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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); ...

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more event handling

More Event Handling


Anonymous Listeners

Pop menus

Validating User Input

remember event handling
Remember event handling..

public class MyWindow extends JFrameimplements ActionListener{

public MyWindow(){


ok= new JButton("OK");ok.addActionListener(this);panel.add(ok);



// event handlerpublic void actionPerformed (ActionEvent e){

System.exit(0); // i.e. what happens




User action that results in Event Listener Type

User clicks a button, presses Enter while ActionListener

typing in a text field, chooses a menu item

User closes a frame (main window) WindowListener

User presses a mouse button while over MouseListener

a Component

User moves the mouse over a component MouseMotionListener

Component gets the keyboard focus FocusListener

Table or list selection changes ListSelectionListener

Any property in a component changes, PropertyChangeListenersuch 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 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

  • 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
  • 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{…
    • }
    • }

public class MyWindow extends JFrame {

private JButton saveButton;

private JMenuItem newMenuItem; private JTextField nameTextField;

public MyWindow(){ …

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..

  • 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
Listeners with multiple event handlers
  • MouseListener:
    • mouseClicked(MouseEvent e)
    • mouseEntered(MouseEvent e)
    • mouseExited(MouseEvent e)
    • mousePressed(MouseEvent e)
    • mouseReleased(MouseEvent e)
  • 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)
  • 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….

  • 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
Using Adapters

// create WindowListener class

class MyWindowListener extends WindowAdapter {

// override event handler public void windowClosing(WindowEvent e) {

// include logic here




// register window listener with the frame// within the frame’s constructor

this.addWindowListener(new MyWindowListener());



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
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
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

Adapter class is instantiated. No name…

..and relevant method that you want your i/f to respond to is called

  • 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)

what component is associated with the it, and the location to display it…

using popup menus
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
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 MouseListener that triggers the pop up menu

Note that we’re using an adapter class

// 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()) {, e.getX(), e.getY());




register mouselistener
Register MouseListener
  • Instantiate Listener
  • Register listener with the component that popup is invoked on

myPopupListener = new PopupListener(popup);


e.g. the invoker might be a panel..

validating user input
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  floatfloat Float.parseFloat(String s)
    • Must handle the NumberFormatException
validating user input1
Validating User Input

public void actionPerformed (ActionEvent e){

// extract string from textfieldString str = field.getText().trim();


// 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 msgJOptionPane.showMessageDialog(null,"You did not enter a number"); }


An example…

Trim() moethod gets rid of leading and trailing whitespaces..

looked at
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…