This classes that represent events are at the core of java'sevent hanflimg mechanism.This we begin our study of eventhandling with a tour of the events classes .As you will see ,thet provide a consistent easy to use means of encapsulating events ,At the root of the java events c;ass hierachy is Eventobject ,Which in java.util .It is the super class for all events .
Object getSource()
This method is defined in the Event Object class. It returns the object that originated the event. So if the user has clicked a Push button, the method getSource returns the object corresponding to Push button is generated.
int getID()
This method returns the integer value corresponding to the type of event. For example if the user has clicked on the mouse, the method getID returns the integer defined as MouseEvent.MOUSE_CLICKED.
ItemEvent is generated when an item is selected or deselected. Following components generate ItemEvents.
CheckBox : When the state of the CheckBox is changed.
CheckBoxMenuItem : When the state of a MenuItem is changed.
Choice : When the state of a ChoiceBox is changed.
List : When an item in list is selected or deselected.
TextEvent is generated when the contents of text component are changed. The components that generate TextEvent are TextArea and TextField.
Now I will describe the GUI components corresponding to low level event classes.
FocusEvent : This event is generated when a component gains or looses focus. Focus may be gained by bringing the mouse over a component (or by using the tab key). The component that has the focus receives all user keyboard events. Focus events are generated by objects of Component class and all its subclasses.
KeyEvent and MouseEvent are subclasses of abstract InputEvent class. Both these events are generated by objects of type Component class and its subclasses. The KeyEvent is generated when the user presses or releases a key on the keyboard. The MouseEvent is generated when the user presses the mouse or moves the mouse.
WindowEvent are generated for the Window class and its subclasses. These events are generated if an operation is performed on a window. The operation could be closing of window, opening of window, activating a window etc.
PaintEvent is generated when a component needs to be repainted (for example when an application which is in front is closed and the component needs to be redrawn.) PaintEvents are internally handled by AWT, and cannot (and should not) be handled by you in the application.
ComponentEvent are also generated by objects of type Component class and its subclasses. This event is generated when a component is hidden, shown, moved or resized.
ContainerEvents are generated when a component is added or removed from a container. ComponentEvent and ContainerEvent are handled by AWT and are not normally handled by the user.
Table Event types and corresponding EventSource & EventListener
A listener is an object that is notified when an event occur .It has two major requirements .First it must have been registerd with one or meore source to recieve notifications about specific types of events .Second it must implemented methods to recieve and process thes notifications .The methods that recieves and process events are defied in a set of interdaces found in java.awt.event .For example the MouseMotionListener interface defines two methods to recieve notifications whenthe mouse is dragged or moved .Any object may recieve and process notifications when the mouse is dragged or moved .Any object may recieve and process one or both of these events if it provides n implementation of this interface .
Table Event Listener Interfaces and corresponding methods which it defines
Object getSource()
This method is defined in the Event Object class. It returns the object that originated the event. So if the user has clicked a Push button, the method getSource returns the object corresponding to Push button is generated.
int getID()
This method returns the integer value corresponding to the type of event. For example if the user has clicked on the mouse, the method getID returns the integer defined as MouseEvent.MOUSE_CLICKED.
- AWT event classes
- The subclasses of ATW Event can be categorized into two groups - Semantic events and low-level events. Semantic events directly correspond to high level user interactions with a GUI component. Clicking of a button is an example of a semantic event. Following event classes are semantic classes.
- ActionEvent
AdjustmentEvent
ItemEvent
TextEvent - Multiple low-level events may get generated for each high level user event. Following event classes are low level event classes.
ComponentEvent
ContainerEvent
FocusEvent
KeyEvent
MouseEvent
PaintEvent
WindowEvent
ItemEvent is generated when an item is selected or deselected. Following components generate ItemEvents.
CheckBox : When the state of the CheckBox is changed.
CheckBoxMenuItem : When the state of a MenuItem is changed.
Choice : When the state of a ChoiceBox is changed.
List : When an item in list is selected or deselected.
TextEvent is generated when the contents of text component are changed. The components that generate TextEvent are TextArea and TextField.
Now I will describe the GUI components corresponding to low level event classes.
FocusEvent : This event is generated when a component gains or looses focus. Focus may be gained by bringing the mouse over a component (or by using the tab key). The component that has the focus receives all user keyboard events. Focus events are generated by objects of Component class and all its subclasses.
