Chapter 7 Event-Driven Programming and Basic GUI Objects

Topics Graphical User Interfaces Inheritance GUI components Interfaces Events and Listeners

Introduction This chapter covers the graphical user interface (GUI). In Java, GUI-based programs are implemented by using classes from the javax.swing and java.awt packages. The Swing classes provide greater compatibility across different operating systems. They are fully implemented in Java, and behave the same on different operating systems.

GUI Components The javax.swing package contains a number of GUI components. –JFrame –JMenu –JLabel –JButton –JTextField –ImageIcon –JTextArea

Sample GUI Program

Introduction AWT classes are implemented by using the native GUI objects. Swing classes support many new functionalities not supported by AWT counterparts. Do not mix the counterparts in the same program because of their differences in implementation.

Introduction To build an effective GUI using objects from the Swing and AWT packages, we must learn a new style of program control called event-driven programming. An event occurs when the user interacts with a GUI object. In event-driven programs, we program objects to respond to these events by defining event-handling methods.

Creating a Subclass of JFrame As we know from Chapter 1, inheritance is a feature we use to define a more specialized class from an existing class. The existing class is the superclass and the specialized class is the subclass.

Inheritance hierarchy of JOptionPane From the API documentation. java.lang.Object - java.awt.Component - java.awt.Container - javax.swing.JComponent - javax.swing.JOptionPane

Creating a Subclass of JFrame To create a customized user interface, we often define a subclass of the JFrame class. The JFrame class contains rudimentary functionalities to support features found in any frame window.

Default JFrame Window A default JFrame window appears at the top left corner of the screen. You may not notice this because it is so small. It looks different on computers running different operating systems

Creating a Subclass of JFrame To define a subclass of another class, we declare the subclass with the reserved word extends. class Ch7JFrameSubclass1 extends JFrame {... }

Creating a Subclass of JFrame We will also add the following default characteristics: –The title is set to My First Subclass. –The program terminates when the close box is clicked. –The size of the frame is 300 pixels wide by 200 pixels high. –The frame is positioned at screen coordinate (150, 250). These properties are set inside the default constructor.

Size and Location of a JFrame How an instance of Ch7JFrameSubclass1 will appear on the screen.

Inheritance Every method of a superclass is inherited by its subclass. Because the subclass-superclass relationships are formed into an inheritance hierarchy, a subclass inherits all methods defined in its ancestor classes.

Another Subclass of JFrame Next we will define another subclass called Ch7JFrameSubclass2, which will have a white background. We will define this class as an instantiable main class so we don’t have to define a separate main class.

Using the Content Pane To make the background white, we must access the frame’s content pane, the area of the frame excluding the title and menu bars and the border. We access the content pane by calling the frame’s getContentPane method. We change the background color by calling the content pane’s setBackground method.

Ch7JFrameSubclass2

Using Buttons in a JFrame There are two approaches to placing GUI objects on a frame’s content pane. 1.One approach uses a layout manager, an object that controls the placement of the GUI objects. 2.The other approach uses absolute positioning to explicitly specify the position and size of GUI objects on the content pane.

Absolute Positioning of Components To use absolute positioning, set the layout manager of a frame’s content pane to none by passing null to the setLayout method. contentPane.setLayout(null); Set the size and position of a button by calling the button’s setBounds method: okButton.setBounds( 75, 125, 80, 30); –The first two arguments specify the button’s position. –The last two arguments specify the width and height of the button. To make a button appear on the frame, add it to the content pane by calling the add method. contentPane.add(okButton);

Button Placement

Handling Button Events 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.

Event Handling in Java

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.

Events 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 events they generate are ignored.

Listener Classes 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.

Listeners and Event Sources A single listener can be associated to multiple event sources. Likewise, multiple listeners can be associated to a single event source.

Event Handling 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 appropriate method in the listener. In the case of action events, the method called is actionPerformed. How does the system know it can call this method for any listener class?

Interfaces 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. An interface cannot be instantiated. You use an interface by writing a class that implements an interface.

Implementing Interfaces A class that implements a Java interface must provide the method body for all the abstract methods defined in the 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. –We use ActionListener as a parameter type in the addActionListener method –An object from any class that implements ActionListener can be used as an argument.

