JComboBox：如何解决 JComboBox 和 ItemListener 中的异常

Question

import java.awt.Font;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
public class AddBook extends JPanel implements ActionListener
{
    JLabel book_name_lbl,author_lbl,isbn_lbl,available_lbl,new_lbl,total_book_lbl,title_lbl;
    JTextField isbn_text,available_text,new_text,total_book_text;
    JComboBox book_name_box,author_box;
    JButton submit_btn,exit_btn;
        Connection conn;
        Statement smtp;
        ResultSet rs;
        PreparedStatement psmtp;
        String book,auther;
    public AddBook(Connection c)
    {
            conn = c;

        setLayout(new GridBagLayout());

        GridBagConstraints gbc1 = new GridBagConstraints();
        gbc1.insets = new Insets(10,10,10,10);

        title_lbl = new JLabel("Book Record Update");
        title_lbl.setFont(new Font("Goudy Old Style",Font.BOLD,30));
        gbc1.gridx = 0;
        gbc1.gridy = 0;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.gridwidth = 2;
        gbc1.insets = new Insets(10,3,30,3);
        gbc1.anchor = GridBagConstraints.CENTER;
        add(title_lbl,gbc1);

        book_name_lbl = new JLabel("Book Name :");
        gbc1.gridx = 0;
        gbc1.gridy = 1;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.gridwidth = 1;
        gbc1.insets = new Insets(10,10,10,10);
        gbc1.anchor = GridBagConstraints.WEST;
        add(book_name_lbl,gbc1);

        book_name_box = new JComboBox();
                book_name_box.addItem("Select book");
            try
        {
            smtp = conn.createStatement();
            rs = smtp.executeQuery("select BOOK_NAME from BOOK_NAME");

            while(rs.next())
            {
                String book_name = rs.getString("BOOK_NAME");
                                book_name_box.addItem(book_name);
            }
        }
        catch(SQLException e)
        {
            e.printStackTrace();
        }
               book_name_box.addActionListener(this);

        gbc1.gridx = 1;
        gbc1.gridy = 1;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.EAST;
        gbc1.fill = GridBagConstraints.HORIZONTAL;
        add(book_name_box,gbc1);

        author_lbl = new JLabel("Author Name :");
        gbc1.gridx = 0;
        gbc1.gridy = 2;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.WEST;
        add(author_lbl,gbc1);
// author combox add
                author_box = new JComboBox();
                author_box.addItem("Select Author");
                 author_box.addActionListener(this);
        gbc1.gridx = 1;
        gbc1.gridy = 2;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.EAST;
        add(author_box,gbc1);

        isbn_lbl = new JLabel("ISBN :");
        gbc1.gridx = 0;
        gbc1.gridy = 3;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.WEST;
        add(isbn_lbl,gbc1);

        isbn_text = new JTextField(15);
                isbn_text.setEditable(true);
                isbn_text.setEditable(true);
        gbc1.gridx = 1;
        gbc1.gridy = 3;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.EAST;
        add(isbn_text,gbc1);

        new_lbl = new JLabel("New Book :");
        gbc1.gridx = 0;
        gbc1.gridy = 4;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.WEST;
        add(new_lbl,gbc1);

        new_text = new JTextField(15);
        gbc1.gridx = 1;
        gbc1.gridy = 4;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.EAST;
        add(new_text,gbc1);

        available_lbl = new JLabel("Available Book :");
        gbc1.gridx = 0;
        gbc1.gridy = 5;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.WEST;
        add(available_lbl,gbc1);

        available_text = new JTextField(15);
                available_text.setEditable(false);
        gbc1.gridx = 1;
        gbc1.gridy = 5;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.EAST;
        add(available_text,gbc1);

        total_book_lbl = new JLabel("Total Book :");
        gbc1.gridx = 0;
        gbc1.gridy = 6;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.WEST;
        add(total_book_lbl,gbc1);

        total_book_text = new JTextField(15);
                total_book_text.setEditable(false);
        gbc1.gridx = 1;
        gbc1.gridy = 6;
        gbc1.ipadx = 0;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.EAST;
        add(total_book_text,gbc1);

        submit_btn = new JButton("Submit");
                //submit_btn.setEnabled(false);
        gbc1.gridx = 0;
        gbc1.gridy = 7;
        gbc1.ipadx = 20;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.WEST;
        add(submit_btn,gbc1);

        exit_btn = new JButton("Reset");
        gbc1.gridx = 1;
        gbc1.gridy = 7;
        gbc1.ipadx = 20;
        gbc1.ipady = 0;
        gbc1.anchor = GridBagConstraints.CENTER;
        gbc1.fill = GridBagConstraints.NONE;
        add(exit_btn,gbc1);

    }
    public void actionPerformed(ActionEvent e)
    {
        if((JComboBox) e.getSource()==book_name_box)
        {
                        try
                        {
                            while(author_box.getItemCount()!=0)
                            {
                author_box.removeItemAt(0);
                            }
                            author_box.addItem("Select Author");
                            book=(String) book_name_box.getSelectedItem();
                          if(!(book.equals("")) || !(book.equals("Select book")))
                            {

