JAVA EVENT CLASSES

     Event classes are the classes that represent events at the core of java’s event handling mechanism.

    At the root of the Java event class hierarchy is EventObject, which is in java.util. It is the superclass of all events. Its constructor is:

EventObject (Object src)

Here, src is the objects that generates the event.

     EventObject contains two objects: getSource() and toString().

The getSource() method returns the source of the event. Its general form is:

Object getSource()

toString() method returns the string equivalent of the event.

   The class AWTEvent, defined within the java.awt package, is a subclass of EventObject. It the superclass of all AWT-based events used by delegation event model.

The main classes in java.awt.event:

The ActionEvent Class:

 The ActionEvent class defines four integer constants that can be used to identify any modifiers associated with an action event: ALT_MASK, CTRL_MASK, META_MASK, and SHIFT_MASK. There is integer constant ACTION_PERFORMED which can be used to identify action events.

 ActionEvent has three constructors:

ActionEvent (Object src, int type, String cmd)

ActionEvent (Object src, int type, String cmd, int modifiers)

ActionEvent (Object src, int type, String cmd, long when, int modifiers)

Here, src is a reference to the object that generated this event. The type of the event is specified by type and its command string is cmd. The argument modifiers indicate which modifier keys (ALT, CTRL, META, and/or SHIFT) were pressed when the event was generated. The parameter specifies when the event occurred.

 Command name for the invoking ActionEvent object by using the getActionCommand() method:

String getActionCommand()

For example, when a button is pressed, an action event is generated that has a command name equal to the label on that button. 

 The getModifier() method returns a value that indicates which modifier keys were pressed when the event was generated.

int getModifier()

 The getWhen() method returns the time at which event took palce.

long getWhen()

The AdjustmentEvent Class:

 There are five types of AdjustmentEvent. The AdjustmentEvent class defines integer constants that can be used to identify them.

BLOCK_DECREMENT	The user clicked inside the scroll bar to decrease its value.
BLOCK_INCREMENT	The user clicked inside the scroll bar to increase its value.
TRACK	The slider was dragged.
UINT_DECREMENT	The button at the end of scroll bar was clicked to decrease its value.
UNIT_INCREMENT	The button at the end of scroll bar was clicked to increase its value.

In addition there is an integer constant, ADJUSTMENT_VALUE_CHANGED that indicates that a change has occurred.

 AdjustmentEvent has a constructors:

AdjustmentEvent (Adjustable src, int id, int type, int data)

Here, src is a reference to the object that generated this event. The type of the adjustment is specified by type and its associated data is data.

 The getAdjustable() method returns object that generated the event.

Adjustable get Adjustable()

For example, when a button is pressed, an action event is generated that has a command name equal to the label on that button. 

 The getAdjustmentType() method returns the type of adjustment event.

int getAdjustmentType()

 The getValue() method returns the amount of adjustment.

long getValue()

The ComponentEvent Class:

 There are four types of AdjustmentEvent. The AdjustmentEvent class defines integer constants that can be used to identify them.

COMPONENT_HIDDEN	The component was hidden.
COMPONENT_MOVED	The component was moved.
COMPONENT_RESIZED	The component was resized.
COMPONENT_SHOWN	The component became visible.

 ComponentEvent has a constructors:

ComponentEvent (Component src, int type)

Here, src is a reference to the object that generated this event. The type of the adjustment is specified by type.

 The ComponentEvent is the superclass either directly or indirectly of ContainerEvent, FocusEvent, KeyEvent, MouseEvent, and WindowEvent.

 The getComponent() method returns the component that generated the event.

Component getComponent ()

The ContainerEvent Class:

 There are two types of container events. The ContainerEvent class defines int constants that can be used to identify them: COMPONENT_ADDED and COMPONENT_REMOVED. They indicate that a component has been added to or removed from the container.

 ContainerEvent is a subclass of ComponentEvent and has a constructor:

ContainerEvent (Component src, int type, Component comp)

Here, src is a reference to the object that generated this event. The type of the event is specified by type and the component that has been added to or removed from the container class.

 The getContainer() method returns a reference to the container that generated the event.

Container getContainer ()

 The getChild() method returns a reference to the component that was added to or removed from the container.

Component getChild()

The FocusEvent Class:

 FocusEvents are identified by integer constants FOCUS_GAINED and FOCUS_LOST.

