GUI and Event-Driven Programming

1. GUI and Event-Driven Programming Part 2

2. Event Handling • An action involving a GUI object, such as clicking a button, is called an event. • The mechanism to process events is called event handling. • The event-handling model of Java is based on the concept known as the delegation-based event model. • With this model, event handling is implemented by two types of objects: • event source objects • event listener objects.

3. Event Sources and Listeners • An event source object is a GUI object where an event occurs. An event source generates events. • An event listener object is an object that includes a method that gets executed in response to the generated events. When an event is generated, the system notifies the relevant event listener objects.

4. Events and Event Listeners • There are many different kinds of events, but the most common one is an action event. • For the generated events to be processed, we must associate, or register, event listeners to the event sources. • If event sources have no registered listeners, the generated events are ignored.

5. Events and Event Listeners • An object that can be registered as an action listener must be an instance of a class that is declared specifically for the purpose. We call such classes action listener classes. • To associate an action listener to an action event source, we call the event source’s addActionListener method with the action listener as its argument.

6. Events and Event Listeners • A single listener can be associated to multiple event sources. • Likewise, multiple listeners can be associated to a single event source. • When an event source generates an event, the system checks for matching registered listeners. • If there is no matching listener, the event is ignored. • If there is a matching listener, the system notifies the listener by calling the listener’s corresponding method.

7. Action Listener Classes • In the case of action events, the method called is actionPerformed. • To ensure the programmer includes the necessary actionPerformed method in the action listener class, the class must be defined in a specific way. import java.awt.event.*; public class ButtonHandler implements ActionListener { ... }

8. The ActionListener interface • ActionListener is an interface, not a class. • Like a class, an interface is a reference data type, but unlike a class, an interface includes only constants and abstract methods. • An abstract method has only the method header, or prototype; it has no method body. • A class that implements a Java interface must provide the method body to all the abstract methods defined in the interface.

9. The ActionListener interface • By requiring an object we pass as an argument to the addActionListener method to be an instance of a class that implements the ActionListener interface, the system ensures that this object will include the necessary actionPerformed method. • Implementation of an interface is Java’s mechanism for providing a form of multiple inheritance

10. Example • Suppose we want to change the title of the frame, depending on which button is clicked • We can do this using the actionPerformed method. The method model is: public void actionPerformed(ActionEvent evt) { String buttonText = get the text of the event source; JFrame frame = the frame that contains this event source; frame.setTitle(“You clicked “ + buttonText); }

11. Handling Button Events • The first statement may be handled in one of two ways. • The first way is via the getActionCommand method of the action event object evt: String buttonText = evt.getActionCommand(); • The second way is via the getSource method of the action event object evt.: JButton clickedButton = (JButton) evt.getSource(); String buttonText = clickedButton.getText();

12. Handling Button Events • A frame window contains nested layers of panes. The topmost pane is called the root pane. • To find the frame that contains the event source, we • get the root pane to which the event source belongs, then • get the frame that contains this root pane. JRootPane rootPane = clickedButton.getRootPane(); Frame frame = (JFrame) rootPane.getParent(); • The frame can be the event listener of the GUI objects it contains. cancelButton.addActionListener(this); okButton.addActionListener(this);

13. Swing Classes for Text Handling • The Swing GUI classes JLabel, JTextField, and JTextArea all deal with text. • A JLabel object displays uneditable text. • A JTextField object allows the user to enter a single line of text. • A JTextArea object allows the user to enter multiple lines of text. It can also be used for displaying multiple lines of uneditable text.

14. Text Fields and Action Events • An instance of JTextField generates action events when the object is active and the user presses the ENTER key. • The actionPerformed method must determine which of the event sources (for example, a button or a text field) generated the event. • The instanceof operator determines the class to which the event source belongs. The general model is thus: if (event.getSource() instanceof JButton ){ // event source is either cancelButton // or okButton ... } else { // event source must be inputLine ... }

15. Text Fields and Action Events • The getText method of JTextField may be used to retrieve the text that the user entered. public void actionPerformed(ActionEvent event) { if (event.getSource() instanceof JButton){ JButton clickedButton = (JButton) event.getSource(); String buttonText = clickedButton.getText(); setTitle(“You clicked ” + buttonText); } else {// the event source is inputLine setTitle(“You entered ‘” + inputLine.getText() + “’”); } }

16. JLabel Objects • A JLabel object is useful for displaying a label indicating the purpose of another object, such as a JTextField object. prompt = new JLabel(“Please enter your name”); • JLabel objects may also be used to display images. • When a JLabel object is created, we can pass an ImageIcon object instead of a string. • To create the ImageIcon object, we must specify the filename of the image.

17. JLabels and Images • We declare the data member private JLabel image; • then create it in the constructor as public TextFrame2 () { ... image = new JLabel(new ImageIcon(“cat.gif”); // assumes the .gif is/in the same directory as the program image.setBounds(10, 20, 50, 50); contentPane.add(image); ... }

18. Example with JLabel, JButton and JTextField objects

19. TextArea & Related Methods • A TextArea is like a TextField, but can accommodate more text • Methods include: • setBorder(BorderFactory); // creates a border around a JTextArea object • setEditable(boolean); // enables/disables changes to a JTextArea object • The append(String) method allows us to add text to the text area without replacing old content. • Using the setText (String) method of JTextArea will replace old content with new content.

20. Example frame with JTextArea object

21. Adding Scrollbars to a JTextArea JScrollPane scrollText = new JScrollPane(textArea); contentPane.add(scrollText);

22. Menus • The javax.swing package contains three useful menu-related classes: JMenuBar, JMenu, and JMenuItem. • MenuBar is a bar where the menus are placed. • Menu (such as File or Edit) is a single item in the MenuBar. • MenuItem (such as Copy, Cut, or Paste) is an individual menu choice in the Menu.

23. Menus • When a MenuItem is selected, it generates a menu action event. • We process menu selections by registering an action listener to menu items. • The following example will create the two menus shown and illustrate how to display and process the menu item selections.

24. Menus • We will follow this sequence of steps: • Create a JMenuBar object and attach it to a frame. • Create a JMenu object. • Create JMenuItem objects and add them to the JMenu object. • Attach the JMenu object to the JMenuBar object.

25. Menus • We create a fileMenu object as fileMenu = new Menu(“File”); • The argument to the Menu constructor is the name of the menu. • Menu items appear from the top in the order in which they were added to the menu.

26. Menus • To create and add a menu item New to fileMenu, we execute item = new MenuItem(“New”); item.addActionListener(this); fileMenu.add(item); • And to add a horizontal line as a separator between menu items, we execute fileMenu.addSeparator();

27. Menus • We then attach the menus to a menu bar. • We create a MenuBar object in the constructor, call the frame’s setMenuBar method to attach it to the frame, and add the two Menu objects to the menu bar. MenuBar menuBar = new MenuBar(); setMenuBar(menuBar); menuBar.add(fileMenu); menuBar.add(editMenu);