Chapter 9: Visual Programming Basics

1. Chapter 9: Visual Programming Basics Object-Oriented Program Development Using Java: A Class-Centered Approach

2. Object-Oriented Program Development Using Java: A Class-Centered Approach 2 Objectives Event-Based Programming Creating a Swing-Based Window Adding a Window Closing Event Handler Adding a Button Component Common Programming Errors

3. Object-Oriented Program Development Using Java: A Class-Centered Approach 3 Event-Based Programming Event-based programs provide fully functioning GUIs An event is initiated by a user action A program must: Correctly assess which specific event has occurred Provide the appropriate code to perform an action based on the identified event

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5. Object-Oriented Program Development Using Java: A Class-Centered Approach 5 Event-Based Programming (continued) Actions that trigger events include: Placing the mouse pointer over a button and clicking the left mouse button Using the TAB key until the desired button is highlighted with a dotted line then pushing the Enter key Pressing an accelerator key The sequence of events in a program are controlled by the user

6. Object-Oriented Program Development Using Java: A Class-Centered Approach 6 Event-Based Programming (continued) Programmer provides: Code to create GUI Code to appropriately process events Java provides a set of objects for coding GUIs: AWT Older GUI components Swing Newer GUI components

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8. Object-Oriented Program Development Using Java: A Class-Centered Approach 8 The Event-Based Model Operating system: Has total control of computer Never relinquishes control to any executing programs Most executing programs spend the majority of their time in a sleep type of mode When an event occurs: The operating system passes event information to the appropriate application Permits the application to take action

9. Object-Oriented Program Development Using Java: A Class-Centered Approach 9 Containment Hierarchy Hierarchy of component placement Consists of one and only one top-level container Any number of other intermediate containers And/or atomic components JFrame is most commonly used as a top-level container Heavyweight components are responsible for interfacing with the operating system

10. Object-Oriented Program Development Using Java: A Class-Centered Approach 10 Containment Hierarchy (continued) A content pane is an internal component provided by each top-level container All of the visible components displayed by a GUI must be placed on a content pane Menu bar: Can also be added to a top-level container Placed outside of a content pane

11. Object-Oriented Program Development Using Java: A Class-Centered Approach 11 Containment Hierarchy (continued) Layout manager: Defines how components are positioned and sized within a container�s content pane Default placement can always be changed By explicitly specifying another layout manager Lightweight components: Intermediate containers and atomic components Do not interface with the operating system

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15. Object-Oriented Program Development Using Java: A Class-Centered Approach 15 Creating a Swing-Based Window Two predominant approaches: Construct a GUI as a separate class using Swing components Construct a GUI object using Swing components from within the main() method

16. Object-Oriented Program Development Using Java: A Class-Centered Approach 16 Creating a Swing-Based Window (continued) Create JFrame: JFrame mainFrame = new JFrame("First GUI Window"); Setting size: mainFrame.setSize(300,150);

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18. Object-Oriented Program Development Using Java: A Class-Centered Approach 18 Creating a Swing-Based Window (continued) Display JFrame: Use show() Or setVisible(true)

19. Object-Oriented Program Development Using Java: A Class-Centered Approach 19 Look and Feel Refers to: How a GUI appears on screen How a user interacts with it Swing package: Supports four look and feel types If no look and feel is specified, the default Java look and feel is used

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22. Object-Oriented Program Development Using Java: A Class-Centered Approach 22 Adding a Window Closing Event Handler GUI creation process includes: Phase 1: Construct a component so that it appears visually Phase 2: Provide an event handler for the component Event handler Object that responds appropriately when an event occurs

23. Object-Oriented Program Development Using Java: A Class-Centered Approach 23 The Event Delegation Model Requires two basic elements: Component to generate an event Event handler or listener object The component delegates responsibility to a listener object for doing something when an event is generated Registration statement Glue that attaches an event to a specific event handler

24. Object-Oriented Program Development Using Java: A Class-Centered Approach 24 Phase 2: Provide an Event Handler for the Component Step 1: Write the code for an event handler class Known as the listener class Step 2: Create an instance of the event handler class Means instantiating an object of the class using the new operator Created object is known as a listener object Step 3: Register the listener object created in step 2