 FocusEvent is a subclass of ComponentEvent and has these constructor:

FocusEvent (Component src, int type)

FocusEvent (Component src, int type, boolean temporaryFlag)

FocusEvent (Component src, int type, boolean temporaryFlag, Component other)

Here, src is a reference to the object that generated this event. The type of the event is specified by type. The argument temporaryFlag is set to true if the focus event is temporary. Otherwise it is set to false. The other component involved in the focus change, called the opposite component, is passed in other. Therefore, if a FOCUS_GAINED event occurred, other will refer to the component that lost focus.

 The getOppositeComponent() method used to determine the other component.

Component getOppositeComponent ()

 The isTemporary() method indicates that if this focus change is temporary.

boolean isTemporary()

The method returns true if the change is temporary.

The InputEvent Class:

 The abstract class InputEvents is a subclass of ComponentEvent and is a superclass for component input events. Its subclasses are KeyEvent and MouseEvent.

 InputEvent defines several integer constants that represent any modifiers, such as the control key being pressed, that might be associated with the event. The following eight modifiers are defined:

ALT_MASK	BUTTON2_MASK	META_MASK
ALT_GRAPH_MASK	BUTTON3_MASK	SHIFT_MASK
BUTTON1_MASK	CTRL_MASK

 However, because of possible conflict between the modifiers used by keyboard events and mouse events, and other issues, the following extended modifier values were addes:

ALT_DOWN_MASK	BUTTON2_DOWN _MASK	META_DOWN _MASK
ALT_GRAPH_DOWN _MASK	BUTTON3_DOWN _MASK	SHIFT_DOWN _MASK
BUTTON1_DOWN _MASK	CTRL_DOWN _MASK

 To test if a modifier was pressed at the time an event is generated, use the isAltDown(), isAltGraphDown(), isControlDown(), isMetaDown(), and isShiftDown() method. The forms of these methods are shown below:

boolean isAltDown() 

boolean isAltGraphDown()

boolean isControlDown()

boolean isMetaDown()

boolean isShiftDown()

The getModifier() method is used to obtain a value that contains all of the original modifier flags.

int getModifiers()

The getModifierEx() method is used to obtain the extended modifiers.

int getModifiersEx()

The ItemEvent Class:

 There are two types of item events which are identified by two integer constants:

DESELECTED	The user deselected an item.
SELECTED	The user selected an item.

ItemEvent defiens one integer constant, ITEM_STATE_CHANGED that signifies a change of state.

 ItemEvent uses the constructor:

ItemEvent (ItemSelectable src, int type, Object entry, int state)

Here, src is a reference to the object that generated this event. The type of the event is specified by type. The specific item that generated the item event is passed in entry. The current state of that item is in state.

 The getItem() method returns a reference to the item that generated an event.

Object getItem ()

 The getItemSelectable() method returns a reference to the ItemSelectable object that generated an event.

ItemSelectable getItemSelectable ()

 The getStateChange() method returns the state change(i.e, SELECTED or DESELECTED) for an event:

int getStateChange ()

The KeyEvent Class:

 There are three types of key events, which are identified by these integer constants: KEY_PRESSED, KEY_RELEASED, and KEY_TYPED. The first two events are generated when a key is pressed or released. The last event occurs when a character is generated.

 Many other integer constants that are defined by KeyEvent:

VK_ALT	VK_DOWN	VK_LEFT	VK_RIGHT
VK_CANCLE	VK_ENTER	VK_ PAGE_DOWN	VK_SHIFT
VK_CONTROL	VK_ESCAPE	VK_PAGE_UP	VK_UP

The VK constants specifies virtual key codes.

 KeyEvent is a subclass of InputEvent and uses the constructor:

KeyEvent (Component src, int type, long when, int modifiers, int code, char ch)

Here, src is a reference to the object that generated this event. The type of the event is specified by type. The system time at which the key is pressed is passed in when. The modifier argument specifies which modifiers were pressed when this key event occurred. The virtual key code is passed in code. The character equivalent is passed in ch. If no valid character exist, then ch contains CHAR_UNDEFINED. For KEY_TYPED events, code will contain VK_UNDEFINED.

 The getKeyChar() method returns the character that was entered and getKeyCode() method returns the key code.

char getKeyChar ()

int getKeyCode ()

The MouseEvent Class:

 There are eigth types of mouse events. MouseEvent class defines the following integer constants:

MOUSE_CLICKED	The user clicked the mouse.
MOUSE_DRAGGED	The user dragged the mouse.
MOUSE_ENTERED	The mouse entered a component.
MOUSE_EXITED	The mouse exited from a component.
MOUSE_MOVED	The mouse moved.
MOUSE_PRESSED	The mouse was pressed.
MOUSE_RELEASED	The mouse was released.
MOUSE_WHEEL	The mouse wheel was moved.

