Event Handling in Java [Explore Java]

Event Handling in Java

Table of Contents
a) Introduction to Event
b) What is Event Handling?
c) Event Delegation Model (EDM)
d) Event Listener and its Description
e) Event Class and its Description

Introduction

Now after the Layout Managers, in this chapter we will learn about Event, Event types, and also learn to handle the Event. You have noticed that in our previous examples I have added the buttons but that button does not perform any kind of activity. So, to make the GUI interactive Event should be handled. It also helps for making easy user interface.

What is Event Handling?

Event handling is the mechanism of controlling the events and decide what to do if an event occurs, Each and every time if an event occurs then the set of code executes ( which is called Event Handler ) and performs the things as per the set of codes. Event is handled by using the Event Delegation Model (EDM). Before moving forward let us learn about the Event Delegation Model (EDM). It consists of: Event Source, Event and Event Listener.

Event Source: Event source is the source from which the event is generated. It means that if a button is clicked than the button is the event source and so on. There are different source for generating events and we will discuss further in the other chapters.

Event: Event is the change in the state of an object. Events are generated through event source which means if a user wants to interact with the Graphical User Interface (GUI) then the user may click to the button and after he clicks the button, action event is generated. And also there are Key Event, Mouse Event, Window Event etc and all these will be discussed in further classes.

Event Listener:  It performs the action after the getting the command of the event. According to the event, the event listener does the rest things. For every event listener there is its corresponding method that is used.

Now let us know about the Event Class and Its Descriptions;

Event lListener	Description
ActionEvent	Generated when a button is pressed, a list item is double clicked, a menu item is selected, enter key is pressed in  a text field, item in combo box is clicked
AdjustmentEvent	Generated when a scroll bar is manipulated
ComponentEvent	Generated when a component is hidden, moved, resized or shown
ContainerEvent	Generated when a component is added to/ removed from a container
FocusEvent	Generated when a component gains or loses keyboard focus
ItemEvent	Generated when a check box , list item  or combo box item is clicked.
KeyEvent	Generated when input is received from the keyboard
MouseEvent	Generated when mouse is clicked, pressed, or released. Also generated when a mouse enters or exits a component
MouseWheelEvent	Generated when the mouse wheel is moved
TextEvent	Generated when the value of text area or text field is changed
WindowEvent	Generated when a window is activated, closed, deactivated, deiconified, iconified, opened or quit.

The Adapter class and Listener Interface used in event handling are;

Adapter Classes	Listener Interfaces with their methods
ComponentAdapter	ComponentListener void componentResized(ComponentEvent e) void componentMoved(ComponentEvent e) void componentShown(ComponentEvent e) void componentHidden(ComponentEvent e)
ContainerAdapter	ContainerListener void componentAdded(ContainerEvent e) void componentRemoved(ContainerEvent e)
FocusAdapter	FocusListener void focusGained(FocusEvent e) void focusLost(FocusEvent e)
KeyAdapter	KeyListener void keyPressed(KeyEvent e) void keyReleased(KeyEvent e) void keyTyped(KeyEvent e)
MouseAdapter	MouseListener void mouseClicked(MouseEvent e) void mouseEntered(MouseEvent e) void mouseExited(MouseEvent e) void mousePressed(MouseEvent e) void mouseReleased(MouseEvent e)
MouseMotionAdapter	MouseMotionListener void mouseDragged(MouseEvent e) void mouseMoved(MouseEvent e)
WindowAdapter	WindowListener void windowActivated(WindowEvent e) void windowClosed(WindowEvent e) void windowClosing(WindowEvent e) void windowDeactivated(WindowEvent e) void windowDeiconified(WindowEvent e) void windowIconified(WindowEvent e) void windowOpened(WindowEvent e)
x	ActionListener void actionPerformed(ActionEvent e)
x	ItemListener void itemStateChanged(ItemEvent e)
x	AdjustmentListener void adjustmentValueChanged(AdjustmentEvent e)
x	TextListener void textChanged(TextEvent e)

Advantage of using Adapter Class over Listener Interface is that we can override only those method which is required to override and do not take the burden of other methods.
[ Note : The ‘x’ sign represent that it does not have Adapter class because they have only one method.]
This chapter includes the theoretical description of the Event Handling. In the next chapter we will discuss further about the event handling with examples.