KeyEvent and MouseEvent are subclasses of abstract InputEvent class. Both these events are generated by objects of type Component class and its subclasses. The KeyEvent is generated when the user presses or releases a key on the keyboard. The MouseEvent is generated when the user presses the mouse or moves the mouse.
WindowEvent are generated for the Window class and its subclasses. These events are generated if an operation is performed on a window. The operation could be closing of window, opening of window, activating a window etc.
PaintEvent is generated when a component needs to be repainted (for example when an application which is in front is closed and the component needs to be redrawn.) PaintEvents are internally handled by AWT, and cannot (and should not) be handled by you in the application.
ComponentEvent are also generated by objects of type Component class and its subclasses. This event is generated when a component is hidden, shown, moved or resized.
ContainerEvents are generated when a component is added or removed from a container. ComponentEvent and ContainerEvent are handled by AWT and are not normally handled by the user.
Table Event types and corresponding EventSource & EventListener
| Event Type | Event Source | Event Listener interface |
| ActionEvent | Button, List, MenuItem, TextField | ActionListener |
| AdjustmentEvent | Scrollbar | AdjustmentListener |
| ItemEvent | Choice, Checkbox, CheckboxMenuItem, List | ItemListener |
| TextEvent | TextArea, TextField | TextListener |
| ComponentEvent | Component | ComponentListener |
| ContainerEvent | Container | ContainerListener |
| FocusEvent | Component | FocusListener |
| KeyEvent | Component | KeyListener |
| MouseEvent | Component | MouseListener, MouseMotionListener |
| WindowEvent | Window | WindowListener |
A listener is an object that is notified when an event occur .It has two major requirements .First it must have been registerd with one or meore source to recieve notifications about specific types of events .Second it must implemented methods to recieve and process thes notifications .The methods that recieves and process events are defied in a set of interdaces found in java.awt.event .For example the MouseMotionListener interface defines two methods to recieve notifications whenthe mouse is dragged or moved .Any object may recieve and process notifications when the mouse is dragged or moved .Any object may recieve and process one or both of these events if it provides n implementation of this interface .
Table Event Listener Interfaces and corresponding methods which it defines
| Event Listener interface | Event Listener Methods |
| ActionListener | actionPerformed(ActionEvent evt) |
| AdjustmentListener | adjustmentValueChanged(AjustmentEvent evt) |
| ItemListener | itemStateChanged(ItemEvent evt) |
| TextListener | textValueChanged(TextEvent evt) |
| ComponentListener | componentHidden(ComponentEvent evt), componentMoved(ComponentEvent evt), componentResized(ComponentEvent evt), componentShown(ComponentEvent evt) |
| ContainerListener | componentAdded(ContainerEvent evt), componentRemoved(ContainerEvent evt) |
| FocusListener | focusGained(FocusEvent evt), focusLost(FocusEvent evt) |
| KeyListener | keyPressed(KeyEvent evt), keyReleased(KeyEvent evt), keyTyped(KeyEvent evt) |
| MouseListener | mouseClicked(MouseEvent evt), mouseEntered(MouseEvent evt), mouseExited(MouseEvent evt), mousePressed(MouseEvent evt), mouseReleased(MouseEvent evt) |
| MouseMotionListener | mouseDragged(MouseEvent evt), mouseMoved(MouseEvent evt) |
| WindowListener | windowActivated(WindowEvent evt), windowClosed(WindowEvent evt), windowClosing(WindowEvent evt), windowDeactivated(WindowEvent evt), windowDeiconified(WindowEvent evt), windowIconified(WindowEvent evt), windowOpened(WindowEvent evt) |
- Adapter classes
- Event adapters facilitate implementing listener interfaces. Many event listener interfaces have more than one event listener methods. For such interfaces, Java technology defines adapter classes. These have empty implementation (stubs) of all the event listener methods defined in the interface they implement. A listener can subclass the adapter and override only stub methods for handling events of interest. The table below lists the low level event listener interfaces and their adapters.
Table Event Listener Interfaces and their corresponding adapter classes.
Event Listener interface Event Listener Adapter ComponentListener ComponentAdapter ContainerListener ContainerAdapter FocusListener FocusAdapter KeyListener KeyAdapter MouseListener MouseAdapter MouseMotionListener MouseMotionAdapter WindowListener WindowAdapter
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