 MouseEvent class is a subclass of InputEvent. It uses the constructor:

MouseEvent (Component src, int type, long when, int modifier, int x,int y, int clicks, boolean triggersPopup)

Here, src is a reference to the object that generated this event. The type of the event is specified by type. The system time at which the mouse event occurred is passed in when. The modifier argument specifies which modifiers were pressed when this mouse event occurred. The coordinates of the mouse are passed in x and y. The click count is passed in clicks.The triggerPopup flag indicates if this event causes a popup menu to appear on this platform.

 The getX() and getY() methods returns the x and y coordinates of the mouse within the component when the event occurred.

int getX()

int getY()

Another method getPoint() method is used to obtain the coordinates of the mouse:

Point getPoint()

 The translatePoint() method changes the location of the event.

void translatePoint (int x, int y)

Here, the arguments x and y are added to the coordinates of the event.

 The getClickCount() method returns the number of mouse clicks for an event:

int getClickCount ()

 The isPopupTrigger() method tests if this event causes a pop-up menu to appear on this platform.

boolean isPopupTrigger()

 The getButton() method that represent the button that caused the event:

int getButton()

NOBUTTON

BUTTON1

BUTTON2

BUTTON3

NOBUTTON indicates that no button was pressed or released.

The MouseWheelEvent Class:

 MouseWheelEvent is a subclass of MouseEvent. Mouse wheels are used for scrolling. MouseWheelEvent defines two integer constants:

WHEEL_BLOCK_SCROLL	A page-up or page-down scroll event occurred.
WHEEL_UNIT_SCROLL	A line-up or line-down scroll event occurred.

MouseWheelEvent uses the constructor:

MouseWheelEvent (Component src, int type, long when, int modifier, int x,int y, int clicks, boolean triggersPopup, int scrollHow, int amont, int count)

Here, src is a reference to the object that generated this event. The type of the event is specified by type. The system time at which the mouse event occurred is passed in when. The modifier argument specifies which modifiers were pressed when this mouse event occurred. The coordinates of the mouse are passed in x and y. The number of clicks the wheel has rotated is passed in clicks. The triggerPopup flag indicates if this event causes a popup menu to appear on this platform. The scrollHow value must be either WHEEL_UNIT_SCROLL or WHEEL_BLOCK_SCROLL. The number of units to scroll is passed in amount. The count parameter indicates the number of rotational units that the wheel moved.

 The getWheelEvent() method returns number of rotational units:

int getWheelRotation()

A positive value is returned if the wheel moved counterclockwise. The negative value is returned if the wheel moved clockwise.

 The getScrollType() method returns either WHEEL_UNIT_SCROLL or WHEEL_BLOCK_SCROLL.

int getScrollType ()

If WHEEL_UNIT_SCROLL is returned getScrollAmount() can be called to obtain the number of units scrolled:

int getScrollAmount()

The TextEvent Class:

 TextEvents are defines the integer constant TEXT_VALUE_CHANGED.

 TextEvent has the constructor:

TextEvent (Object src, int type)

Here, src is a reference to the object that generated this event. The type of the event is specified by type.

 The text event object does not include the characters currently in the text components that generated the event.

The WindowEvent Class:

 WindowEvent class defines the following integer constants that can be used to identify them:

WINDOW_ACTIVATED	The window was activated.
WINDOW_CLOSED	The window has been closed.
WINDOW_CLOSING	The user requested that the window be closed.
WINDOW_ DEACTIVATED	The window was deactivated.
WINDOW_DEICONIFIED	The window was deiconified.
WINDOW_GAINED_FOCUS	The window gained input focus.
WINDOW_ICONIFIED	The window was iconified.
WINDOW_LOST_FOCUS	The window lost input focus.
WINDOW_OPENED	The window was opened.
WINDOW_STATE_CHANGED	The state of the window changed.

 WindowEvent class is a subclass of ComponentEvent. It uses the constructor:

WindowEvent (Window src, int type)

WindowEvent (Window src, int type, Window other)

WindowEvent (Window src, int type, int fromState, int toState)

WindowEvent (Window src, int type, Window other, int fromState, int toState)

Here, src is a reference to the object that generated this event. The type of the event is specified by type. other specifies the opposite window when a focus or activation event occurs. The fromState specifies the prior state of the window, and the toState specifies the next state of the window.

 The getWindow() method returns the object that generated the event.

Window getWindow()

 WindowEvent also defines methods that returns the opposite window (When a focus or activation event has occurred), the previous window state and the current window state:

Window getOppositeWindow()

int getOldState()

int getNewState()