UMass Lowell Computer Science Java and Distributed Computing Prof. Karen Daniels Fall, 2000 Lecture 15 Java Fundamentals Java2 Graphical User Interfaces (GUIs) Fri. 10/13/00

Homework Status 1 Fri, 9/8 Fri, 9/15 Part 1 Mon, 9/18 Part 2 Mon, 9/18 Part 2 2Fri, 9/15 Fri, 9/22 Part 1 & Part 2 3Fri, 9/22 Fri, 9/29Part 1 & Part 2 4Fri, 10/6Fri, 10/13Part 1 & Part 2 5Fri, 10/13Fri, 10/20Part 1 & Part 2 HW# Assigned Due Content

Overview ä Events ä Widgets (GUI components) ä Containers ä Labels ä Buttons ä Text fields ä Select lists

GUIs Are Important ä GUIs need to provide a skillful blend of: ä User productivity - tasks are accomplished quickly ä User safety - mistakes are minimized ä Intuitiveness - training/learning effort is minimized

GUIs Are Hard to Build ä Event loop processing is a new, somewhat complicated paradigm ä Testing is a challenge (hard to exercise all user options) ä Layout requires task-based human factors considerations

Events ä The user interacts with your program by initiating “events” ä Mouse move ä Mouse button (up/down) ä Keyboard key (up/down)

Event-driven Programming ä Set up the user environment ä Create and initialize widgets ä Register event handlers with specific widgets ä Enter the “event loop” ä Wait for an event (listen) ä Handle the event (respond)

Handling Events ä Get the user input (if appropriate) ä Perform appropriate processing based on user action ä Refresh the user interface

Event-driven Programming in Java ä The event loop is hidden from you (you show() the top-level window) ä Java puts event information into an instance of a class derived from AWTEvent (depending on event type) ä Event instances are “sent” to widgets automatically ä Widgets pass off events to the appropriate handler(s)

Java Uses Event Delegation ä Widgets are represented by instances of various Java library classes ä Widgets maintain a list of event handlers (an EventListenerList) ä Event handlers are EventListeners (i.e., implement an EventListener interface) and are keyed to specific event types (they “listen” for certain types of events)

Event Delegation (concluded) ä EventListeners are programmer-selected classes that are distinct from the widget ä EventListeners are interfaces ä Widgets attach implementations of these listener interfaces ä Such implementations are often done via an anonymous inner class ä These implementations “handle” the event

Example of an Event Listener public class MyFrame extends JFrame {... // inside of a constructor, for example jTextField1 = new JTextField(); jTextField1.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent evt) { public void actionPerformed(ActionEvent evt) { jTextField1ActionPerformed(evt); jTextField1ActionPerformed(evt); } } ); // end of addActionListener ); // end of addActionListenergetContentPane().add(jTextField1);...}

Example of an Event Listener public class MyFrame extends JFrame {... private void jTextField1ActionPerformed (ActionEvent evt) { String inputString = jTextField1.getText(); String inputString = jTextField1.getText(); myColor = myStringToColor(inputString); myColor = myStringToColor(inputString); updateWidgets(); updateWidgets(); Container c = getContentPane(); Container c = getContentPane(); c.repaint(); c.repaint();}...}

Java Events ä Java aggregates low-level user events to make life easier for the developer ä Depending on the “focus” (“active” widget), e.g.: ä Button selected ä Text entered ä List item selected ä Menu item selected (we won’t discuss Java menus in this course) ä Item dragged (we won’t discuss Java drag and drop in this course)

Types of Java Events ä ActionEvent ä AdjustmentEvent ä ItemEvent ä ComponentEvent ä ContainerEvent ä FocusEvent ä PaintEvent ä WindowEvent ä Input Event ä KeyEvent ä MouseEvent

Types of Java Event Listeners ä ActionListener ä AdjustmentListener ä ItemListener ä ComponentListener ä ContainerListener ä FocusListener ä WindowListener ä KeyListener ä MouseListener ä MouseMotionListener ä TextListener

