Event Classes - Java Tutorials

The classes that represent events are at the core of Java’s event handling mechanism. Thus, we begin our study of event handling with a tour of the event classes. As you will see, they provide a consistent, easy-to-use means of encapsulating events. At the root of the Java event class hierarchy is EventObject, which is in java.util. It is the 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. 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 several types of events that are generated by various user interface elements. Below enumerates the most important of these event classes and provides a brief description of when they are generated. The most commonly used constructors and methods in each class are described in the following sections.

Main Event Classes in java.awt.event

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 super class 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. (Added by Java 2, version 1.4)

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.

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. 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. The third constructor was added by Java 2, version 1.4. 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( )

Java 2, version 1.4 added the method getWhen( ) that returns the time at which the event took place. This is called the event’s timestamp. The getWhen( ) method is shown here.

      long getWhen( )

Timestamps were added by ActionEvent to help support the improved input focus subsystem implemented by Java 2, version 1.4.

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 equals ADJUSTMENT_VALUE_CHANGED. The type of the event 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:

      Adjustable getAdjustable( )

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)

      Focus Event(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. The third constructor was added by Java 2, version 1.4. You can determine the other component by calling getOppositeComponent( ), shown here.

      Component getOppositeComponent( )

The opposite component is returned. This method was added by Java 2, version 1.4. 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, Java 2, version 1.4 added the following extended

modifier values.

      ALT_DOWN_MASK

      ALT_GRAPH_DOWN_MASK 

      BUTTON1_DOWN_MASK

      BUTTON2_DOWN_MASK 

      BUTTON3_DOWN_MASK 

      CTRL_DOWN_MASK

      META_DOWN_MASK 

      SHIFT_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 called getModifiersEx( ), which is shown here.

      int getModifiersEx( )

This method was also added by Java 2, version 1.4.

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 (i.e., 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 key presses result in characters. For example, pressing the SHIFT key 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_ENTER 

      VK_ESCAPE 

      VK_CANCEL 

      VK_UP

      VK_DOWN 

      VK_LEFT 

      VK_RIGHT 

      VK_PAGE_DOWN

      VK_PAGE_UP 

      VK_SHIFT 

      VK_ALT 

      VK_CONTROL

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 are two of its constructors:

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

      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 (Java 2, v1.4).

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. Java 2, version 1.4 adds a second constructor which also allows the button that caused the event to be specified.

The most commonly used methods in this class are getX( ) and getY( ). These return the X and Y coordinates of the mouse 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( )

Java 2, version 1.4 added 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.

The MouseWheelEvent Class

The MouseWheelEvent class encapsulates a mouse wheel event. It is a subclass of MouseEvent and was added by Java 2, version 1.4. 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.

MouseWheelEvent defines the following constructor.

      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. (Added by Java 2, version 1.4.)

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. Java 2, version 1.4 adds the next three constructors.

      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 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. The most 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( )

Java 2, version 1.4, adds methods that return the opposite window (when a focus event has occurred), the previous window state, and the current window state. These method are shown here:

      Window getOppositeWindow()

      int getOldState()

      int getNewState()