嘿伙计们我试图从我创建的jdbc表中获取数据并在我创建的GUI上显示它们。这是一个显示电影名称的程序。我的表由7列组成，它们是：
MovieID, title, genre, year, mpaaRating, directors, company。我的工作只是展示movieID标题年份和导演。我创建的第一个GUI界面包含三个按钮，每个按钮都会将您带到不同的界面，并且它们可以正常工作。我的主界面代码是：
public class MovieGUI extends JFrame implements ActionListener {
private static final int FRAME_WIDTH = 600;
private static final int FRAME_HEIGHT= 600;
private static final int FRAME_X_ORIGIN = 200;
private static final int FRAME_Y_ORIGIN = 50;
private static final String dbPrefix = "xxxxx";
private static final String netID = "xxxxx";
private static final String hostName = "xxxxx";
private static final String databaseURL = "jdbc:mysql://"+hostName+"/"+dbPrefix+netID;
private static final String password = "xxxxx";
private Connection connection = null;
private Statement statement = null;
private ResultSet resultSet = null;
private JLabel movies[] = new JLabel[20];
private JLabel promptName;
private JTextField fieldName;
private JButton browseMoviesButton, findNameButton, findRatingButton;
public MovieGUI(){
setTitle("Menu");
setResizable(false);
setSize(FRAME_WIDTH, FRAME_HEIGHT);
setLocation(FRAME_X_ORIGIN, FRAME_Y_ORIGIN);
Container contentPane = getContentPane();
contentPane.setLayout(null);
contentPane.setBackground(Color.red);
promptName = new JLabel();
promptName.setText("Movie Rating Services");
promptName.setBounds(250, 35, 150, 100);
contentPane.add(promptName);
browseMoviesButton = new JButton("Browse Movies");
browseMoviesButton.setBounds(240,120,150,50);
contentPane.add(browseMoviesButton);
browseMoviesButton.addActionListener(this);
findNameButton = new JButton("Find movies given name");
findNameButton.setBounds(230,220,180, 80);
contentPane.add(findNameButton);
findNameButton.addActionListener(this);
findRatingButton = new JButton("Find movie by rating");
findRatingButton.setBounds(230,350,180, 80);
contentPane.add(findRatingButton);
findRatingButton.addActionListener(this);
setDefaultCloseOperation(EXIT_ON_CLOSE);
}
public static void main(String args[]){
MovieGUI obj = new MovieGUI();
try{
//connects to database
Class.forName("com.mysql.jdbc.Driver");
System.out.println("databaseURL =" + databaseURL);
obj.connection = (Connection) DriverManager.getConnection(databaseURL, netID, password);
System.out.println("Connection Established");
obj.statement = (Statement) obj.connection.createStatement();
String sql;
}
catch(SQLException e){
System.out.println("command did not go through");
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
MovieGUI frameObj = new MovieGUI();
frameObj.setVisible(true);
}
public void actionPerformed (ActionEvent event){
if (event.getSource() instanceof JButton){
FindName nameObj = new FindName();
BrowseMovies browseObj = new BrowseMovies();
FindRating ratingObj = new FindRating();
JButton clickedButton = (JButton) event.getSource();
JRootPane rootPane = clickedButton.getRootPane();
JFrame frame = (JFrame) rootPane.getParent();
String buttonText = clickedButton.getText();
if (buttonText.equals("Browse Movies")){
browseObj.setVisible(true);
frame.setVisible(false);
try {
statement = (Statement) connection.createStatement();
String sql;
sql = "SELECT movieID, title, year, directors FROM MOVIE";
resultSet = statement.executeQuery(sql);
int columns = resultSet.getMetaData().getColumnCount();
StringBuilder message = new StringBuilder();
while (resultSet.next()) {
for (int i = 0; i < movies.length; i++){
for (int j = 1; j <= columns; j++) {
message.append(resultSet.getString(j) + "\t\t");
}//end nested for loop
message.append("\n");
movies[i].setText(message.toString());
}//end for loop
}//end while loop
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}//end catch
for (int i = 0; i < movies.length; i++){
movies[i].setBounds(150,60,300,50);
browseObj.add(movies[i]);
}
}
else if(buttonText.equals("Find movies given name")){
nameObj.setVisible(true);
frame.setVisible(false);
}
else if (buttonText.equals("Find movie by rating")){
ratingObj.setVisible(true);
frame.setVisible(false);
}
}
}
}
当我点击浏览电影按钮时，它应该转到下一个界面并在我的表格中显示电影的上述属性，其中有20个。
我的BrowseMovies界面代码是：
public class BrowseMovies extends JFrame {
private static final int FRAME_WIDTH = 600;
private static final int FRAME_HEIGHT= 600;
private static final int FRAME_X_ORIGIN = 200;
private static final int FRAME_Y_ORIGIN = 50;
private JLabel promptName;
public BrowseMovies(){
setTitle("Movie List");
setResizable(false);
setSize(FRAME_WIDTH, FRAME_HEIGHT);
setLocation(FRAME_X_ORIGIN, FRAME_Y_ORIGIN);
Container contentPane = getContentPane();
contentPane.setLayout(null);
contentPane.setBackground(Color.blue);
promptName = new JLabel();
promptName.setText("Movie Rating Services -- Browse Movies");
promptName.setBounds(200, 35, 250, 100);
contentPane.add(promptName);
}//GUI constructor
}
我遇到的问题是browseMovies界面只显示promptName而没有其他内容，我在控制台中收到一个很长的错误。错误是：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at project5.MovieGUI.actionPerformed(MovieGUI.java:111)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
我不知道出了什么问题。我猜这个问题是在actionPerformed方法的try块中的某个地方，但我似乎无法找到它。任何帮助将不胜感激。
答案 0 :(得分：0)
您必须使用实际的JLabel对象初始化数组。 下面给出了第一次迭代中的空指针
public class TestSwing {
public static void main(String[] args) {
JLabel movies[] = new JLabel[20];
for (int j = 0; j < movies.length; j++) {
System.out.println("Loop Counter : " + j);
movies[j].setText("Counter : " + j);
}
}
}
你必须做类似的事情
public class TestSwing {
public static void main(String[] args) {
JLabel movies[] = new JLabel[20];
for (int j = 0; j < movies.length; j++) {
System.out.println("Loop Counter : " + j);
movies[j] = new JLabel(); // initialize your JLable
movies[j].setText("Counter : " + j);
}
}
}
注意：这只是代码的摘录，我删除了一些代码只是为了更好地澄清它