ActionListener Classes ActionListener is an interface, not a class. To ensure that the programmer includes the necessary actionPerformed method in an action listener class, the class must implement the ActionListener interface. import java.awt.event.*; class ButtonHandler implements ActionListener {... } In the actionPerformed method, we need to define what should happen when an event occurs.

The actionPerformed method To change the title of the frame, depending on which button is clicked, we use 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); }

Finding the Source of an Event We can get the text from the event source using the getActionCommand method of the action event object evt: String buttonText = evt.getActionCommand(); We can get a reference to the event source itself using the getSource method of the action event object evt.: JButton clickedButton = (JButton) evt.getSource(); String buttonText = clickedButton.getText(); –Note that the object returned by the getSource method may be an instance of any Component class, so we type cast the returned object to a proper class in order to use the desired method.

Identifying the Frame 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();

Chapter7JButtonFrame A sample window when it is first opened and after the CANCEL button is clicked.

Frames as Listeners A frame window contains nested layers of panes. –The topmost pane is called the root pane. The frame can be the event listener for the GUI objects it contains. cancelButton.addActionListener(this); okButton.addActionListener(this); –The Frame can call its setTitle method directly. The frame class must implement ActionListener

Classes for Text The Swing GUI classes JLabel, JTextField, and JTextArea all deal with text. –A JLabel object displays uneditable (by the user) 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.

JTextField Class 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 three event sources in our example (two buttons and one text field) generated the event.

What class does an Object Come From? 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... }

JTextField 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() + “’”); }

JLabel 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”);

Images in JLabel Objects JLabel objects may also be used to display images. –They can display text, image or both To create a JLabel with an image instead of text, we pass an ImageIcon object instead of a string. To create the ImageIcon object, we must specify the filename of the image.

JLabel We declare the data member private JLabel image; then create it in the constructor as public Ch7TextFrame2{... image = new JLabel(new ImageIcon(“cat.gif”); //note that this assumes the.gif is //in the same directory as the program image.setBounds(10, 20, 50, 50); contentPane.add(image);... }

Ch7TextFrame2 A window with one text JLabel, one image JLabel, one JTextField, and two JButton objects.

JTextArea Next we will declare a JTextArea object: private JTextArea textArea; and add statements to create it textArea = new JTextArea(); textArea.setBounds(50, 5, 200, 135); textArea.setBorder(BorderFactory.create LineBorder(Color.red)); textArea.setEditable(false); contentPane.add(textArea);

Putting Borders on Components by default the rectangle indicating the boundary of the JTextArea object is not shown on the screen. We call the createLineBorder method of the BorderFactory class, because We disable editing of the text area in the statement textArea.setEditable(false);

Modifying Text in a JTextArea The append method allows us to add text to the text area without replacing old content. Using the setText method of JTextArea will replace old content with new content. See Ch7TextFrame3

Scroll Bars To add scroll bars to the JTextArea object, we amend the earlier code as follows: textArea = new JTextArea(); textArea.setEditable(false); JScrollPane scrollText = new JScrollPane(textArea); textArea.setBounds(50, 5, 200, 135); textArea.setBorder(BorderFactory.createLine Border(Color.red)); contentPane.add(scrollText); See TextFrame4

Menus The javax.swing package contains three useful menu-related classes 1.JMenuBar is a bar where the menus are placed. 2.JMenu (such as File or Edit) is a single item in the MenuBar. 3.JMenuItem (such as Copy, Cut, or Paste) is an individual menu choice in the Menu.

Using Menus When a MenuItem is selected, it generates a menu action event. We process menu selections by registering an action listener to menu items.

How to Implement Menus 1.Create a JMenuBar object and attach it to a frame. 2.Create a JMenu object. 3.Create JMenuItem objects and add them to the JMenu object. 4.Attach the JMenu object to the JMenuBar object.

Creating a Menu 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.

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

Creating the Menu Bar We create a JMenuBar object JMenuBar menuBar = new JMenuBar(); Then call the frame’s setMenuBar method to attach it to the frame setMenuBar(menuBar); Then add the two JMenu objects to the menu bar. menuBar.add(fileMenu); menuBar.add(editMenu);