SCJP Java Programming Exam Cram Notes : events

It is useful to divide the event classes into Semantic events and Low-level events.

Semantic Events

These classes are used for high-level semantic events, to represent user interaction with GUI. ActionEvent, AdjustmentEvent, ItemEvent, TextEvent

Event Class	Source	Event Types
ActionEvent	Button - when clicked List - when doubleclicked MenuItem - when clicked TextField - when Enter key is pressed	ACTION_PERFORMED
AdjustmentEvent	Scrollbar - when adjustments are made	ADJUSTMENT_VALUE_CHANGED
ItemEvent	CheckBox - when selected or deselected CheckboxMenuItem - same as checkbox Choice - when an item is selected or deselected List - when an item is selected or deselected	ITEM_STATE_CHANGED
TextEvent	TextFieldTextArea	TEXT_VALUE_CHANGED

Methods defined in the events to get the information about them.

Event Class	Method	Description
ActionEvent	String getActionCommand	Returns the command name associated with this action
ActionEvent	int getModifiers	Returns the sum of modifier constants corresponding to the keyboard modifiers held down during this action. SHIFT_MASK, ALT_MASK, CTRL_MASK, META_MASK
AdjustmentEvent	int getvalue	Returns the current value designated by the adjustable component
ItemEvent	Object getItem	Returns the object that was selected or deselected Label of the checkbox
ItemEvent	int getStateChange SELECTED DESELECTED	Returned value indicates whether it was a selection or a de-selection that took place, given by the two constants in ItemEvent.

Low-level Events These classes are used to represent low-level input or window operations. Several low-level events can constitute a single semantic event. ComponentEvent, ContainerEvent, FocusEvent, KeyEvent, MouseEvent, PaintEvent, WindowEvent

Event Class	Source	Event Types
ComponentEvent	All components	COMPONENT_SHOWN, COMPONENT_HIDDEN, COMPONENT_MOVED, COMPONENT_RESIZED AWT handles this event automatically. Programs should not handle this event.
ContainerEvent	All components	COMPONENT_ADDED, COMPONENT_REMOVED AWT handles this event automatically. Programs should not handle this event.
FocusEvent	All components	FOCUS_GAINED, FOCUS_LOST Receiving focus means the component will receive all the keystrokes
InputEvent	All components	This is an abstract class. Parent of KeyEvent and MouseEvent. Constants for key and mouse masks are defined in this class.
KeyEvent	All components	KEYPRESSED, KEYRELEASED, KEYTYPED (when a character is typed)
MouseEvent	All components	MOUSE_PRESSED, MOUSE_RELEASED, MOUSE_CLICKED, MOUSE_DRAGGED, MOUSE_MOVED, MOUSE_ENTERED, MOUSE_EXITED
PaintEvent	All components	This event occurs when a component should have its paint()/update() methods invoked. AWT handles this event automatically. Programs should not handle this event. This event is not supposed to be handled by the event listener model. Components should override paint/update methods to get rendered.
WindowEvent	All windows	This event is generated when an important operation is performed on a window. WINDOW_OPENED, WINDOW_CLOSING, WINDOW_CLOSED, WINDOW_ICONIFIED, WINDOW_DEICONIFIED, WINDOW_ACTIVATED, WINDOW_DEACTIVATED

Methods defined in the events to get the information about them.

Event Class	Method	Description
ComponentEvent	Component getComponent	Returns a reference to the same object as getSource, but the returned reference is of type Component.
ContainerEvent	Container getContainer	Returns the container where this event originated.
ContainerEvent	Component getChild	Returns the child component that was added or removed in this event
FocusEvent	boolean isTemporary	Determines whether the loss of focus is permanent or temporary
InputEvent	long getWhen	Returns the time the event has taken place.
InputEvent	int getModifiers	Returns the modifiers flag for this event.
KeyEvent	int getKeyCode	For KEY_PRESSED or KEY_RELEASED events, this method can be used to get the integer key-code associated with the key. Key-codes are defined as constants is KeyEvent class.
KeyEvent	char getKeyChar	For KEY_TYPED events, this method returns the Unicode character that was generated by the keystroke.
MouseEvent	int getX	Return the position of the mouse within the originated component at the time the event took place
	int getY
	Point getPoint
	int getClickCount
WindowEvent	Window getWindow	Returns a reference to the Window object that caused the event to be generated.

Event Listeners

Each listener interface extends java.util.EventListener interface.

There are 11 listener interfaces corresponding to particular events. Any class that wants to handle an event should implement the corresponding interface.

Listener interface methods are passed the event object that has all the information about the event occurred. Then the listener classes should be registered with the component that is the source/originator of the event by calling the addXXXListener method on the component. Listeners are unregistered by calling removeXXXListener method on the component.

A component may have multiple listeners for any event type.

A component can be its own listener if it implements the necessary interface. Or it can handle its events by implementing the processEvent method. (This is discussed in explicit event enabling section).

All registered listeners with the component are notified (by invoking the methods passing the event object). But the order of notification is not guaranteed (even if the same component is registered as its own listener). Also the notification is not guaranteed to occur on the same thread. Listeners should take cautions not to corrupt the shared data. Access to any data shared between the listeners should be synchronized.

Same listener object can implement multiple listener interfaces.

Event listeners are usually implemented as anonymous classes.

