GUIs in Java Swing, Events

1. GUIs in JavaSwing, Events CS2110, SW Development Methods Readings:MSD, Chapter 12 Lab Exercise

2. Why Study GUIs in CS2110 • First, why not? • Complex topic, complex library • Many classes, methods • Hard to do this well initially • Reasons we study GUIs • Again, example of inheritance in a framework, software reuse, etc. • Event-driven programming • An important form of program-control

3. Swing • Swing is a Java library (framework) for creating GUIs • Part of a larger JFC (Java Foundation classes) • Replaces but uses an older library, AWT • Another, newer alternative: SWT • Used in Eclipse • Swing apps will use look-and-feel of the system they’re running on • Or can be set by the program

4. Learning Swing • Important things to learn • Swing components • E.g. buttons, text-fields, frames, etc. • How to organize and arrange them • Containers, layout managers • How to make things change when something happens • Event-based programming

5. Containment Hierarchy • Top-level container: • place for other Swing components to paint themselves • e.g., JFrame, JDialog, Japplet • Intermediate container: • simplify positioning of atomic components • e.g., JPanel, JSplitPane, JTabbedPane

6. Components and Containers • See pages 809-816 in text • All Swing GUI objects are Components • Some are also Containers • Example: JFrame, JPanel, etc • You place other Components inside Containers • Example: a JFrame has buttons, text-fields, etc. • Example: a JPanel is part of a window, in which we organize GUI components

7. What We Do with Containers • Add components to them • Determine how these items will be arranged on the screen • Layout control • We associate a Swing layout-manager with each container • Layout is very hard to do at the beginning • So we won’t sweat it in CS2110

8. Non-Containers • Atomic components: • self-sufficient components that present information to and get input from the user • e.g., JButton, JLabel, JList, JComboBox, JTextField, JTable

9. Swing • Componentsand containers: • superclassesand interfaces • extendsand implements © O’Reilly 1999

10. Top-Level Containers • JFrame example: • contains a single component JRootPane, which has a JMenuBar (optional) and a content pane • add non-menu components to its content panel • theFrame.add( aButton ) • Pre Java 5.0 • theFrame.getContentPane().add( aButton )

11. Events and User Interaction • Programs so far:main() is called and runs to completion • Swing programs: • “main” class has (or may be) a JFrame • main() sets up the GUI and ends • Program keeps running, waiting for and responding to “events” • Button pressed, text typed, window closed, etc • Program is event-driven

12. Demo

13. Events • Two approaches to event handling • read-evaluation loop (client-written loop) • notification-based (callbacks) • Swing uses the 2nd approach

14. Events • Swing: • objects communicate by “firing” and “handling”events (event objects) • (conventional method call) • events are sent from a single source object to one or more registered listener objects

15. Events • Swing: • different event sources produce different kinds of eventse.g., a JButton object, when clicked, generates an ActionEvent object, which is handled by an ActionListener (an object whose class implements this interface)

16. MSD book, p. 837

17. Events • Handling: • create a component • e.g., a JButton • add it to the GUI • e.g., to a JPanel • register a listener to be notified when the component generates an event • e.g., interface ActionListener • define the callback method • e.g., actionPerformed()

18. Event Handling • class MyListener implements ActionListener { … public void actionPerformed( ActionEvent event ) {// react to event … }} • …// instantiate event listenerActionListener listener = new MyListener();…// instantiate event sourceJButton button = new JButton( “Hello” );…// register event listener with event sourcebutton.addActionListener( listener );

19. Common Technique (e.g. Jigloo) • Use anonymous class to create a listener object “on the fly” // register event listener with event sourcebutton1.addActionListener( new ActionListener() { public void actionPerformed(ActionEvent evt) { handlerForButton1Press(evt); //see below }}); ... // and later you write the handler methodpublic void handlerForButton1Press(evt) { // your code to handle button press}

20. What’s this? Java Anonymous Classes • There’s a Java technique called anonymous classes • One of several types of nested class definition • You’ll very often see it in GUI programming (Swing) and with threads • Situation: • Sometimes Java’s design encourages us to create some thing that might be used just once • That thing needs to be wrapped up in a class, say because we need a function object • What couldn’t we just declare it at the same place we use it? Why create a separate file?

21. Creating and Using an Anonymous Class • Example: sort a list of Strings by their length Collections.sort ( stringList, new Comparator() { public int compare( Object o1, Object o2 ) { return ((String) o1).length() - ((String) o2).length(); } }) ; • We’ve created a new Comparator “on the fly” • new creates a new instance, but what kind? • Some object that implements Comparator • Object not named, and its “true” class not named! • What must a Comparator have? compare() • We defined it right here, where it’s used!

22. Anonymous Classes: Comments • Anonymous classes are unlike other classes • They have no name • Typically only implement methods in their interface or superclass. No new methods! • Since they have no name, can only define and use them at one point in your code! • Hard to understand at first? Sure! • Naming an abstraction is important for human understanding! • Sorting, a Collection, Comparing • Advice • Keep them very short (and simple)! • Be ready to understand them when you see them in Swing and with threads