Object Oriented Programming. Interface Event Handling

Interface Event Handling

Using interface, we specify what a class must do, but not how it does this.

An interface is syntactically similar to a class, but it lacks instance variables and its methods are declared without any body.

An interface is defined with an interface keyword.

Two types of access:◦ public – interface may be used anywhere in a program◦ default – interface may be used in the current package only

Interface methods have no bodies – they end with the semicolon after the parameter list. They are essentially abstract methods.

An interface may include variables, but they must be final, static and initialized with a constant value.

In a public interface, all members are implicitly public.

A class implements an interface if it provides a complete set of methods defined by this interface.◦ any number of classes may implement an interface◦ one class may implement any number of interfaces

Each class is free to determine the details of its implementation.

Implementation relation is written with the implements keyword.

Declaration of the Callback interface:

Client class implements the Callback interface:

An event is an action initiated by the user interacting with the program.

Examples◦ Keyboard events - pressing a key, holding a key, releasing a key◦ Mouse events - moving the mouse, clicking the mouse◦ GUI events - clicking on a button, resizing a window, closing a window,

opening a window An event in Java is an object of a particular event class, that

represents some user actions to which the GUI might respond

Low level events represent direct communication from the user Low level event examples (all the event classes listed below belong

to the java.awt.event package0:◦ key event - a keyboard key pressed or released - in the KeyEvent class◦ focus event – a component got focus, lost focus – in the FocusEvent class◦ mouse event - the mouse is moved or dragged, a mouse button is pressed or

released, the mouse cursor enters or exits a component - in the MouseEvent class

◦ component event - a component is hidden, shown, resized, or moved – in the ComponentEvent class

◦ container event - a component is added to or removed from a container in the ContainerEvent class

◦ window event - a window is opened, closed, activated, deactivated, etc. - in the WindowEvent class

High level events usually involve one or more low level events

High Level Event examples◦ action event - do a command – ActionEvent class◦ adjustment event - represents scrollbar motions such as a value

was adjusted – AdjustmentEvent class◦ item event - occurs when the user selects a checkbox, choice, or

list item, i.e. item state has changed –ItemEvent class◦ text event – represents a text component content (value) change –

TextEvent class

When the user clicks the mouse on a button, then releases it, the button gets two or three separate, low level mouse events◦ one for mouse down ◦ one for mouse up◦ possibly one for mouse drag (if the user moves the mouse

while the button is pressed) However, the button then fires one high level event

only - ActionEvent

Events are organized into hierarchy of event classes Event classes contain data relevant to a particular

event type An event is an object of one of the event classes

java.util.EventObject

java.awt.AWTEvent

AdjustmentEvent ComponentEventActionEvent TextEventItemEvent

WindowEventInputEventFocusEventContainerEvent

MouseEventKeyEvent

PaintEvent

java.lang.Object

The type of an event depends on its source Example of event sources:◦ the keyboard◦ the mouse◦ the GUI components – buttons, text fields, windows

Event source is an object with the ability to determine when an event has occurred

In event driven programming the events “drive” the execution of the program, e.g. the code is executed when events are activated

The program interacts with the user and generates events based on the external user actions

Java Visual (Graphical) programming and Visual Basic programming are event driven

When writing applets that are using events in Java we have to import the “events” package java.awt.event.*;

Java uses delegation-based model for event handling

Java uses event listener to register an event and event handler to respond to the event

The use of event listeners in event handling is called delegation event model

An external user’s action on a source object (e.g. the event source) activates an event

An event listener object (e.g. an object interested in the event source) receives the event. This object is an instance of a class that implements a specific EventListener interface ◦ Example: ActionEvent --> ActionListener

The source maintains a list containing all the listener objects that have registered to be notified of events of that type

The transmission of an event from an event source to an event listener involves invoking a method on the listener object by which the source notifies the listener of the occurrence of an event of a specific type ◦ Example: method actionPerformed (ActionEvent e)◦ An EventListener interface declares one or more methods

which must be defined in the listener class, and which are invoked by the event source in response to each specific event type handled by the interface Example:

EventListener method actionPerformed (ActionEvent e)

The Java standard class library contains several classes that represent typical events

Components, such as an applet or a button, generate (fire) an event when it occurs

Objects, called listeners, wait for events to occur. A listener object is an instance of a class that implements a specific listener interface

A number of listener interfaces are pre-defined and each interface declares the appropriate methods for a specific class of events

Component

This object maygenerate an event

Listener

This object waits for andresponds to an event

Event

When an event occurs, the component callsthe appropriate method of the listener,

passing an (event) object that describes the event

Each event is represented by an object that gives information about the event and identifies the event source.

Each event source can have multiple listeners registered on it. A single listener can register with multiple event sources.

event source

event listener 1

event listener 2

event listener 3

A listener object can be registered on a source object to be notified of the occurrence of all events of the specific class for which the listener object is designed

The occurrence of an event defined by the specified class will automatically invoke the matching method in the listener object

The code in the body of the method is designed by the programmer to perform the desired action when the event occurs

We can create a listener object by writing a class that implements a particular listener interface

The Java standard class library contains several interfaces that correspond to particular event categories

After creating the listener, we add the listener to the component that might generate the event to set up a relationship between the component, generating the event and the event listener

Register an event listener◦ “listens” for events generated by GUI components

◦an object of a class from the package java.awt.event Implement an event handler◦ a method that is automatically called in response to a particular

type of event

For each event class there is a corresponding listener interface defined in Java and corresponding listener methods (handlers) in the listener interface

Example : ◦ for the event class ActionEvent ◦ the listener is ActionListener ◦ and the listener method (handler) is actionPerformed (ActionEvent e )

Commonly used AWT event Listeners◦ActionListener◦AdjustmentListener◦ FocusListener◦ ItemListener◦KeyListener◦MouseListener◦MouseMotionListener◦MouseWheelListener◦WindowListener◦WindowFocusListener◦WindowStateListener

Every event handler requires three separate steps1. In the declaration for the event handler class, we specify that the

class either implements a listener interface or extends a class that implements a listener interface

public class MyClass implements ActionListener { … }

2. Code that registers an instance of the event handler class as a listener upon one or more components

someComponent.addActionListener(instanceOfMyClass);

3. Code that implements the methods in the listener interfacepublic void actionPerformed(ActionEvent e) { //code that reacts to the action... }

Adapter Class exists as convenience for creating a listener object. Extend this class to create a listener for a particular listener

interface and override the methods for the events of interest. It defines null methods for all of the methods in the listener

interface, so you can only have to define methods for events you care about.

Commonly used adapter classes:◦ FocusAdapter◦ KeyAdapter◦ MouseAdapter◦ MouseMotionAdapter◦ WindowAdapter