListener{  private String[] OPPs={"中北大学","北京邮电大学","西安电子科技大学"};  private String[] explains={"中北大学通信工程排名60","北京邮电大学通信工程排名第一","西安电子科技大学通信工程排名第二"};  private LinkedList<String> explainList=new LinkedList<String>(Arrays.asList(explains));//增添到俩链接表  private JPanel comboBoxPanel,buttonPanel,button2Panel,textPanel,textAreaPanel;  private JComboBox comboBox;  private JButton newItemButton ,exitButton,button2;  private JTextArea explainTextArea,displayTextArea;  private Border loweredBorder;  private JTextField textField;  public OOPPanel()//构造函数  {   Border loweredBorder=BorderFactory.createBevelBorder(BevelBorder.LOWERED);   createGUIComponents();  }  public void createGUIComponents()  {   setLayout(new GridBagLayout());//创建网络包布局管理   GridBagConstraints c=new GridBagConstraints();   JPanel comboBoxPanel=new JPanel();//创建下拉列表控制板   comboBoxPanel.setBorder(BorderFactory.createTitledBorder(loweredBorder,"选择一所大学准备考研"));   comboBoxPanel.setLayout(new GridLayout(1,2));   JComboBox comboBox=new JComboBox(OPPs);   comboBoxPanel.add(comboBox);   c=setupConstraints(0,0,2,1,GridBagConstraints.WEST);   add(comboBoxPanel,c);   comboBox.addActionListener(this);   JPanel buttonPanel =new JPanel();   buttonPanel.setLayout(new FlowLayout(FlowLayout.LEFT));//嵌套一个流程布局并靠左对齐   JButton newItemButton=new JButton("newItem");   JButton exitButton=new JButton("exit");   buttonPanel.add(newItemButton);   buttonPanel.add(exitButton);   c=setupConstraints(0,3,2,1,GridBagConstraints.WEST);   add(buttonPanel,c);   exitButton.addActionListener(this);   newItemButton.addActionListener(this);   JPanel textPanel =new JPanel();//创建文本字段控制面板   textPanel.setLayout(new GridLayout(1,2));   JLabel label=new JLabel("new term");   JTextField textField=new JTextField(8);   textPanel.add(label);   textPanel.add(textField);   c=setupConstraints(0,0,2,1,GridBagConstraints.WEST);   add(textPanel,c);   textPanel.setVisible(false);//默认为不显示   JPanel textAreaPanel=new JPanel();   textAreaPanel.setLayout(new FlowLayout(FlowLayout.LEFT));   explainTextArea=setupTextArea(explainTextArea,3,15,true);//调用自定义方法处理文本窗口   textAreaPanel.add(explainTextArea);   c=setupConstraints(0,1,2,3,GridBagConstraints.WEST);   add(textAreaPanel,c);   textAreaPanel.setVisible(false);   JPanel button2Panel=new JPanel();//创建第二个按钮控制板   JButton button2=new JButton("submit");   button2Panel.setLayout(new FlowLayout(FlowLayout.LEFT));   button2Panel.add(button2);   button2.addActionListener(this);   c=setupConstraints(2,3,1,1,GridBagConstraints.WEST);   add(button2Panel,c);   button2Panel.setVisible(false);   displayTextArea=setupTextArea(displayTextArea,5,20,true);   comboBox.setSelectedIndex(0);   updateTextArea(0);   c=setupConstraints(3,0,3,5,GridBagConstraints.WEST);   add(displayTextArea,c);  }  private JTextArea setupTextArea(JTextArea textArea, int rows, int cols,boolean editable)  {   textArea =new JTextArea(rows,cols);   textArea.setLineWrap(true);//将超长的行折回到下一行   textArea.setWrapStyleWord(true);//按字折会   textArea.setEditable(editable);   textArea.setBorder(BorderFactory.createTitledBorder(loweredBorder,"Explaination"));   return textArea;  }  private void updateTextArea(int index)  {   displayTextArea.setText(explainList.get(index));   displayTextArea.setVisible(true);  }  private GridBagConstraints setupConstraints(int gridx, int gridy,int gridwidth, int gridheight, int anchor)  {   GridBagConstraints c=new GridBagConstraints();   c.gridx=gridx;   c.gridy=gridy;   c.gridwidth=gridwidth;   c.gridheight=gridheight;   c.insets=new Insets(5,5,5,5);//默认组件周围空隙5个像素   c.ipadx=c.ipady=0;//设置组件内部周边空隙为5个像素   c.anchor=anchor;//微调组件在网格空隙   return c;  }  public void actionPerformed (ActionEvent e)  {   Object source=e.getSource();   if(source==exitButton)   {    System.exit(0);    System.out.println("exitbutton");   }   else if(source==comboBox)   {    int index=comboBox.getSelectedIndex();    updateTextArea(index);   }   else if(source==newItemButton)   {    comboBoxPanel.setVisible(false);    buttonPanel.setVisible(false);    displayTextArea.setVisible(false);    textPanel.setVisible(true);    textAreaPanel.setVisible(true);    button2Panel.setVisible(true);   }   else if(source==button2)   {    comboBox.addItem(textField.getText());    explainList.addLast(explainTextArea.getText());    comboBoxPanel.setVisible(false);    buttonPanel.setVisible(false);    displayTextArea.setVisible(false);    textPanel.setVisible(true);    textAreaPanel.setVisible(true);    button2Panel.setVisible(true);   }  }}