More Event Handling Adapters Anonymous Listeners Pop menus Validating User Input

Remember event handling.. public class MyWindow extends JFrame implements ActionListener { public MyWindow(){... ok = new JButton("OK"); ok.addActionListener(this); panel.add(ok);... } // event handler public void actionPerformed (ActionEvent e){ System.exit(0); // i.e. what happens }

Examples… User action that results in EventListener Type User clicks a button, presses Enter whileActionListener typing in a text field, chooses a menu item User closes a frame (main window)WindowListener User presses a mouse button while overMouseListener a Component User moves the mouse over a componentMouseMotionListener Component gets the keyboard focusFocusListener Table or list selection changesListSelectionListener Any property in a component changes,PropertyChangeListener such as the text on a button So when designing a GUI, you’ve got to figure out which listeners you need, for what components/user actions

ActionEvents ActionEvents are generated from –a JButton –a JMenuItem –a JTextField (if user presses Enter) All ActionEvents are handled by same event handler public void actionPerformed(ActionEvent e) How to distinguish between components generating the events? –i.e. supposing have 3 buttons, 2 menu items on the same screen? Which one was clicked? –Use the event to determine which component created it getSource() method returns the object that created the event

ActionEvents public void actionPerformed(ActionEvent e) { if (e.getSource() instanceof JButton) { if (e.getSource() == saveButton) { ………………… } else if (e.getSource() instanceof JMenuItem) { if (e.getSource() == newMenuItem) {…………… } else if (e.getSource() instanceof JTextField) { if (e.getSource() == nameTextField {………. } } etc

ActionEvents public void actionPerformed(ActionEvent e){ if (e.getSource() instanceof JButton) { if (e.getSource() == saveButton) {… } else if (e.getSource() instanceof JMenuItem) { if (e.getSource() == newMenuItem){… } else if (e.getSource() instanceof JTextField){ if (e.getSource() == nameTextField{… } Components that the user will interact can be set up as instance variables of the class. Then Can access in the action performed Method. More on code structure later.. public class MyWindow extends JFrame { private JButton saveButton; private JMenuItem newMenuItem; private JTextField nameTextField; public MyWindow(){ … …

Adapters Certain events can be generated by multiple actions, –e.g. opening, closing, minimising, maximising, activating, deactivating a window all create a WindowEvent –e.g. clicking a mouse, moving a mouse over a component, pressing and releasing the mouse buttons all create a MouseEvent the Listener has to handle all actions –many event handlers, one for each type of event (because different things happen as a response to each event…)

Listeners with multiple event handlers Your window may only need to handle one event but as it implements a listener interface it must override all event handler methods..! That’s the rule of implementing interfaces…. WindowListener: windowActivated(WindowEvent e) windowClosed(WindowEvent e) windowClosing(WindowEvent e) windowDeactivated(WindowEvent e) windowDeiconified(WindowEvent e) windowIconified(WindowEvent e) windowOpened(WindowEvent e) MouseListener: mouseClicked(MouseEvent e) mouseEntered(MouseEvent e) mouseExited(MouseEvent e) mousePressed(MouseEvent e) mouseReleased(MouseEvent e) MouseMotionListener: mouseMoved(MouseMotionEvent e) mouseDragged(MouseMotionEvent e)

public class MyWindow extends JFrame implements WindowListener{ //instance variables //constructor //createContentPane //createMenubar //event handlers public void windowActivated(WindowEvent e){ }// do nothing public void windowClosed(WindowEvent e){ }// do nothing windowClosing(WindowEvent e){ //include code here } windowDeactivated(WindowEvent e){ }//do nothing windowDeiconified(WindowEvent e){ } // do nothing windowIconified(WindowEvent e){ }// do nothing windowOpened(WindowEvent e){ }// do nothing} All I want to do is respond to closing a window….

Adapters To facilitate this: –an abstract Adapter class is created which implements empty methods for all event handlers each xxxListener interface has an associated xxxAdapter class E.g. WindowListener has the WindowsAdapter class –the Listener class then becomes a subclass of the Adapter class and overrides only the event handlers of interest –Think of the adapter class as implementing a set of dummy methods to save you work..

