Explain Event Classes AWTEvent, ActionEvent Class, The ComponentEvent Class The ContainerEvent Class, The FocusEvent Class, The InputEvent Class, The ItemEvent Class, The KeyEvent Class, The MouseEvent Class, The MouseWheelEvent Class, WindowEvent
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Event Classes
The classes that represent events are at the core of Java’s event handling mechanism. Thus, a
discussion of event handling must begin with the event classes. It is important to understand,
however, that Java defines several types of events and that not all event classes can be discussed
in this chapter. The most widely used events are those defined by the AWT and those defined by
Swing. This chapter focuses on the AWT events. (Most of these events also apply to Swing.)
Several Swing-specific events are described in Chapter 29, when Swing is covered.
At the root of the Java event class hierarchy is
EventObject
, which is in
java.util
.I
tisthe
superclass for all events. Its one constructor is shown here:
EventObject(Object src )
Here, src is the object that generates this event.
EventObject
contains two methods:
getSource( )
and
toString( )
.
The
getSource( )
method
returns the source of the event. Its general form is shown here:
Object getSource( )
As expected,
toString( )
returns the string equivalent of the event.
The class
AWTEvent
, defined within the
java.awt
package, is a subclass of
EventObject
.
It is the superclass (either directly or indirectly) of all AWT-based events used by the delegation
event model. Its
getID( )
method can be used to determine the type of the event. The signature
of this method is shown here:
int getID( )
Additional details about
AWTEvent
are provided at the end of Chapter 24. At this point, it
is important to know only that all of the other classes discussed in this section are subclasses
of
AWTEvent
.
To summarize:
•
EventObject
is a superclass of all events.
•
AWTEvent
is a superclass of all AWT events that are handled by the delegation
event model.
The package
java.awt.event
defines many types of events that are generated by various
user interface elements. Table 22-1 shows several commonly used event classes and provides
a brief description of when they are generated. Commonly used constructors and methods in
each class are described in the following sections.
The ActionEvent Class
An
ActionEvent
is generated when a button is pressed, a list item is double-clicked, or a
menu item is selected. 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
. In addition, there is an integer constant,
ACTION_
PERFORMED
, which can be used to identify action events.
Event Class
Description
ActionEvent
Generated when a button is pressed, a list item is double-clicked, or a menu
item is selected.
AdjustmentEvent
Generated when a scroll bar is manipulated.
ComponentEvent
Generated when a component is hidden, moved, resized, or becomes visible.
ContainerEvent
Generated when a component is added to or removed from a container.
FocusEvent
Generated when a component gains or loses keyboard focus.
InputEvent
Abstract superclass for all component input event classes.
ItemEvent
Generated when a check box or list item is clicked; also occurs when a choice
selection is made or a checkable menu item is selected or deselected.
KeyEvent
Generated when input is received from the keyboard.
MouseEvent
Generated when the mouse is dragged, moved, clicked, pressed, or released;
also generated when the mouse enters or exits a component.
MouseWheelEvent
Generated when the mouse wheel is moved.
TextEvent
Generated when the value of a text area or text field is changed.
WindowEvent
Generated when a window is activated, closed, deactivated, deiconified,
iconified, opened, or quit.
TABLE 22-1
Main Event Classes in
java.awt.event
ActionEvent
has these 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 indicates which modifier keys
(ALT, CTRL, META, and/or SHIFT) were pressed when the event was generated. The when
parameter specifies when the event occurred.
You can obtain the command name for the invoking
ActionEvent
object by using the
getActionCommand( )
method, shown here:
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
getModifiers( )
method returns a value that indicates which modifier keys (ALT, CTRL,
META, and/or SHIFT) were pressed when the event was generated. Its form is shown here:
int getModifiers( )
The method
getWhen( )
returns the time at which the event took place. This is called the
event’s timestamp. The
getWhen( )
method is shown here:
long getWhen( )
The AdjustmentEvent Class
An
AdjustmentEvent
is generated by a scroll bar. There are five types of adjustment events.
