GUIs and Events

1. GUIs and Events Rick Mercer

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

3. 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

4. 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

5. Get a window to show itself • Code to tell a JFrame to show itself: • // Construct window with a title • JFrame aWindow = new JFrame("Graffiti"); • // Make sure the program terminates when window closes • aWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); • aWindow.setVisible(true);

6. Some JFrame messages • Set the size of the window with • aWindow.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

7. 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 window object

8. Add components to a window • Add the previously constructed components to one of the five areas of a JFrame • aWindow.add(clickMeButton, BorderLayout.NORTH); • aWindow.add(aLabel, BorderLayout.CENTER); • aWindow.add(textEditor, BorderLayout.SOUTH);

9. 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

10. Null layout manager easier to layout components • We will explicitly state where each component goes on a Container, which can be a JFrame or JPanel • Code Demo: Add these components to a GUI • JLabel, JButton, JTextField

11. 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…

12. Java's Event Model • Java can let the operating system notify graphical components of user interaction • JButton objects notified when user clicks it • JTextField objects with focus knows when the user enters text and your program can respond • A menu item know that a user has selected it • Event driven programs respond to many things • mouse clicks, mouse movements • clicks on hyperlinks, buttons, menu items • Users pressing any key, selecting a list item, moving a slider bar, …

13. Example: Action Events • The buttons, text fields, and menu items do not perform the actions • Instead JButton, JTextField, JMenuItem objects send actionPerformed messages to other objects, for example • We write code to respond to the above events in actionPerformed methods • This requires a class that implements the ActionListener interface, for example

14. Event Driven Program 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, …

15. 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

16. A Java GUI: Rick's model • Preview for writing a GUI with events 1. Have a class that extendsJFrame so your class IS-A JFrame. so you inherit many methods already working! 2. Add mainto start up the GUI (could be separate file) 3. Add instance variables – include graphical components that will be needed by two or more methods (a JTextField, Timer, or JList will be needed by listeners later 4. Lay out a GUI and initialize instance variables in the constructor

17. The first 4 steps • import java.awt.*; • importjavax.swing.*; • publicclassSomeEventsextendsJFrame { • publicstaticvoid main(String[] args) { • // Construct an instance of this class and show it • SomeEvents window = newSomeEvents(); • window.setVisible(true); • } • // A graphical component to be "listened" to • privateJButtonaButton; • publicSomeEvents() { • layoutGUI(); • } • private void layoutGUI(); • // Lay out the GUI, initialize instance variables • // Have this object send some messages to itself • this.setSize(200, 100); • this.setTitle("Listen to button");

18. 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)

19. 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)

20. 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 • But there is no ActionListener class! • What can we do?????

21. 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

22. 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 exact purpose: to have listener objects respond to user interactions with GUI components

23. Have a class that implements ActionListener // 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."); } // 6. Register the instance of the listener so the // component can later send messages to that object ButtonListeneraListener = newButtonListener(); aButton.addActionListener(aListener); Caution: this is easy to forget. It is an error no one will tell you about

24. Polymorphism through 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 • public void addActionListener(ActionListener aL) Adds the specified action listener to receive action events from JButtons, TextFields, ... Parameter: aL - an instance of a class that implements the ActionListener interface

25. 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

26. Listen to JTextField • Add to the current GUI • Add a JTextField so when the user enters a String, show it in Upper case • Need JTextArea methods getText and setText

27. JList • Code demo: • Show a JList GUI component with a ListModel set as the model • List to clicks on the elements in the JList • Begin with code on the next slide

28. public class ShowJListListModel extends JFrame { • public static void main(String[] args) { • JFrame window = new ShowJListListModel(); • window.setVisible(true); • } • private JList guiList; • private ListModel allStrings; • public ShowJListListModel() { • setSize(380, 200); • setLocation(100, 100); • setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); • // List must implement ListModel so it can be shown in a JList • allStrings = new MyLittleList(); • guiList = new JList(allStrings); • guiList.setFont(new Font("Arial", Font.BOLD, 13)); • this.getContentPane().add(guiList, BorderLayout.CENTER); • } • }

29. Demo javax.swing.Timer • Count down to 10 • A timer is constructed and begun like this • timer = new Timer(1000, new TimerListener()); • timer.start(); • Timers need an ActionListener argument • privateclass TimerListener implements ActionListener { • publicvoid actionPerformed(ActionEvent ae) { • // Do whatever is here when the timer sends • // this listener an actionPerformed message • } • }