GUIs in Java Swing, 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 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 Components and containers: –superclasses and interfaces –extends and 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 )

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 11

Demo 12

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 events e.g., a JButton object, when clicked, generates an ActionEvent object, which is handled by an ActionListener (an object whose class implements this interface)

MSD book, p

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 listener ActionListener listener = new MyListener(); … // instantiate event source JButton button = new JButton( “Hello” ); … // register event listener with event source button.addActionListener( listener );

Common Technique (e.g. Jigloo) Use anonymous class to create a listener object “on the fly” // register event listener with event source button1.addActionListener( new ActionListener() { public void actionPerformed(ActionEvent evt) { handlerForButton1Press(evt); //see below } });... // and later you write the handler method public void handlerForButton1Press(evt) { // your code to handle button press } 19

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?

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!

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