The
AdjustmentEvent
class defines integer constants that can be used to identify them. The
constants and their meanings are shown here:
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.
UNIT_DECREMENT
The button at the end of the scroll bar was clicked to decrease its value.
UNIT_INCREMENT
The button at the end of the 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.
Here is one
AdjustmentEvent
constructor:
AdjustmentEvent(Adjustable src , int id , int type , int data )
Here, src is a reference to the object that generated this event. The id specifies the event. The
type of the adjustment is specified by type, and its associated data is data.
The
getAdjustable( )
method returns the object that generated the event. Its form is
shown here:
The type of the adjustment event may be obtained by the
getAdjustmentType( )
method. It
returns one of the constants defined by
AdjustmentEvent
. The general form is shown here:
int getAdjustmentType( )
The amount of the adjustment can be obtained from the
getValue( )
method, shown here:
int getValue( )
For example, when a scroll bar is manipulated, this method returns the value represented
by that change.
The ComponentEvent Class
A
ComponentEvent
is generated when the size, position, or visibility of a component is
changed. There are four types of component events. The
ComponentEvent
class defines
integer constants that can be used to identify them. The constants and their meanings are
shown here:
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 this constructor:
ComponentEvent(Component src , int type )
Here, src is a reference to the object that generated this event. The type of the event is
specified by type.
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. It is
shown here:
Component getComponent( )
The ContainerEvent Class
A
ContainerEvent
is generated when a component is added to or removed from a container.
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 this constructor:
ContainerEvent(Component src , int type , Component comp )
Here, src is a reference to the container 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 is comp.
You can obtain a reference to the container that generated this event by using the
getContainer( )
method, shown here:
Container getContainer( )
The
getChild( )
method returns a reference to the component that was added to or removed
from the container. Its general form is shown here:
Component getChild( )
The FocusEvent Class
A
FocusEvent
is generated when a component gains or loses input focus. These events are
identified by the integer constants
FOCUS_GAINED
and
FOCUS_LOST
.
FocusEvent
is a subclass of
ComponentEvent
and has these constructors:
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 component 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
.
(A temporary focus event occurs as a result of another user
interface operation. For example, assume that the focus is in a text field. If the user moves
the mouse to adjust a scroll bar, the focus is temporarily lost.)
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. Conversely, if a
FOCUS_LOST
event occurred, other will refer to the component
that gains focus.
You can determine the other component by calling
getOppositeComponent( )
,
shown here:
Component getOppositeComponent( )
The opposite component is returned.
The
isTemporary( )
method indicates if this focus change is temporary. Its form
is shown here:
boolean isTemporary( )
The method returns
true
if the change is temporary. Otherwise, it returns
false
.
The InputEvent Class
The abstract class
InputEvent
is a subclass of
ComponentEvent
and is the 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. Originally, the
InputEvent
class defined the following eight values to represent the modifiers:
ALT_MASK
BUTTON2_MASK
META_MASK
ALT_GRAPH_MASK
BUTTON3_MASK
SHIFT_MASK
BUTTON1_MASK
CTRL_MASK
However, because of possible conflicts between the modifiers used by keyboard events and
mouse events, and other issues, the following extended modifier values were added:
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
When writing new code, it is recommended that you use the new, extended modifiers rather
than the original modifiers.
To test if a modifier was pressed at the time an event is generated, use the
isAltDown( )
,
isAltGraphDown( ) isControlDown( ) isMetaDown( )
,
,
, and
isShiftDown( )
methods. The
forms of these methods are shown here:
boolean isAltDown( )
boolean isAltGraphDown( )
boolean isControlDown( )
boolean isMetaDown( )
boolean isShiftDown( )
You can obtain a value that contains all of the original modifier flags by calling the
getModifiers( )
method. It is shown here:
int getModifiers( )
You can obtain the extended modifiers by calling
getModifiersEx( )
, which is shown here:
int getModifiersEx( )
The ItemEvent Class
An
ItemEvent
is generated when a check box or a list item is clicked or when a checkable menu
item is selected or deselected. (Check boxes and list boxes are described later in this book.)
