Event Driven Programs with a Graphical User Interface

Event Driven Programs with a Graphical User Interface Rick Mercer

Event-Driven Programming with Graphical user Interfaces • Most applications have graphical user interfaces to respond to user desires

A Few Graphical Components • A Graphical User Interface (GUI) presents a graphical view of an application to users. • To build a GUI application, you must: • Have a well-tested model that is independent of the view • Make graphical components visible to the user • Ensure the correct things happen for each event • user clicks button, moves mouse, presses enter key, ... • Let's first consider some of Java's GUI components: • windows, buttons, and text fields

Classes in the swing package • The javax.swingpackage has components that show in a graphical manner JFrame: window with title, border, menu, buttons JButton: A component that can "clicked" JLabel: A display area for a small amount of text JTextField: Allows editing of a single line of text

Get a window to show itself • import javax.swing.JFrame; • publicclass FirstGUI extends JFrame { • publicstaticvoid main(String[] args) { • // Construct an object that has all the methods of JFrame • JFrame aWindow = new FirstGUI(); • aWindow.setVisible(true); • } • // Set up the GUI • public FirstGUI() { • // Make sure the program terminates when window closes • this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); • // Set the title of this instance of FirstGUI • this.setTitle("Graffiti"); • // … more to come … • } • }

Some JFrame messages • Set the size of the window with • // this not necessary if message is in • // a FirstGUI method like this constructor • setSize(220, 100); • The first int is the width of the window in pixels • the second int is the height of the window in pixels

Building components • So far we have an empty window • Let us add a button, a label, and an editable line • First construct three graphical components • JButton clickMeButton = • new JButton("Nobody is listening to me"); • JLabel aLabel = • new JLabel("Button above, text field below"); • JTextField textEditor = • new JTextField("You can edit this text "); • Next, add these objects to a JFrame

Add components to a window • Could use the default BorderLayout and add components to one of the five areas of a JFrame • add(clickMeButton, BorderLayout.NORTH); • add(aLabel, BorderLayout.CENTER); • add(textEditor, BorderLayout.SOUTH);

The 5 areas of BorderLayout • By default, JFrame objects have only five places where you can add components • a 2nd add wipes out the 1st • There are many layout managers • We will use null for layout: • Must set the size and location of each component

Null layout manager easier to layout components • Explicitly state where each component goes • // Add three graphical components using a null layout • setLayout(null); // Controversial ... • clickMeButton.setSize(200, 25); • clickMeButton.setLocation(0, 0); • add(clickMeButton); • // Add a label not needed by other methods in this class • JLabelaLabel=newJLabel("Button above, text field below"); • aLabel.setSize(200, 25); • aLabel.setLocation(0, 25); • add(aLabel); • textEditor.setSize(200, 25); • textEditor.setLocation(0, 50); • add(textEditor);

So what happens next? • You can layout a real pretty GUI • You can click on buttons, enter text into a text field, move the mouse, press a key • And NOTHING happens • So let’s make something happen…

Java's Event Model • Java lets the operating system notify graphical components of user interaction • JButton objects know when a user clicks it • JTextField objects with focus know when the user presses the enter (return) key • Event driven programs respond to many things • mouse clicks, mouse movements • clicks on hyperlinks or menu items • Users pressing any key, selecting a list item, moving a slider bar, checking a radio button, …

Example: Action Events • The buttons and text fields do not execute code • Instead JButton and JTextField objects send actionPerformed messages to other objects that have been registered to “listen” • We write the code we want to execute in actionPerformed methods • This requires a class that implements an interface, for example • Registering an instance of that class to listen

Event Driven Programs with GUIs • Key elements of an event-driven GUI • Graphical components • The screen elements that a user manipulates with the mouse and keyboard JFrame JLabel JButton JScrollbar JMenuItem JTextField JTextArea JList ... • Layout managers • Govern how the components appear on the screen • Examples FlowLayout GridLayout null layout • Events • Signal that a user interacted with the GUI • Examples: mouse clicks, keys pressed, hyperlink selected, time expires on a timer, …

Java's Event Model • JFrame • 1 Layout Graphical Components • JFrame • JButton • 4 Users interact with these graphical components • JMenuItem • JTextField • JButton • 3 You register objects that waits for messages from graphical components addActionListener • 2 You write classes that implement the correct interface • ActionListener • Listener • Listener • Listener

