Events

1. Events • Anything that happens in a GUI is an event. For example: • User clicks a button, presses return when typing text, or chooses a menu item (ActionEvent) • Mouse button is clicked or released, or mouse cursor moves into a component (MouseEvent) • User changes an object's text (TextEvent) • A window receives focus, is closed, or iconified (WindowEvent) • and many more

2. Event Objects • Each event is represented by an object that: • gives information about the event, and • identifies the event source • An event source is typically a component, but other kinds of objects can be sources as well

3. Event Object Hierarchy (Partial) Object EventObject AWTEvent ActionEvent TextEvent ComponentEvent WindowEvent InputEvent MouseEvent

4. Listeners • In order to respond to an event, an object must be notified when the event occurs • In order to be notified, the object must: • be registered as an event listener on the appropriate event source, and • implement the appropriate listener interface • Recall: an interface is a set of methods that must be defined by any class that implements it

5. Event Listener Interface Hierarchy (Partial) EventListener ActionListener MouseListener TestListener WindowListener Note correspondence to event objects.

6. ActionEvent and ActionListener ActionEvent Button ActionListener The ActionListener interface has just one method: public void actionPerformed(ActionEvent e) { <code to execute when an action occurs> } Any class that implements the ActionListener interface must define the actionPerformed method.

7. Example import java.awt.*; import javax.swing.*; public class Main { public static void main(String[] args) { ListenerTest test = new ListenerTest(); } } public class ListenerTest { JFrame frame; Container contentPane; JButton button; public ListenerTest() { frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); contentPane = frame.getContentPane(); button = new JButton("EXIT"); contentPane.add(button, BorderLayout.CENTER); frame.setVisible(true); } }

8. Event Handling • The previous example will put button in a frame, but it will not respond to any clicks yet • In order to handle an ActionEvent, the button must add an ActionListener object • We make the ListenerTest class into an action listener by having it implement the ActionListener interface: • declare that it does so with an implements clause, and • define the required actionPerformed method • What ActionListener object should be added to button?

9. Modified Example import java.awt.*; import java.awt.event.*; import javax.swing.*; public class ListenerTest implements ActionListener { JFrame frame; Container contentPane; JButton button; public ListenerTest() { frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); contentPane = frame.getContentPane(); button = new JButton("EXIT"); button.addActionListener(this); contentPane.add(button, BorderLayout.CENTER); frame.setVisible(true); } public void actionPerformed(ActionEvent e) { frame.dispose(); System.exit(0); } }

10. Notes on the Example • It is a good idea to dispose any top-level Swing window object when you are done with it • Why? Because these objects all extend heavyweight AWT windows which: • are allocated by the O.S. and not by Java • might take graphics card memory that is not automatically given back when Java program exits • I created a couple of dozen JFrames under KDE and exited without disposing: • eventually crashed the JVM

11. Notes on the Example (cont'd) • System.exit(0) signals a normal exit • java.awt and java.awt.event are separate packages; anytime you do event handling you must import the latter • Since the ListenerTest class implements the ActionListener interface, any instance of it can be added to button, including this • Other methods of the form addXXXListener are available where XXX signifies other types of events

12. Another Approach • The button could add any appropriate ActionListener, not just this • We could, for example, define another class, say, MyActionListener, that implements the ActionListener interface • Then we could create an object of type MyActionListener and add it to the button • Then the ListenerTest class itself would not have to implement the ActionListener interface

13. The MyActionListener Class import java.awt.event.*; public class MyActionListener implements ActionListener { JFrame frame; public MyActionListener(JFrame f) { frame = f; } public void actionPerformed(ActionEvent e) { frame.dispose(); System.exit(0); } }

14. Modified ListenerTest Class import java.awt.*; // java.awt.event.* no longer import javax.swing.*; // necessary public class ListenerTest { // no longer implements // ActionListener JFrame frame; Container contentPane; JButton button; public ListenerTest() { frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); contentPane = frame.getContentPane(); button = new JButton("EXIT"); MyActionListener al = new MyActionListener(frame); button.addActionListener(al); contentPane.add(button, BorderLayout.CENTER); frame.setVisible(true); } }