Using Adapters... // register window listener with the frame // within the frame’s constructor this.addWindowListener(new MyWindowListener());... // create WindowListener class class MyWindowListener extends WindowAdapter { // override event handler public void windowClosing(WindowEvent e) { // include logic here }

What if you adapter class can’t be used? To use an adapter class, your class has to inherit from it Since java supports single inheritance, can be restrictive – A solution is to use an anonymous listener… But be aware of performance issues – using anonymous listeners, you’re loading an extra class – increases start up time and memory requirements

Anonymous Listeners Listeners can be defined anonymous (i.e. with no name) –Class is created “on the fly” (i.e. placed where it is needed, where the listener is registered with the component) –anonymous  cannot be used by another component –responsible for only a single component  no need to include getSource() etc to identify where the event was generated –Used frequently with WindowListeners.. Why do you think? Syntax is component.addXXXListener( [whole definition of XXXListener goes here] );

Anonymous Listener Example addWindowListener( //begin implementation of Listener new WindowAdapter() { // begin implementation of required method public void windowClosing(WindowEvent e){ // include cleanup code here } // end required method } end Listener ); // end addWindowListener..and relevant method that you want your i/f to respond to is called Adapter class is instantiated. No name…

Pop up menus

PopupMenus Pop-up menus are free floating menus that are associated with an underlying component (the invoker) E.g. right clicking on your mouse as you read this… How would you implement..? Create a menu, decide what makes it appear (e.g. mouse right click ?.. so listen for this and show the menu), say exactly where on screen it should pop up, what events happen when I select menu options, etc.

Brought into existence by a pop-up trigger event that occurs while the mouse is over the invoker –trigger event is platform dependent –normally a MouseEvent –could be mouse pressed or mouse released event Pop-up menu created using JPopupMenu class –JMenuItems and separators added as before Pop-up menu is displayed using show() void show(Component origin, int x, int y) PopupMenus what component is associated with the it, and the location to display it…

Using Popup Menus Create the component – the popup menu –Register listeners with the menu items so you know when user has clicked on them Add event handling for menu items of the popup menu Create a MouseListener (using MouseAdapter) to listen for the trigger (user right click) Add event handling code to display the popup when requested Register MouseListener with appropriate component (normally some container)

Create Popup Menu private JPopupMenu createPopUp(){ // create pop-up menu JPopupMenu popup = new JPopupMenu(); // add menu items and associated listener menuItem = new JMenuItem(“whatever…”); menuItem.addActionListener(myActionListener); popup.add(menuItem); return popup; } Invoke this method in the JFrame constructor to create the popup menu

Create MouseListener that triggers the pop up menu // create PopupListener (a MouseListener) class PopupListener extends MouseAdapter{ JPopupMenu popup;// instance variable PopupListener(JPopupMenu popup){ this.popup = popup; } // include appropriate event handlers // for relevant acions e.g. mousePressed public void mousePressed(MouseEvent e) { if (e.isPopupTrigger()) { popup.show(e.getComponent(), e.getX(), e.getY()); } Note that we’re using an adapter class

Register MouseListener myPopupListener = new PopupListener(popup); Instantiate Listener Register listener with the component that popup is invoked on invoker.addMouseListener(myPopupListener); e.g. the invoker might be a panel..

Validating User Input In GUI user enters text into JTextFields Use String getText() to extract text from field May need to extract numbers from returned String –Use Envelop classes – wrapper classes for each primitive datatype Integer  int int Integer.parseInt(String s) Double  double double Double.parseDouble(String s) Float  float float Float.parseFloat(String s) –Must handle the NumberFormatException

Validating User Input public void actionPerformed (ActionEvent e){ // extract string from textfield String str = field.getText().trim(); try{ // try to extract num from string double num = Double.parseDouble(str); // successfully extracted the number // do something with it... } // unsuccessful extract so handle the error catch (NumberFormatException e){ // display error msg JOptionPane.showMessageDialog(null, "You did not enter a number"); } } An example… Trim() moethod gets rid of leading and trailing whitespaces..

Looked at… Implementing listeners –E..g MouseListener, WindowListener –As interfaces (just implement in your frame) –As adapter classes (when there are many method you don’t need) –As anonymous listeners – when inheriting from adapter class not good.. Creating Pop Menus Example of validating user input –Remember that field text is extracted as a String – convert if needs to be treated as a number…