A Java GUI: Rick's model • We’ve begin by setting up the GUI 1. FirstGUIextendsJFrame It IS-A JFrame so we inherit many methods 2. mainstarts up the GUI (could be separate file) 3. Added instance variables – graphical components that will be needed by two or more methods (a JTextField, JButton, or JList will be needed by listeners later 4. Lay out a GUI and initialize instance variables in the constructor

But no one is "Listening" • Okay, now we have a GUI • but when run, nothing happens • Wanted: An object to listen to the button that understands a specific message such as • actionPerformed • Also need to tell the button who it can send the actionPerfomed message to • Register the listener with this method addActionListener(ActionListener al)

Handling Events 5. Add a private inner class that can listen to the event that the graphical component will generate Your class must implement a listener interface to guarantee that it has the expected methods: First up: ActionListener 6. Register the listener object to the GUI component (JButton JTextField) so it can later send actionPerformed messages to that listener when the use clicks the button or presses enter. events occur anytime in the future--the listener is listening (waiting for user generated events such as clicking a button or entering text into a text field)

ActionEvent / ActionListener • When a JButton object is clicked, it constructs an ActionEvent object and sends it to the actionPerformed method of its listeners • We can usually ignore that parameter, other times we can't • To register a listener to a JButton, send an addActionListener message to button • public void addActionListener(ActionListener al) • You need an ActionListener object • There is no ActionListener class! • What can we do?????

Implement an interface Then your object can be treated as if it were anActionListener Polymorphism in action: We can have any number of actionPerformed methods that do whatever they are supposed to. The Button does not care what happened

Inner class • Add an inner class • inner classes have access to the enclosing classes' instance variables • Make it private since no one else needs to know about it • Java added inner classes for the very purpose: to have listener objects respond to user interactions with GUI components

And register the listener withaddActionListener • // 6. Register the instance of the listener so the • // component can later send messages to that object • ButtonListeneraListener = newButtonListener(); • clickMeButton.addActionListener(aListener); • } // End constructor • // 5. inner class to listen to events • privateclassButtonListenerimplementsActionListener { • // No constructor needed here. • // Must have this method to implement ActionListener • publicvoidactionPerformed(ActionEventanActionEvent) { • System.out.println("Button was clicked."); • } • } • Caution: this is easy to forget. It is an error no one will tell you about

Another benefit of interfaces • Can have many ActionListener objects • Any class that implements ActionListener • may need a different class for every button and text field in the GUI • But they all can be treated as ActionListener objects • They can be passed as arguments to this method • publicvoid addActionListener(ActionListener aL) Adds an action listener to receive action events from JButtons, TextFields, ... aL - an instance of a class that implements the ActionListener interface

Assignment Compatible • Can pass instances of classes implementing an interface to the interface type parameter addActionListener(ActionListener anyListener) addActionListener(new ButtonListener()); addActionListener(new TextFieldListener()); • ButtonListener and TextFieldListener must implement interfaceActionListener

Listen to JTextField • Add to the current GUI • Have the button click toggle the JTextField form upper to lower case • Need methods getText and setText

JList and DefaultListModel • Code demo: • Show a JList GUI component with a DefaultListModel set as the model • will contain a list of strings • Respond to selections by showing the selected string in a dialog box

public class ShowJListListModel extends JFrame { • public static void main(String[] args) { • JFrame window = new ShowJListListModel(); • window.setVisible(true); • } • private JList guiList; • private DefaultListModel allStrings; • public ShowJListListModel() { • setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); • setSize(250, 300); • setLocation(100, 100); • setLayout(null); • initializeAllStrings(); • guiList = new JList(allStrings); • guiList.setSelectedIndex(allStrings.getSize() - 1); • guiList.setSize(150, 250); • guiList.setLocation(20, 10); • guiList.setFont(new Font("Arial", Font.BOLD + Font.ITALIC, 14)); • add(guiList); • registerListeners(); • } • private void registerListeners() { • ListSelectionListener listener = new GuiListListListener(); • guiList.addListSelectionListener(listener); • } • private class GuiListListListener implements ListSelectionListener { • public void valueChanged(ListSelectionEvent arg0) { • String str = allStrings.get(guiList.getSelectedIndex()).toString(); • System.out.println(str + " selected"); • } • } • private void initializeAllStrings() { • allStrings = new DefaultListModel(); • allStrings.addElement("Riley"); allStrings.addElement("Dakota"); • allStrings.addElement("Casey"); allStrings.addElement("Angel"); allStrings.addElement("Reese"); • }

Event Driven Programs with a Graphical User Interface