In short, such a thing. I began to learn Java through a video course : having come to lesson 167 , I encountered the problem of how to load elements into a component of a graphical interface. We write the keyboard simulator, (in the lessons I use NetBeans, I use Eclipse, the codes when creating the JFrame form are different, but so far I have managed it). We have three classes:

  • MainWindow (no import specified)
  • ExerciseGUI (no import specified)
  • Lessons
 public class MainWindow extends JFrame { private ExerciseGUI exercise; private JPanel contentPane; private void exit() { int result = JOptionPane.showConfirmDialog (null, "Do you really want to exit?", "Information", JOptionPane.YES_NO_OPTION, JOptionPane.QUESTION_MESSAGE ); if(result != JOptionPane.NO_OPTION) { dispose(); System.exit(0); } } private void centered() { Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize(); Dimension frameSize = this.getSize(); if(frameSize.height > screenSize.height) { frameSize.height = screenSize.height; } if(frameSize.width > screenSize.width) { frameSize.width = screenSize.width; } this.setLocation((screenSize.width - frameSize.width) / 2 ,(screenSize.height - frameSize.height) / 2); } public static void main(String[] args) { EventQueue.invokeLater(new Runnable() { public void run() { try { MainWindow frame = new MainWindow(); frame.setVisible(true); } catch (Exception e) { e.printStackTrace(); } } }); } public MainWindow() { addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent evt) { exit(); } }); this.setTitle("Название программы"); this.setSize(500, 400); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); this.centered(); exercise = new ExerciseGUI(); exercise.pack(); exercise.validate(); exercise.reveal(new Lessons("Lessons 1", "Please type" , "fjkd fkll jklh assdf")); exercise.setResizable(true); exercise.setVisible(true); JMenuBar menuBar = new JMenuBar(); setJMenuBar(menuBar); JMenu file_name = new JMenu("File"); file_name.setMnemonic('F'); menuBar.add(file_name); JMenuItem file_exit = new JMenuItem("Exit"); file_exit.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent evt) { exit(); } }); file_name.add(file_exit); JMenu help_menu = new JMenu("Help"); help_menu.setMnemonic('H'); menuBar.add(help_menu); JMenuItem help_about = new JMenuItem("About"); help_about.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { JOptionPane.showConfirmDialog (null, "Under Construction...", "Разработка", JOptionPane.OK_OPTION, JOptionPane.INFORMATION_MESSAGE); } }); help_menu.add(help_about); JMenuItem help_on_typist = new JMenuItem("Help"); help_on_typist.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent arg0) { JOptionPane.showConfirmDialog (null, "Under Construction...", "Думаем ещё", JOptionPane.OK_OPTION, JOptionPane.INFORMATION_MESSAGE); } }); help_menu.add(help_on_typist); contentPane = new JPanel(); contentPane.setBorder(new EmptyBorder(5, 5, 5, 5)); contentPane.setLayout(new BorderLayout(0, 0)); setContentPane(contentPane); JDesktopPane desktopPane = new JDesktopPane(); contentPane.add(desktopPane, BorderLayout.CENTER); desktopPane.add(exercise); } } 
 public class ExerciseGUI extends JInternalFrame { private Lessons lessons; public void reveal (Lessons lessons) { this.lessons = lessons; // выдает ошибку title_label cannot be resolved title_label.setText(lessons.get_title()); descr_label.setText(lessons.get_instructions());//cannot be resolved reference_text_area.setText(lessons.get_text());//cannot be resolved } public static void main(String[] args) { EventQueue.invokeLater(new Runnable() { public void run() { try { ExerciseGUI frame = new ExerciseGUI(); frame.setVisible(true); } catch (Exception e) { e.printStackTrace(); } } }); } public ExerciseGUI() { setResizable(true); setBounds(100, 100, 450, 300); JPanel top_panel = new JPanel(); getContentPane().add(top_panel, BorderLayout.NORTH); JPanel toolbar = new JPanel(); top_panel.add(toolbar); JButton restart_button = new JButton("Restart"); toolbar.add(restart_button); JToggleButton pause_button = new JToggleButton("Pause"); toolbar.add(pause_button); JPanel label_panel = new JPanel(); top_panel.add(label_panel); JLabel title_label = new JLabel("title"); label_panel.add(title_label); JLabel descr_label = new JLabel(""); label_panel.add(descr_label); JPanel text_panel = new JPanel(); getContentPane().add(text_panel, BorderLayout.CENTER); text_panel.setLayout(new CardLayout(0, 0)); JSplitPane dril_pane = new JSplitPane(); dril_pane.setOrientation(JSplitPane.VERTICAL_SPLIT); text_panel.add(dril_pane, "name_11473649550263"); JTextArea reference_text_area = new JTextArea(); dril_pane.setLeftComponent(reference_text_area); JTextArea entry_text_area = new JTextArea(); dril_pane.setRightComponent(entry_text_area); JPanel status_panel = new JPanel(); getContentPane().add(status_panel, BorderLayout.SOUTH); } } 
 public class Lessons { private String title; private String instructions; private String text; public Lessons (String title, String instructions, String text) { this.title = title; this.instructions = instructions; this.text = text; } public String get_title() { if (title !=null) return title; else return "Title"; } public String get_instructions() { if (instructions !=null) return instructions; else return "Instructions"; } public String get_text() { if (text !=null) return text; else return "Text"; } }

Update

 private Lessons lessons; private JLabel title_label; private JLabel descr_label; private JTextArea reference_text_area; public void reveal (Lessons lessons) { this.lessons = lessons; title_label.setText(lessons.get_title()); descr_label.setText(lessons.get_instructions()); reference_text_area.setText(lessons.get_text()); }

Here so added ...

  • If you are given an exhaustive answer, mark it as correct (a daw opposite the selected answer). - Nicolas Chabanovsky ♦
  • Did you fix the designer? or just added class fields? - Vartlok

1 answer 1

You probably do not understand the concept of class fields and the scope of variables, try to read about it or review the lessons. In this particular case, the reveal method doesn't know anything about variables named title_label, descr_label, reference_text_area. In order for it to know about them and they refer to the correct object, you need to declare them as fields of a class, such:

 public class ExerciseGUI extends JInternalFrame { private Lessons lessons; private JLabel title_label; private JLabel descr_label; private JTextArea reference_text_area; ... public ExerciseGUI() { ... title_label = new JLabel("title"); descr_label = new JLabel(""); ... } }
  • Thank you so much for the answer, did as you advised, the errors went away, but when you start it, these are curses: java.lang.NullPointerException at ExerciseGUI.reveal (ExerciseGUI.java:27) at MainWindow. <Init> (MainWindow.java:83) at MainWindow $ 1.run (MainWindow.java:55) at java.awt.event.InvocationEvent.dispatch (Unknown Source) at java.awt.EventQueue.dispatchEventImpl (Unknown Source) ... (there is more) - Evgeny
  • @Evgeny it is obvious that something is null there, what exactly I do not know, did you update the designer too? - Vartlok
  • Thank you very much, your answer really helped, I fixed everything. Launched and even earned as expected :) - Evgeny