Chapter 7 EventDriven Programming and Basic GUI Objects

Chapter 7 Event-Driven Programming and Basic GUI Objects ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

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. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

Fig. 7. 1 Various GUI objects from the javax. swing package. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

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. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

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. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 1 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. http: //java. sun. com/j 2 se/1. 5. 0/docs/api/i ndex. html ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

Fig. 7. 4 How an instance of Ch 7 JFrame. Subclass 1 will appear on the screen. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 1 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. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 1 Creating a Subclass of JFrame Next we will define another subclass called Ch 7 JFrame. Subclass 2, 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. http: //java. sun. com/j 2 se/1. 5. 0/docs/api/i ndex. html ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 1 Creating a Subclass of JFrame 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 get. Content. Pane method. We change the background color by calling the content pane’s set. Background method. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 1 Creating a Subclass of JFrame Load Ch 7 Frame. Subclass 1. java ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 2 Placing Buttons on the Content Pane of a Frame There are two approaches to placing GUI objects on a frame’s content pane. One approach uses a layout manager, an object that controls the placement of the GUI objects. (will use later in course) The other approach uses absolute positioning to explicitly specify the position and size of GUI objects on the content pane. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

Fig. 7. 7 The process of creating a button and placing it on a frame. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 2 Placing Buttons on the Content Pane of a Frame To use absolute positioning, set the layout manager of a frame’s content pane to none by passing null to the set. Layout method. content. Pane. set. Layout(null); ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 2 Placing Buttons on the Content Pane of a Frame We then place two buttons at the position and size we want by calling the button’s set. Bounds method: ok. Button. set. Bounds( 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. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 2 Placing Buttons on the Content Pane of a Frame To make a button appear on the frame, add it to the content pane by calling the add method. content. Pane. add(ok. Button); Load code: Ch 7 Button. Frame. Java ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 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. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events 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. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events 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. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button 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 generated events are ignored. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events 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 add. Action. Listener method with the action listener as its argument. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events A single listener can be associated to multiple event sources. Likewise, multiple listeners can be associated to a single event source. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events 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. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events In the case of action events, the method called is action. Performed. To ensure the programmer includes the necessary action. Performed method in the action listener class, the class must be defined in a specific way. import java. awt. event. *; class Button. Handler implements Action. Listener {. . . } ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events A class that implements a Java interface must provide the method body to all the abstract methods defined in the interface. By requiring an object we pass as an argument to the add. Action. Listener method to be an instance of a class that implements the Action. Listener interface, the system ensures that this object will include the necessary action. Performed method. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events To change the title of the frame, depending on which button is clicked, we use the action. Performed method. The method model is: public void action. Performed(Action. Event evt){ String button. Text = get the text of the event source; JFrame frame = the frame that contains this event source; frame. set. Title(“You clicked “ + button. Text); } ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events View source code. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events The second way is via the get. Source method of the action event object evt. : JButton clicked. Button = (JButton) evt. get. Source(); String button. Text = clicked. Button. get. Text(); Note that the object returned by the get. Source method may be an instance of any class, so we type cast the returned object to a proper class in order to use the desired method. ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

7. 3 Handling Button Events 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. JRoot. Pane root. Pane = clicked. Button. get. Root. Pane(); Frame frame = (JFrame) root. Pane. get. Parent(); ©The. Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.