Event Adapters ä Convenience classes that implement event listener interfaces ä Provide “stubs” for methods you don’t want to implement yourself ä You just override the implementation of event handlers of interest ä For example, the WindowListener interface declares windowActivated, windowClosed, windowClosing, windowDeactivated, windowIconified, windowDeiconified, windowOpened methods that the WindowAdapter implements

Example of an Event Adapter public class MyFrame extends JFrame {... // inside of a constructor, for example addWindowListener (new WindowAdapter () { public void windowClosing (WindowEvent evt) { public void windowClosing (WindowEvent evt) { System.exit (0); System.exit (0); } } ); // end of addWindowListener call ); // end of addWindowListener call...}

Processing GUI Events in Java ä Implement an event handler ä Define a class that implements an event-listener interface (i.e., defines the appropriate event handling method(s)) ä Create an instance of the class ä Register the event handler with the widget, e.g.: ä addActionListener() ä addItemListener()

Another Example // define a public class MyFrame that extends JFrame private JButton myButton1; private boolean state1; // instance variables // in the constructor myButton1 = new JButton(“Press Me”); ButtonHandler handler = new ButtonHandler(); myButton1.addActionListener (handler); addWindowListener (new WindowAdapter () { public void windowClosing (WindowEvent evt) {System.exit(0); } public void windowClosing (WindowEvent evt) {System.exit(0); } } ); ); // elsewhere in the class, define method main for class MyFrame public static void main(String args[]) { MyFrame myFrame1 = new MyFrame(); MyFrame myFrame1 = new MyFrame(); myFrame1.show(); myFrame1.show();}

Another Example (2) // elsewhere in the class, define an inner class private class ButtonHandler implements ActionListener { public void actionPerformed(ActionEvent evt) public void actionPerformed(ActionEvent evt) { if (state1) { if (state1) { myButton1.setText("State2”); myButton1.setText("State2”); state1 = false; state1 = false; } else { else { myButton1.setText("State1”); myButton1.setText("State1”); state1 = true; state1 = true; } }

java.awt.Component ä The parent class for almost all user interface widgets ä Includes methods for: ä Adding various event listeners ä Getting/setting location & size ä Handling text (fonts,...) ä Painting a Graphics object ä Processing focus, visibility, colors, cursors,...

javax.swing.JComponent ä Parent class for most Swing components ä Supports different look-and-feels ä Mechanisms for tool tips, mnemonics, internationalization, borders, scrolling,...

java.awt.Container ä A collection of (holder for) other widgets ä You usually work with one of its subclasses (like JFrame) ä You add() other widgets to a container ä Can have a layout policy ä Set before add widgets ä Only one layout policy at a time ä Has benefits and disadvantages

Layout Policies ä Set with setLayout() method ä FlowLayout (left to right, top to bottom) ä BorderLayout (5 regions) ä GridLayout (arbitrary sized matrix) ä Others (Box, Card, GridBag,...)

java.awt.Frame ä Top-level window (implemented via native window) ä Has a title ä Can have a menubar ä Controls cursors

javax.swing.JFrame ä Extends Frame ä Platform-neutral window ä Children must be added to the content pane (Container) ä Has default close operation

javax.swing.JPanel ä Panels are sub-containers (containers within containers) ä Used to organize user interface into major sections ä Fairly lightweight

javax.swing.JLabel ä Extends JComponent ä Non-editable text and/or Image ä Generally does not change even under program control ä Often used with a text entry field ä Very important human factors element

javax.swing.JButton ä Extends AbstractButton ä Implements a simple user selection paradigm ä Typically has a title ä Can also have an Icon ä Several types ä Simple push button (command button) ä Select one of many radio button ä Two-state check box

javax.swing.JTextField ä Extends JTextComponent ä Basic one-line text entry ä Can be uneditable, typically editable ä Can set size by number of characters ä Includes text processing mechanisms (font, alignment,...)

javax.swing.JList ä Extends JComponent ä Multiple or sing-select, potentially scrolla ble, list ä Need to embed in a JScrollPane to get scrollbars ä Includes mechanisms to set, clear selected item(s), set number of visible items, modify colors, convert point to list index, modify list data,... ä Different selection modes (single, single contiguous, multiple discrete)