There are two types of item events, which are identified by the following integer constants:
DESELECTED
The user deselected an item.
SELECTED
The user selected an item.
In addition,
ItemEvent
defines one integer constant,
ITEM_STATE_CHANGED
, that
signifies a change of state.
ItemEvent
has this constructor:
ItemEvent(ItemSelectable src , int type , Object entry , int state )
Here, src is a reference to the component that generated this event. For example, this might
be a list or choice element. 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 can be used to obtain a reference to the item that generated an
event. Its signature is shown here:
Object getItem( )
The
getItemSelectable( )
method can be used to obtain a reference to the
ItemSelectable
object that generated an event. Its general form is shown here:
ItemSelectable getItemSelectable( )
Lists and choices are examples of user interface elements that implement the
ItemSelectable
interface.
The
getStateChange( )
method returns the state change (that is,
SELECTED
or
DESELECTED
) for the event. It is shown here:
int getStateChange( )
The KeyEvent Class
A
KeyEvent
is generated when keyboard input occurs. 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 any key is pressed or released. The
last event occurs only when a character is generated. Remember, not all keypresses result
in characters. For example, pressing SHIFT does not generate a character.
There are many other integer constants that are defined by
KeyEvent
.
For example,
VK_0
through
VK_9
and
VK_A
through
VK_Z
define the ASCII equivalents of the numbers and
letters. Here are some others:
VK_ALT
VK_DOWN
VK_LEFT
VK_RIGHT
VK_CANCEL
VK_ENTER
VK_PAGE_DOWN
VK_SHIFT
VK_CONTROL
VK_ESCAPE
VK_PAGE_UP
VK_UP
The
VK
constants specify virtual key codes and are independent of any modifiers, such as
control, shift, or alt.
KeyEvent
is a subclass of
InputEvent
. Here is one of its constructors:
KeyEvent(Component src , int type , long when , int modifiers , int code , char ch )
Here, src is a reference to the component that generated the event. The type of the event is
specified by type. The system time at which the key was pressed is passed in when. The modifiers
argument indicates which modifiers were pressed when this key event occurred. The virtual
key code, such as
VK_UP VK_A
,
, and so forth, is passed in code. The character equivalent
(if one exists) is passed in ch. If no valid character exists, then ch contains
CHAR_UNDEFINED
.
For
KEY_TYPED
events, code will contain
VK_UNDEFINED
.
The
KeyEvent
class defines several methods, but the most commonly used ones are
getKeyChar( )
, which returns the character that was entered, and
getKeyCode( )
, which
returns the key code. Their general forms are shown here:
char getKeyChar( )
int getKeyCode( )
If no valid character is available, then
getKeyChar( )
returns
CHAR_UNDEFINED
. When
a
KEY_TYPED
event occurs,
getKeyCode( )
returns
VK_UNDEFINED
.
The MouseEvent Class
There are eight types of mouse events. The
MouseEvent
class defines the following integer
constants that can be used to identify them:
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
is a subclass of
InputEvent
. Here is one of its constructors:
MouseEvent(Component src , int type , long when , int modifiers ,
int x , int y , int clicks , boolean triggersPopup )
Here, src is a reference to the component that generated the event. The type of the event is
specified by type. The system time at which the mouse event occurred is passed in when. The
modifiers argument indicates which modifiers were pressed when a mouse event occurred.
The coordinates of the mouse are passed in x and y. The click count is passed in clicks. The
triggersPopup flag indicates if this event causes a pop-up menu to appear on this platform.
Two commonly used methods in this class are
getX( )
and
getY( )
.
These return the X and
Y coordinates of the mouse within the component when the event occurred. Their forms are
shown here:
int getX( )
int getY( )
Alternatively, you can use the
getPoint( )
method to obtain the coordinates of the mouse.
It is shown here:
Point getPoint( )
It returns a
Point
object that contains the X,Y coordinates in its integer members:
x
and
y
.