Event Type	Event Source	Listener Registration and removal methods provided by the source	Event Listener Interface implemented by a listener
ActionEvent	Button List MenuItem TextField	addActionListener removeActionListner	ActionListener
AdjustmentEvent	Scrollbar	addAdjustmentListener removeAdjustmentListner	AdjustmentListener
ItemEvent	Choice List Checkbox CheckboxMenuItem	addItemListener removeItemListner	ItemListener
TextEvent	TextField TextArea	addTextListener removeTextListner	TextListener
ComponentEvent	Component	add ComponentListener remove ComponentListner	ComponentListener
ContainerEvent	Container	addContainerListener removeContainerListner	ContainerListener
FocusEvent	Component	addFocusListener removeFocusListner	FocusListener
KeyEvent	Component	addKeyListener removeKeyListner	KeyListener
MouseEvent	Component	addMouseListener removeMouseListner	MouseListener
MouseEvent	Component	addMouseMotionListener removeMouseMotionListner	MouseMotionListener
WindowEvent	Window	addWindowListener removeWindowListner	WindowListener

Event Listener interfaces and their methods:

Event Listener Interface	Event Listener Methods
ActionListener	void actionPerformed(ActionEvent evt)
AdjustmentListener	void adjustmentValueChanged(AdjustmentEvent evt)
ItemListener	void itemStateChanged(ItemEvent evt)
TextListener	void textValueChanged(TextEvent evt)
ComponentListener	void componentHidden(ComponentEvent evt) void componentShown(ComponentEvent evt) void componentMoved(ComponentEvent evt) void componentResized(ComponentEvent evt)
ContainerListener	void componentAdded(ContainerEvent evt) void componentRemoved(ContainerEvent evt)
FocusListener	void focusGained(FocusEvent evt) void focusLost(FocusEvent evt)
KeyListener	void keyPressed(KeyEvent evt) void keyReleased(KeyEvent evt) void keyTyped(KeyEvent evt)
MouseListener	void mouseClicked(MouseEvent evt) void mouseReleased(MouseEvent evt) void mousePressed(MouseEvent evt) void mouseEntered(MouseEvent evt) void mouseExited(MouseEvent evt)
MouseMotionListener	void mouseDragged(MouseEvent evt) void mouseMoved(MouseEvent evt)
WindowListener	void windowActivated(WindowEvent evt) void windowDeactivated(WindowEvent evt) void windowIconified(WindowEvent evt) void windowDeiconified(WindowEvent evt) void windowClosing(WindowEvent evt) void windowClosed(WindowEvent evt) void windowOpened(WindowEvent evt)

Event Adapters

Event Adapters are convenient classes implementing the event listener interfaces. They provide empty bodies for the listener interface methods, so we can implement only the methods of interest without providing empty implementation. They are useful when implementing low-level event listeners.

There are 7 event adapter classes, one each for one low-level event listener interface.

Obviously, in semantic event listener interfaces, there is only one method, so there is no need for event adapters

Event adapters are usually implemented as anonymous classes.

Explicit Event Enabling

How events are produced and handled?

OS dispatches events to JVM. How much low-level processing is done by OS or JVM depends on the type of the component. In case of Swing components JVM handles the low-level events.

JVM creates event objects and passes them to the components.

If the event is enabled for that component, processEvent method in that component (inherited from java.awt.Component) is called. Default behavior of this method is to delegate the processing to more specific processXXXEvent method. Then this processXXXEvent method invokes appropriate methods in all registered listeners of this event.

All the registered listeners of the event for the component are notified. But the order is not guaranteed.

This delegation model works well for pre-defined components. If the component is customized by sub-classing another component, then it has the opportunity to handle its own events by implementing appropriate processXXXEvent methods or the processEvent method itself.

To handle its own events, the subclass component must explicitly enable all events of interest. This is done by calling enableEvents method with appropriate event masks in the constructor. Enabling more than one event requires OR'ing corresponding event masks. These event masks are defined as constants in java.awt.AWTEvent.

If the component wants to also notify the registered listeners for the event, then the overriding methods should call the parent version of the methods explicitly.

Component class has a method processMouseMotionEvent, even though there is no event called MouseMotionEvent.

Steps for handling events using listeners or by the same component

Delegating to listeners	Handling own events (explicit enabling)
1. Create a listener class, either by implementing an event listener interface or extending an event adapter class. 2. Create an instance of the component 3. Create an instance of the listener class 4. Call addXXXListener on the component passing the listener object. (This step automatically enables the processing of this type of event. Default behavior of processEvent method in the component is to delegate the processing to processXXXEvent and that method will invoke appropriate listener class methods.)	1. Create a subclass of a component 2. Call enableEvents(XXX_EVENT_MASK) in the constructor. 3. Provide processXXXEvent and/or processEvent in the subclass component. If also want to notify the listeners, call parent method. 4. Create an instance of the subclass component

Delegating to listeners

Handling own events (explicit enabling)

1. Create a listener class, either by implementing an event listener interface or extending an event adapter class.
2. Create an instance of the component
3. Create an instance of the listener class
4. Call addXXXListener on the component passing the listener object. (This step automatically enables the processing of this type of event. Default behavior of processEvent method in the component is to delegate the processing to processXXXEvent and that method will invoke appropriate listener class methods.)

1. Create a subclass of a component
2. Call enableEvents(XXX_EVENT_MASK) in the constructor.
3. Provide processXXXEvent and/or processEvent in the subclass component. If also want to notify the listeners, call parent method.
4. Create an instance of the subclass component

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SCJP Java Programming Exam Cram Notes : events

12. Events