Interaction with a GUI

1. Interaction with a GUI Interaction with GUI

2. Topics • Event handling in java with buttons • Dialogs • Validating user input Interaction with GUI

3. JOptionPane Dialog Interaction with GUI

4. JOptionPane • JOptionPane is an example of a dialog box. • It is used to get limited information from the user • Displays a message • Either Yes/No or OK/Cancel • Maybe simple text input • It is modal. This means that once it is displayed, the user has to interact with it before they can do anything else with the application. Interaction with GUI

5. JOptionPane examples - 1 • The simplest version displays a message • JOptionPane.showMessageDialog(null, "Goodbye"); • The first parameter is often ‘null’, meaning that the message is not attached to a parent Component, such as a JFrame. Interaction with GUI

6. JOptionPane examples - 2 • An input dialog gets text from the user • If the user presses cancel, the value returned is null String speed = JOptionPane.showInputDialog(null, "Speed in miles per hour?"); Interaction with GUI

7. Validating input • You could use this for any input, not just JOptionPanes try { double mph = Double.parseDouble(speed); double kph = 1.621 * mph; JOptionPane.showMessageDialog(null, "KPH = " + kph); finish = true; } catch (NumberFormatException nfe) { JOptionPane.showMessageDialog(null, "'" + speed + "' is not a number", "Invalid input", JOptionPane.ERROR_MESSAGE ); } Interaction with GUI

8. JOptionPane examples - 3 • This example sets the title & changes the icon JOptionPane.showMessageDialog(null, "'" + speed + "' is not a number", "Invalid input", JOptionPane.ERROR_MESSAGE ); Interaction with GUI

9. JOptionPane - summary • You can • Show a message, input or confirm dialog box • Link the dialog to a parent component, such as a frame, or not (essential) • Provide a message (essential) • Provide a title (optional) • Change the default set of option buttons (optional) • Change the style of the message (optional) • Includes a default icon (warning, error etc ) • For more details • See java tutorial and java documentation • My example provided on Vision Interaction with GUI

10. Back to buttons Interaction with GUI

11. User Interface Events • There are many user interface events. • Today we are interested in button clicks • Other events might be…… • Mouse pressed, released, clicked, dragged,... • Text value changed • Window closed, activated • Check box, list box, combo box changed • etc Interaction with GUI

12. An Event-driven program • An event driven program waits for an event and then reacts to it • The user, not the programmer, controls the order in which events occur • This is the natural model for a program with a GUI • The programmer specifies which events they wish to handle • Events which are not handled are ignored • The java.awt.event package is used to • Listen for events • Transfer control to the correct place in the program Interaction with GUI

13. Java Event Handling Model ActionEvent Listening Class is registered to listen for Button Event source Event handler fires action event passes action event details to Operating System Interaction with GUI

14. The event source • The event source (e.g. Button) • Can generate the event (e.g. button clicks) JButton search; search = new JButton("Search"); • Manages the listeners – knows which class contains the methods which will handle the event • The method-handling class is specified using the addActionListenermethod • In our case, the method which handles the event will be in the same class i.e. the GUI class, and is referred to as ‘this’ • search.addActionListener(this) ; Interaction with GUI

15. The listeners • The event listener class must provide standard methods to handle the event • The class does this by implementing the appropriate Listener interface • For JButtons, there is an ActionListener interface • This interface describes the required method actionPerformed which handles button clicks. • All the information about the button click is provided in an ActionEvent object, which is a parameter to the actionPerformed method. Interaction with GUI

16. actionPerformed method • In the StaffList example, our frame class implements the interface and provides the actionPerformed method. public class StaffListGUI extends JFrame implements ActionListener { . . . public void actionPerformed(ActionEvent e) { if (e.getSource() == showListById) { displayList.setText(staffList.listByID()); } else if (e.getSource() == showListByName ) { displayList.setText(staffList.listByName()); } else if (e.getSource() == search) { . . . } } Interaction with GUI

17. Java Event Model • A typical program • System calls the main method • Main method calls constructors to set up the GUI • Main terminates • Nothing happens until the user causes an event • System calls the listener method • Listener method handles the event and terminates • Nothing happens until the next event..... Interaction with GUI

18. What can go wrong with listeners? • What if you do not register the listener class with the button? (i.e. no addActionListener ) • What if your GUI class implements ActionListener, but does not have an actionPerformed method? • What if you provide the actionPerformed method and do not implement ActionListener? • What if you forget to import the java.awt.event package? Interaction with GUI

19. Listener problems – answers - 1 • What if you do not register the listener class with the button? (i.e .no addActionListener ) • Nothing happens when you click on the button • What if your GUI class implements ActionListener, but does not have an actionPerformed method? • Compilation error, if the class implements an interface, it must provide the required methods Interaction with GUI

20. Listener problems - answers • What if you provide the actionPerformed method and do not implement ActionListener? • If you don’t add actionListeners either, the actionPerformed method is just another method in your class, but will never be called • If you do add actionListeners, the compiler recognises that your class doesn’t implement ActionListener, and complains. • What if you forget to import the java.awt.event package? • The compiler doesn’t recognise any of the event handling classes and interfaces and their methods Interaction with GUI

21. Summary : Enabling a GUI class to handle ActionEvents • Import java.awt.event.* • Add implements ActionListener to the class header • use the addActionListener method for each component that is to fire an event: myButton.addActionListener(this) • write an actionPerformed (ActionEvent e) method containing the event handling code. You may need to determine which event occurred. if (e.getSource() == myButton) Interaction with GUI

22. StaffList application Action Listener StaffList Demo JFrame Other Awt and swing classes StaffListGUI StaffList Interface N.B. StaffList and Staff classes could be used by different interface => Flexible design StaffList Staff Interaction with GUI

23. Closing an application • Sometimes you want to do some final processing just before closing. • Disable the window closing button setDefaultCloseOperation(this.DO_NOTHING_ON_CLOSE); • Add your own button to the GUI close = new JButton("Close"); close.addActionListener(this); northPanel.add(close); • Do whatever you like when the user presses it. elseif (e.getSource() == close) { JOptionPane.showMessageDialog(this, "Do 'end of program' things instead of showing this"); System.exit(0); } Interaction with GUI

24. Getting data from text fields • If the user inputs data, you get it from the text field and check that it is ok String searchString = searchField.getText().trim(); if(searchString.length() > 0) { Staff person = staffList.findById(searchString); if (person != null ) result.setText(person.toString()); else result.setText("not found"); } else result.setText("no text entered"); Interaction with GUI

25. More data validation • You may need to convert a string to a number • Recall Integer.parseInt discussed under File IO • See update method in StaffListGUI2 • Or check ranges using if/else ...... • You could use a try/catch Interaction with GUI

26. Summary • These 2 lectures provide examples of: • Organising layout • Closing the application from a GUI • Displaying in text areas, labels, text fields • Using JOptionPane methods • Responding to button clicks • Getting data from a text area • You should be able to study the supplied code and use a similar approach in your assignment Interaction with GUI