GUIs with Swing

Principles of Software System Construction

Prof. Jonathan Aldrich
Fall 2011

Slides copyright 2011 by Jeffrey Eppinger, Jonathan Aldrich

Used and adapted by permission
 What makes GUIs different?
• (see example GUIs)

2
 What makes GUIs different?
• The user is in control
– GUI has to react to the user’s actions
– Requires structuring the GUI around reacting to events

3
 Reacting to events - from framework
• Setup phase
– Describe how the GUI window should look
Application
– Use libraries for windows, widgets, and layout
– Embed specialized code for user later event—
drawing
mouse, key,
windows
• Customization (provided during setup) display, …

– New widgets that display themselves in custom ways

– How to react to events Framework
• Execution
get next
– Framework gets events from OS event event

• Mouse clicks, key presses, window becomes visible, etc.

– Framework triggers application code in response OS
• The customization described above

4
 Cookbook Programming
• Typical mode of using a framework
– Let’s you follow a recipe for writing your programs
– All cakes are different, but there are a few basic recipes and
everything else is a slight variation
• Add some cinnamon
• Substitute chocolate chips instead of nuts

• Tends to be most effective way to learn a framework

– Typically infeasible to read the documentation of all operations
– Instead, find a “recipe” similar to what you need to do
– Understand the recipe by reading about the ingredients
• Selective reading of the documentation
– Then you can combine the ingredients in new ways with confidence

5
 Cookbook Programming
• You have a template for your program
• You change things around, but you don’t mess with the
overall structure
• Examples:
public static void main(String[] args) { … }
for (int i=0; i<args.length; i++) { … }
• Many people consider Swing development to be cookbook
programming

6
 A Little History
In the beginning…
• There was Java. It was like C++, but simpler and
cleaner.
• Then came HotJava, a Java-based browser
– You could run chunks of Java code called Applets
– It was cool
Netscape & then IE added Java support
• But Applets were a pain
– Browsers had out of date JVMs
– Used the AWT (lots of platform-based non-Java code)
– Didn’t have the look and feel of the rest of the platform
– Couldn’t run as a standalone program with a GUI

7
 Swing
• A new user interface environment
– Implemented in Java
• More consistent across implementations
– Offers different “look and feel” options
• Windows, Unix, and other (Metal)
– Can be a main method or a JApplet
• Still uses AWT for event handling, fonts, etc.
– BTW – still issues with Swing non-native look and feel,
predictable performance
– SWT – An alternate Standard Widget Toolkit (from Eclipse)
addresses this by staying closer to OS windowing support
• but, not standard for Java

8
 Simplest Structure
• You make a Window (a JFrame)
• Make a container (a JPanel)
– Put it in the window
• Add your Buttons, Boxes, etc to the container
– Use layouts to control positioning
– Set up listeners to receive events
– Optionally, write custom widgets with application-specific display logic
• Set up the window to display the container

• Then wait for events to arrive…

9
 Components
Swing has lots of components:
• JLabel • JTextField
• JButton • JTextArea
• JCheckBox • JList
• JChoice • JScrollBar
• JRadioButton • … and more

10
 JFrame & JPanel
• JFrame is the Swing Window
• JPanel (aka a pane) is the container to which you add your
components (or other containers)

11
 Layout Managers
• The default Layout Manager is FlowLayout
– Place items in the container from left to right
– When a line is full, FlowLayout goes to the next

12
 More Layout Options
• GridLayout
• GridBagLayout
• Explicit Placement

13
Example

14
 Question
• How do you make a button work?

15
 Events in Swing
• An event is when something changes
– Button clicked, scrolling, mouse movement
• Swing (actually AWT) generates an event
• To do something you need to implement a Listener Interface
and register interest

16
 Event Listeners
Swing has lots of event listener interfaces:
• ActionListener • MouseListener
• AdjustmentListener • TreeExpansionListener
• FocusListener • TextListener
• ItemListener • WindowListener
• KeyListener • …and on and on…

17
 ActionListener
• Events for JButtons, JTextFields, etc
– The things we are using
• Implement ActionListener
– Provide actionPerformed method
• In actionPerformed method
– Use event.getSource() to determine which button was clicked, etc.

18
Example

19
 Organizational Tips
• Declare references to components you’ll be manipulating as
instance variables
• Put the code that performs the actions in private “helper”
methods. (Keeps things neat)

20
 GUI design issues
• Interfaces vs. inheritance
– Inherit from JPanel with custom drawing functionality
– Implement the ActionListener interface, register with button
– Why this difference?

• Models and views

21
 GUI design issues
• Interfaces vs. inheritance
– Inherit from JPanel with custom drawing functionality
• Subclass “is a” special kind of Panel
• The subclass interacts closely with the JPanel – e.g. the subclass calls back
with super()
• The way you draw the subclass doesn’t change as the program executes
– Implement the ActionListener interface, register with button
• The action to perform isn’t really a special kind of button; it’s just a way of
reacting to the button. So it makes sense to be a separate object.
• The ActionListener is decoupled from the button. Once the listener is
invoked, it doesn’t call anything on the Button anymore.
• We may want to change the action performed on a button press—so once
again it makes sense for it to be a separate object

• Models and views

22
 For More Information
• Oracle’s Swing tutorials
– http://download.oracle.com/javase/tutorial/uiswing/
• Introduction to Programming Using Java, Ch. 6
– http://math.hws.edu/javanotes/c6/index.html

23