Event Driven Programming

Transcription

1 Event Driven Programming Part 1 Introduction Chapter 12 CS 2334 University of Oklahoma Brian F. Veale 1

2 Graphical User Interfaces So far, we have only dealt with console-based programs Run from the console window Suitable for simple textual I/O, but not very user-friendly Most of the programs we use daily have Graphical User Interfaces Modern Operating Systems usually have GUI interfaces If we can learn to write programs with GUIs, we have a much larger range of possible programs 2

3 Interacting with console and GUI programs Console interaction is typically quite limited MS-DOS only has about 100 commands Some command line programs have far fewer Such as projects 1 and 2 GUIs have incredibly large number of possible interactions NetBeans Microsoft Office products 3

4 Events Wide range of actions with GUI programs Moving the mouse Closing a window Clicking on a button Typing into a dialog box All of these types of interactions have a generic name, event An event is a user interaction with a program with a graphical user interface Setting-up and managing events is a major part of GUI programming 4

5 Event Handling These events mean little unless our program reacts to them We will need to decide which events having meaning to us, and which are unimportant event handling - The act of responding to an event. 5

6 How to handle an event Programs respond to events by calling methods Java defines which method responds to which event by declaring Listeners Listener - An object that has the responsibility to respond to an event generated by a user interaction. 6

7 GUI Program Structure Interaction with a GUI program is controlled by the main event loop This is an infinite loop with a large switch statement When an interaction occurs, the main event loop checks the switch statement for the type of event Inside a case statement for a given event is an indication of which Listener method to call Once the listener method has completed its actions, control goes back to the main event loop In Java, the compiler generates the main event loop. We provide that code that implements the Listener methods. 7

8 Components and Containers Java GUIs consist of components arranged in containers Component: A single user interface element Button Menu item Text field Container: a GUI element that is used to group together and organize other components At least one container is always needed JFrame Combines content area and menu bar Very common type of top-level container Demo: Button.java 8

9 Some important methods Button.java frame.getcontentpane().add(button) Adds the JButton to the container Specifically the content pane portion of the container (not the menu bar) frame.pack() Organizes the components that are inside the JFrame Causes the JFrame to change shape to accommodate the JButton JButton would not be visible without this method call frame.setvisible(true) Tells Java to show the GUI on the screen You won t see anything without this method call 9

10 JLabel Other Important Components Displays textual labels User cannot change the label The program can communicate with the following components to get user input/ineraction JTextField Displays single line of text User can change the text Optionally non-editable JTextArea Displays multiple lines of text Designed for plain text entry only Demo: Label.java, TextField.java, TextArea.java 10

11 Containers GUI programs don t usually have a single component Containers help us organize multiple components Containers can also contain other containers 11

12 Heavyweight vs. Lightweight There are two categories of containers Heavyweight Have their own window Should try to have only one per window displayed Carries extra baggage Examples: JFrame Lightweight Must be displayed in a Heavyweight container Examples: JPanel: the content pane of the demo program. This is returned by frame.getcontentpane() in our demo programs. Other examples: JMenuBar: a container itself, contains JMenu containers (which contain JMenuItems) 12

13 JFrame, JPanel, and JMenuBar (JMenuBar) (JPanel) From page 716 of the Tymann text 13

14 Properties of Containers Some containers have special properties JSplitPane Used to split a space shared by two components JScrollPane Includes scrollbars for when components are too large for the window JTabbedPane Creates an interface for multiple components using the same space These components are accessed via a row of tabs across the top of bottom of the screen 14

15 Containers in Excel What containers exist in Excel as shown below? 15

16 Futility of shaking sticks at Java Components and Containers There are a large number of different components and containers in Swing Most of these are the classes that start with J Say it with me, Read the API Being able to find and learn how to use new methods, components, and containers found in the API documentation is an important skill 16

17 READ the API Download the documentation so you don t need network access to get to the API Scroll down to J2SE 5.0 Documentation and click on download Unzip the file you downloaded. Open your browser, and choose Open File in the file menu. Then, browse to the docs\api directory inside the directory that you saved the contents of the zip file into. 17