15. Anonymous Classes • The MyActionListener class is only used to make one instance • In such cases an alternative is to define an unnamed (anonymous) class ``on the fly'' in the one place where it is used • Java allows the following construction to create an instance of an anonymous class that implements the ActionListener interface: new ActionListener() { public void actionPerformed(ActionEvent e) { <code to execute when the event occurs> } }

16. Another Modified ListenerTest Class import java.awt.*; import java.awt.event.*; // reinstated import javax.swing.*; public class ListenerTest { JFrame frame; Container contentPane; JButton button; public ListenerTest() { // still does not need implements frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); contentPane = frame.getContentPane(); button = new JButton("EXIT"); ActionListener al = new ActionListener() { public void actionPerformed(ActionEvent e) { frame.dispose(); System.exit(0); } }; button.addActionListener(al); contentPane.add(button, BorderLayout.CENTER); frame.setVisible(true); } }

17. Anonymous Objects • Not only is the constructor for the anonymous class used only once, but the one object al that is created is used only once • So we can simplify matters further by making this object anonymous also: ... button = new JButton("EXIT"); button.addActionListener( new ActionListener() { public void actionPerformed(ActionEvent e) { frame.dispose(); System.exit(0); } }); contentPane.add(button, BorderLayout.CENTER); ...

18. WindowEvents and WindowListeners • Window events: opening, closing, activating (receiving focus), deactivating, iconifying, and deiconifying • WindowListener interface methods: • void windowOpened(WindowEvent e) • void windowClosed(WindowEvent e) • void windowClosing(WindowEvent e) • void windowActivated(WindowEvent e) • void windowDeactivated(WindowEvent e) • void windowIconified(WindowEvent e) • void windowDeiconified(WindowEvent e)

19. Window Event Example import java.awt.*; import javax.swing.*; public class ListenerTest { JFrame frame; public ListenerTest() { frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); frame.setVisible(true); } } We would like to add a WindowListener to frame that will cause the JVM to exit when the window is closed.

20. Adding a Window Listener import java.awt.*; import java.awt.event.*; import javax.swing.*; public class ListenerTest { JFrame frame; public ListenerTest() { frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); frame.setVisible(true); frame.addWindowListener(new WindowListener() { public void windowClosing(WindowEvent e) { frame.dispose(); System.exit(0); } public void windowClosed(WindowEvent e) {} //do nothing public void windowOpened(WindowEvent e) {} public void windowActivated(WindowEvent e) {} public void windowDeactivated(WindowEvent e) {} public void windowIconified(WindowEvent e) {} public void windowDeiconified(WindowEvent e) {} }); } }

21. Adapter Classes • When you implement an interface, you are required to define all interface methods, even if you are interested in one or two. • Those you are not interested in still have to be defined, even if they do nothing • The task of defining interface methods to do nothing is done for you through abstract adapter classes • If you extend an adapter class, you only have to define the method(s) you care about; the rest do nothing by default

22. Adapter Class Hierarchy (Partial) Object WindowAdapter MouseAdapter FocusAdapter Implements Implements Implements WindowListener MouseListener FocusListener

23. Using a Window Adapter public class ListenerTest { JFrame frame; public ListenerTest() { frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); frame.setVisible(true); frame.addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) { frame.dispose(); System.exit(0); } }); } }

24. Anonymous Classes Again • In the previous example the anonymous class defined is an extension of the abstract WindowAdapter class • In the example before that, the anonymous class defined is an implementation of the WindowListener interface • In both cases, the new creates an instance of the anonymous class • The compiler creates a separate bytecode file for each anonymous class of the form <classname>$<n>.class • for example, ListenerTest$1.class

25. Text Fields and Listeners • For one-line text input, use JTextField • For multiple-line text, use JTextArea • Both extend the JTextComponent class

26. Text Field Example public class ListenerTest { JFrame frame; JTextField textField; Container contentPane; public ListenerTest() { frame = new JFrame("Test Frame"); frame.setSize(new Dimension(300,200)); frame.setLocation(100,100); frame.addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) { frame.dispose(); System.exit(0); } }); textField = new JTextField(); contentPane = frame.getContentPane(); contentPane.add(textField, BorderLayout.CENTER); frame.setVisible(true); } }

27. Output This field has had some text entering and editing done. Note that the field fills the BorderLayout frame.