The
translatePoint( )
method changes the location of the event. Its form is shown here:
void translatePoint(int x , int y )
Here, the arguments x and y are added to the coordinates of the event.
The
getClickCount( )
method obtains the number of mouse clicks for this event.
Its signature is shown here:
int getClickCount( )
The
isPopupTrigger( )
method tests if this event causes a pop-up menu to appear on this
platform. Its form is shown here:
boolean isPopupTrigger( )
Also available is the
getButton( )
method, shown here:
int getButton( )
It returns a value that represents the button that caused the event. The return value will be
one of these constants defined by
MouseEvent
:
NOBUTTON
BUTTON1
BUTTON2
BUTTON3
The
NOBUTTON
value indicates that no button was pressed or released.
Java SE 6 added three methods to
MouseEvent
that obtain the coordinates of the mouse
relative to the screen rather than the component. They are shown here:
Point getLocationOnScreen( )
int getXOnScreen( )
int getYOnScreen( )
The
getLocationOnScreen( )
method returns a
Point
object that contains both the X and
Y coordinate. The other two methods return the indicated coordinate.
The MouseWheelEvent Class
The
MouseWheelEvent
class encapsulates a mouse wheel event. It is a subclass of
MouseEvent
.
Not all mice have wheels. If a mouse has a wheel, it is located between the left and right
buttons. Mouse wheels are used for scrolling.
MouseWheelEvent
defines these 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.
Here is one of the constructors defined by
MouseWheelEvent
:
MouseWheelEvent(Component src , int type , long when , int modifiers ,
int x , int y , int clicks , boolean triggersPopup ,
int scrollHow , int amount , int count )
Here, src is a reference to the object that generated the event. The type of the event is specified
by type. The system time at which the mouse event occurred is passed in when. The modifiers
argument indicates which modifiers were pressed when the 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 triggersPopup flag indicates if this event causes a pop-up 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.
MouseWheelEvent
defines methods that give you access to the wheel event. To obtain
the number of rotational units, call
getWheelRotation( )
, shown here:
int getWheelRotation( )
It returns the number of rotational units. If the value is positive, the wheel moved
counterclockwise. If the value is negative, the wheel moved clockwise.
To obtain the type of scroll, call
getScrollType( )
, shown next:
int getScrollType( )
It returns either
WHEEL_UNIT_SCROLL
or
WHEEL_BLOCK_SCROLL
.
If the scroll type is
WHEEL_UNIT_SCROLL
, you can obtain the number of units to scroll
by calling
getScrollAmount( )
. It is shown here:
int getScrollAmount( )
The TextEvent Class
Instances of this class describe text events. These are generated by text fields and text areas
when characters are entered by a user or program.
TextEvent
defines the integer constant
TEXT_VALUE_CHANGED
.
The one constructor for this class is shown here:
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
TextEvent
object does not include the characters currently in the text component that
generated the event. Instead, your program must use other methods associated with the text
component to retrieve that information. This operation differs from other event objects
discussed in this section. For this reason, no methods are discussed here for the
TextEvent
class. Think of a text event notification as a signal to a listener that it should retrieve information
from a specific text component.
The WindowEvent Class
There are ten types of window events. The
WindowEvent
class defines integer constants that
can be used to identify them. The constants and their meanings are shown here:
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
is a subclass of ComponentEvent
. It defines several constructors. The
first is
WindowEvent(Window src , int type )
Here, src is a reference to the object that generated this event. The type of the event is type .
The next three constructors offer more detailed control:
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, other specifies the opposite window when a focus or activation event occurs. The
fromState specifies the prior state of the window, and toState specifies the new state that the
window will have when a window state change occurs.
A commonly used method in this class is
getWindow( )
. It returns the
Window
object
that generated the event. Its general form is shown here:
Window getWindow( )
WindowEvent
also defines methods that return the opposite window (when a focus or
activation event has occurred), the previous window state, and the current window state.
These methods are shown here:
Window getOppositeWindow( )
int getOldState( )
int getNewState( )