Java Events and Event Handling

• Events in Java represent all user activity within an application.
• Like Java classes, events are Objects initiated by the user.
• EventObject serves as the parent class for all event objects, descending from the Object class.
• EventObject is the parent of AWTEvent, which is the parent of event classes like ActionEvent and ComponentEvent.

Events and Event Handling

• Common user actions generate specific events.
• Clicking a button results in an ActionEvent.
• Clicking a component triggers a MouseEvent.
• Selecting an item in a list box generates an ItemEvent.
• Clicking a check box item also results in an ItemEvent.
• Changing the text in a text field creates a TextEvent.
• Opening a window produces a WindowEvent.
• Iconifying a window also generates a WindowEvent.
• Pressing a key triggers a KeyEvent.
• GUI programs handle events originating from mouse or keyboard interactions with Components or Containers.

Event Listener

• An event listener is an object with a method that executes in response to generated events.

• Objects can be notified of events by implementing the appropriate interface and registering as an event listener of the event source.

• The class responding to an event needs a method accepting the event object created by the user's action.

• When registering a component to listen for events, a method that reacts to the generated event must be written.

Methods Reacting to Events

• ActionListener uses actionPerformed(ActionEvent).
• AdjustmentListener uses adjustmentValueChanged(AdjustmentEvent).
• FocusListener uses focusGained(FocusEvent) and focusLost(FocusEvent).
• ItemListener uses itemStateChanged(ItemEvent).

AWTEvent Class Methods

• getSource(): Returns the Object involved in the event.
• getComponent(): Returns the Component involved in the event.
• getWindow(): Returns the Window involved in the event.
• getStateChange(): Returns an integer, ItemEvent.SELECTED or ItemEvent.DESELECTED.

MouseListener Methods

• mouseClicked(MouseEvent e): Invoked when a mouse button is clicked on a component.
• mouseEntered(MouseEvent e): Invoked when the mouse enters a component.
• mouseExited(MouseEvent e): Invoked when the mouse exits a component.
• mousePressed(MouseEvent e): Invoked when a mouse button is pressed on a component.
• mouseReleased(MouseEvent e): Invoked when a mouse button is released on a component.

MouseMotionListener Methods

• mouseDragged(MouseEvent e): Invoked when the mouse is pressed on a component and dragged.
• mouseMoved(MouseEvent e): Invoked when the mouse cursor is moved onto a component with no buttons pressed.

Methods and Fields of MouseEvent Class

• getButton(): Returns which mouse button has changed state, if any.

• getClickCount(): Returns the number of mouse clicks associated with the event.

• getX(): Returns the horizontal x-position of the event, relative to the source component.

• getY(): Returns the vertical y-position of the event, relative to the source component.

• BUTTON1: Indicates mouse button #1, used by getButton().

• BUTTON2: Indicates mouse button #2, used by getButton().

• BUTTON3: Indicates mouse button #3, used by getButton().

• NOBUTTON: Indicates no mouse buttons, used by getButton().

• Constants for specific mouse events include MouseEvent.MOUSE_CLICKED, MouseEvent.MOUSE_DRAGGED, MouseEvent.MOUSE_ENTERED, and MouseEvent.MOUSE_EXITED.

InputEvent Class

• getModifiers(): Returns an integer indicating which mouse button was clicked.

KeyEvent Class

• getKeyChar(): Returns the Unicode character entered from the keyboard.
• getClickCount(): Returns the number of clicks

Mouse Events

• The MouseMotionListener interface offers mouseDragged() and mouseMoved() to detect mouse movements.
• The MouseListener interface includes mousePressed(), mouseClicked(), andmouseReleased().
• The MouseInputListener interface implements all methods from both MouseListener and MouseMotionListener.

Other Components of Event Handling

Abstract Button Class

• A GUI application with an button, performs an action when the button is clicked.
• Demonstrates event handling for button clicks, where the button text changes upon clicking.

JList Class

• The JList class displays a list of items like students or files.
• JList objects are created by passing an array of strings.
• Three Selection Modes: Single Selection, Single Interval, and Multiple Interval.

Methods of the JList Class

• getFirstVisibleIndex(): Returns the index of the first visible cell.
• getLastVisibleIndex(): Returns the index of the last visible cell.
• getMinSelectionIndex(): Returns the smallest selected cell index.
• getMaxSelectionIndex(): Returns the largest selected cell index.
• getSelectedIndex(): Returns the first selected index, or -1 if no item is selected.
• setSelectedIndex(int index): Selects a single cell.
• getSelectionMode(): Returns whether single-item or multiple-item selections are allowed.
• setSelectionMode(int selectionMode): Determines whether single-item or multiple-item selections are allowed.
• isSelectionEmpty(): Returns true if nothing is selected.

JScrollPane Class

• JScrollPane can hold components needing more space than allocated.

• The constructor can come in four forms:

• JScrollPane(): Creates an empty pane with scroll bars appearing when needed.

• JScrollPane(Component): Displays the content of a specified component.

• JScrollPane(Component, int, int): Displays a component with specified vertical and horizontal scroll bar policies.

• JScrollPane(int, int): Creates a scroll pane with specified vertical and horizontal scroll bar policies.

• Use class variables defined in ScrollPaneConstants class to force the bar to appear.

• Example of a pane displaying a picture with a vertical, but no horizonal scroll bar.

JScrollPane Methods

• createHorizontalScrollBar(): Returns a default JScrollPane.ScrollBar.
• createVerticalScrollBar(): Returns a default JScrollPane.ScrollBar.
• getHorizontalScrollBar(): Returns the horizontal scroll bar that controls the viewport's horizontal view position.
• getVerticalScrollBar(): Returns the vertical scroll bar that controls the viewport's vertical view position.
• setHorizontalScrollBar(JScrollBar horizontalScrollBar): Adds the scrollbar that controls the viewport's horizontal view position.
• setLayout(LayoutManager layout): Sets the layout manager for this JScrollPane.
• setVerticalScrollBar(JScrollBar verticalScrollBar): Adds the scrollbar that controls the viewport's vertical view position.

Interaction Between Components

• setText(""); // setting text field value
• getText(""); // getting text field value
• requestFocus(); // setting focus(cursor) back to the text field