JComboBox：如何解决 JComboBox 和 ItemListener 中的异常
JComboBox: How to solve exception in JComboBox and ItemListener
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)
{
}
}
}
}
异常
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.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。第一个组合框项目从数据库插入比我 select 来自 book_name_box 的值然后值插入 author_box 然后我 select 作者名称然后值显示在其他字段如 isbn_text , available_text, new_text, total_book_text
请告诉我如何解决。
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at AddBook.itemStateChanged(AddBook.java:235)
鉴于跟踪涉及：
if(!(auther.equals("")) || !(auther.equals("Select Author")))
我预计 auther 即 null。 （顺便说一下，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：
- 检查
SELECTED 事件，否则中止。
在同一代码行中的其他检查之前检查 auther==null，如果 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触发的侦听器。在这种情况下，请参阅 JComboBox.addActionListener(ActionListener)。
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)
{
}
}
}
}
异常
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.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。第一个组合框项目从数据库插入比我 select 来自 book_name_box 的值然后值插入 author_box 然后我 select 作者名称然后值显示在其他字段如 isbn_text , available_text, new_text, total_book_text
请告诉我如何解决。
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at AddBook.itemStateChanged(AddBook.java:235)
鉴于跟踪涉及：
if(!(auther.equals("")) || !(auther.equals("Select Author")))
我预计 auther 即 null。 （顺便说一下，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：
- 检查
SELECTED事件，否则中止。 在同一代码行中的其他检查之前检查
auther==null，如果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触发的侦听器。在这种情况下，请参阅 JComboBox.addActionListener(ActionListener)。