25. Object-Oriented Program Development Using Java: A Class-Centered Approach 25 Coding an Event Handler The event handling class has been coded as a separate nonnested class WindowListener interface: Must be implemented by an event handler class Required by Java for handling window events All of the listed methods must be implemented Even if they consist of empty bodies

26. Object-Oriented Program Development Using Java: A Class-Centered Approach 26 Coding an Event Handler (continued) Add listener to JFrame: mainFrame.addWindowListener(handler); Multiple listener objects can be registered to the same event source A single listener object can be registered to multiple event sources

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28. Object-Oriented Program Development Using Java: A Class-Centered Approach 28 Adapter and Inner Classes Adapter classes: Declare empty event handling methods for a given interface type Can be used as a parent class for a listener class Constructed as abstract classes Inner class: One class is nested inside another class Place the event handler class definition close to other GUI-related code

29. Object-Oriented Program Development Using Java: A Class-Centered Approach 29 Adapter and Inner Classes (continued) Event-handling code guidelines: Listener class should be made as short as possible Code should be placed as close as possible to where an object of the class is actually instantiated

30. Object-Oriented Program Development Using Java: A Class-Centered Approach 30 Anonymous Classes Class without a name Permits placing the event handling class code directly into the statement that creates an instance of the event handling class Example: mainFrame.addWindowListener(new WindowAdapter() {public void windowClosing(WindowEvent e) {System.exit(0);}});

31. Object-Oriented Program Development Using Java: A Class-Centered Approach 31 Anonymous Classes (continued) Should only be used when an event handler consists of a single, short method Always used within a statement that creates an instance of the class

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33. Object-Oriented Program Development Using Java: A Class-Centered Approach 33 Adding a Button Component Adding components to GUI: Phase 1: Construct a component so that it appears visually Step 1: Create a specific component Step 2: Add the component into a container Phase 2: Provide an event handler for the component Step 1: Write code for an event handler class Step 2: Create an instance of the event handler class Step 3: Register the listener object

34. Object-Oriented Program Development Using Java: A Class-Centered Approach 34 Adding a Button When adding lightweight components into a top-level container, they must be added to the container�s content pane Declare and instantiate JButton: private JButton firstButton; firstButton = new JButton("Press me"); Add to the main frame�s content pane: Container c = mainFrame.getContentPane(); c.add(firstButton);

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36. Object-Oriented Program Development Using Java: A Class-Centered Approach 36 Adding ToolTips and Accelerator Keys ToolTip: Single line of text Appears when a user positions the mouse cursor over a GUI component Provides a quick single-line documentation for a component Syntax: objectName.setToolTipText("string value");

37. Object-Oriented Program Development Using Java: A Class-Centered Approach 37 Adding ToolTips and Accelerator Keys (continued) Accelerator key: Called a mnemonic key in Java Any key that initiates an action by pressing the Alt key and a designated letter Syntax: objectName.setMnemonic('letter'); Choose a letter contained in the object�s caption: Will be underlined in caption Otherwise hot-key will be �hidden�

38. Object-Oriented Program Development Using Java: A Class-Centered Approach 38 Adding an Event Handler An event handler for an atomic component is created and registered in the same way as event handlers for a JFrame container ActionListener class is added to JButton Must implement actionPerformed() method Use addActionListener() method to register a handler

39. Object-Oriented Program Development Using Java: A Class-Centered Approach 39 Common Programming Errors Forgetting to include GUI-related import statements Creating an event handler and failing to register it Modifying a GUI class that has previously been compiled and changing its name But forgetting to change the name when an instance of the class is created in the main() method

40. Object-Oriented Program Development Using Java: A Class-Centered Approach 40 Summary Event-based programs execute program code depending on what events occur Java handles events triggered by GUI components using an event delegation model Graphical components are structured into a Swing-based GUI following a containment hierarchy The name of the listener class for each Swing component must be a Java-specified name

41. Object-Oriented Program Development Using Java: A Class-Centered Approach 41 Summary (continued) Each implemented listener class requires a specific set of methods that must be included Even if this means creating empty method bodies Listener classes can be nested inside the class used to instantiate and display GUI components Anonymous classes can be used to construct listener classes Whenever a single-statement listener object instantiation and registration are employed