                                    psmtp = conn.prepareStatement("select AUTHOR_NAME from BOOK_TABLE where BOOK_NAME=?");
                                    psmtp.setString(0,book);
                                    rs=psmtp.executeQuery();
                                    while(rs.next())
                                    {
                                            String auther_name = rs.getString("AUTHOR_NAME").toString();
                                            author_box.addItem(auther_name);
                                    }
                            }
                            else
                            {
                                    isbn_text.setText("");
                                    new_text.setText("");
                                    available_text.setText("");
                                    total_book_text.setText("");
                                    //submit_btn.setEnabled(false);
                            }
                        }
                        catch(SQLException se)
                        {

                        }
        }
                else if((JComboBox) e.getSource()==author_box)
                {
                        try
                        {
                            auther=(String) author_box.getSelectedItem();
                          if(!(auther.equals("")) || auther!=null || !(auther.equals("Select Author")))
                            {
                                String query = "select ISBN, AVAILABLE_BOOK, TOTAL_BOOK from BOOK_TABLE where BOOK_NAME = ? and AUTHOR_NAME = ?";
                                psmtp=conn.prepareStatement(query);
                                psmtp.setString(1,book);
                                psmtp.setString(2, auther);
                                rs = psmtp.executeQuery();
                                while(rs.next())
                                {
                                    String isbn = rs.getString("ISBN").toString();
                                    int ava_book = rs.getInt("AVAILABLE_BOOK");
                                    int total_book = rs.getInt("TOTAL_BOOK");
                                    isbn_text.setText(isbn);
                                    available_text.setText(ava_book+"");
                                    total_book_text.setText(total_book+"");
                                }
                            }
                            else
                            {
                                System.out.println("3");
                                    isbn_text.setText("");
                                    new_text.setText("");
                                    available_text.setText("");
                                    total_book_text.setText("");
                            }
                        }
                        catch(SQLException se)
                        {

                        }
        }
    }
}

例外

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at AddBook.actionPerformed(AddBook.java:233)
        at javax.swing.JComboBox.fireActionEvent(JComboBox.java:1242)
        at javax.swing.JComboBox.contentsChanged(JComboBox.java:1313)
        at javax.swing.AbstractListModel.fireContentsChanged(AbstractListModel.java:100)
        at javax.swing.DefaultComboBoxModel.setSelectedItem(DefaultComboBoxModel.java:88)
        at javax.swing.DefaultComboBoxModel.removeElementAt(DefaultComboBoxModel.java:140)
        at javax.swing.JComboBox.removeItemAt(JComboBox.java:741)
        at AddBook.actionPerformed(AddBook.java:198)
        at javax.swing.JComboBox.fireActionEvent(JComboBox.java:1242)
        at javax.swing.JComboBox.setSelectedItem(JComboBox.java:569)
        at javax.swing.JComboBox.setSelectedIndex(JComboBox.java:605)
        at javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(BasicComboPopup.java:814)
        at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:273)
        at java.awt.Component.processMouseEvent(Component.java:6041)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
        at javax.swing.plaf.basic.BasicComboPopup$1.processMouseEvent(BasicComboPopup.java:480)
        at java.awt.Component.processEvent(Component.java:5806)
        at java.awt.Container.processEvent(Container.java:2058)
        at java.awt.Component.dispatchEventImpl(Component.java:4413)
        at java.awt.Container.dispatchEventImpl(Container.java:2116)
        at java.awt.Component.dispatchEvent(Component.java:4243)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
        at java.awt.Container.dispatchEventImpl(Container.java:2102)
        at java.awt.Window.dispatchEventImpl(Window.java:2440)
        at java.awt.Component.dispatchEvent(Component.java:4243)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)

我想使用数据库。首先从数据库中插入组合框项，然后从 book_name_box 中选择值，然后在 author_box 中插入值，然后选择作者姓名，然后在其他字段中显示值，例如 isbn_text、available_text、new_text、total_book_text

请告诉我如何解决它。

Answer 1

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at AddBook.itemStateChanged(AddBook.java:235)

鉴于该跟踪涉及：

if(!(auther.equals("")) || !(auther.equals("Select Author")))

我希望它是auther，即null。 （顺便说一句，正确的拼写是author。）

有两种可能的方法。

One starts with realizing that the ItemEvent might relate to an ItemEvent.DESELECTED event (the component will fire multiple events when the selection changes from one item to another).当前代码可能：

1. 检查SELECTED 事件，否则中止。

2. 在同一代码行中的其他检查之前检查auther==null，如果是null，则中止。

   if(auther!=null && !(auther.equals("")) || !(auther.equals("Select Author")))
   

但更好的整体方法 (IMO) 是使用仅在选择项目时才会触发的侦听器。在这种情况下，请参阅JComboBox.addActionListener(ActionListener)。