ppt la10 java programming_05

Java Programming(J2EE LC)

Day 5

ER/CORP/CRS/LA10/003

Version 1.00Copyright © 2005, Infosys

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Session Plan

Day 5

– Designing GUI

– Event Handling

– Applets



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The java.awt Package

Java helps the programmer to create GUI with the classes in java.awt

AWT stands for Abstract Window Toolkit



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Class Hierarchy

Component

Button

Label

WindowScrollbar

Canvas

Checkbox

Choice

Container

TextComponent

AppletPanel

Dialog

FrameTextField

TextArea

ScrollPane



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Container Classes

Container classes are classes that can have other components on it

So for creating a GUI, we need at least one Container object

Three types of containers

– Panel : It is a pure container and is not a window in itself. The sole purpose

of a Panel is to organize the components on to a window.

– Frame : It is a fully functioning window with its own title and icons.

– Dialog : It can be thought of as a pop-up window that pops out when message

has to be displayed. It is not a fully functioning window like the Frame.



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A Simple GUI Program

Just creating a Frame container without any components added to it

import java.awt.*;

class SimpleGUI{

public static void main(String args[]) {

Frame f = new Frame(“Simple GUI”);

f.setSize(300, 400);

f.setVisible(true);

}

} Making the Frame visible, without this statement, nothing will be

seen and should be the last statement after all components

have been added

Setting the size of the Frame

Calling the constructor of the Frame class and passing the title of the Frame as a String argument



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import java.awt.*;

class SimpleGUI{

public static void main(String args[]){

Frame myFrame= new Frame(“Simple GUI”);

myFrame.setSize(300, 400);

myFrame.setBackground(Color.pink);

Button myButton = new Button(“Ok”);

myFrame.add(myButton);

myFrame.setVisible(true);

}

}

Adding Components

We can add Components to a Container

Creating a Button object and adding onto the container



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Layout Managers

Each Container has a Layout Manager that will decide the size and position

of the Components placed on it ie each container has its default layout

The programmer can change the default Layout of the Container using the

method setLayout (which layout to follow)

The programmer can also cancel the default Layout of a Container by calling

the method setLayout (null)



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Different Layouts (1 of 2)

There are 5 different layouts available

FlowLayout

– The components are placed horizontally one after another and then move to the next

line

GridLayout

– The components are placed in a grid (rows, columns)

BorderLayout

– 5 components can be added at the most

– The 5 borders are North, South, East, west and Center

– If not specified the default position is center in Border Layout



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Different Layouts (2 of 2)

CardLayout

– The CardLayout places components/containers on top of each other like a deck of

cards

– Only one is visible at a time

– Every card is made visible using the method show()

GridBagLayout

– Most powerful and flexible

– It is a more advanced form of GridLayout where components can be placed

horizontally and vertically

– Components can be of different sizes and they can span multiple cells in the grid



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Absolute Positioning

The programmer may take full control of positioning the components on the

container by doing absolute positioning

– setLayout(null);

– setBounds(x-cord, y-cord, width, height);



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Checkbox

Checkboxes are used when you want to select 0 or more items from a set of items

The method getState() will return true if the Checkbox is selected

…

Checkbox c1 = new Checkbox(“English”);

Checkbox c2 = new Checkbox(“German”);

Checkbox c3 = new Checkbox(“French”);

…

if (c1.getState())

System.out.println(“The user knows English”);



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CheckboxGroup

CheckboxGroup can be used to group some Checkbox objects together, so

that they become Radio Buttons

Radio Buttons are used when we want to select one and only one from a set of

items

…

CheckboxGroup cg = new CheckboxGroup();

Checkbox c1 = new Checkbox(“Morning”, cg, true);

Checkbox c2 = new Checkbox(“Evening”, cg, false);

Checkbox c3 = new Checkbox(“Night”, cg, false);

…



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Choice

Choice is a Drop Down List

This is used to select 0 or more items from a set of items

The method getSelectedItem() will return the item selected by the user

– System.out.println(c.getSelectedItem());

…

Choice c = new Choice();

c.add(“Sunday”);

c.add(“Monday”);

…



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List

List is a scrollable list of items

A list can be used to select 0 or more items from a set of items

The method getSelectedItem will return the item selected by the user

– System.out.println(l.getSelectedItem());

– getSelectedItems() returns a String[] if multiple items are selected

…

List list = new List();

List list = new List(5,true);

list.add(“Sunday”);

list.add(“Monday”);

…



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Label

Label is a simple control used to display some message

– Label l = new Label(“Hello”)

The method setText can be used to set the message on the label

– l.setText(“Hello World”);

The method getText can be used to get the message from the label

– System.out.println(l.getText());



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Scrollbar

Scrollbar is used to increment or decrement some value

The method getValue is used to get the current value of the Scrollbar

– System.out.println(s.getValue());

Scrollbar s = new Scrollbar(Scrollbar.HORIZONTAL,

25, 10, 0, 100);



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TextField and TextArea

TextField and TextArea permits the user to type in some input

TextField is single line whereas TextArea is multi line

The method getText gets the text typed in by the user

– System.out.println(tf.getText());

…

TextField tf = new TextField(10);

TextArea ta = new TextArea(10, 20);

…



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MenuBar, Menu and MenuItem (1 of 3)

The classes MenuBar, Menu and MenuItem help us to create a menu system

for the window

MenuBar is the area in the window where the menu system appears

– MenuBar mb = new MenuBar();

A menu system can be added to a window by setting a MenuBar to this window

– f.setMenuBar(mb); (where f is the Frame reference)



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The different items that appear on the MenuBar are represented by the class

Menu

A Menu can be added to a MenuBar using the add method

…

Menu m1 = new Menu(“File”);

Menu m2 = new Menu(“Edit”);

Menu m3 = new Menu(“Help”);

…

…

mb.add(m1);

mb.add(m2);

…



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The items that drop down when you select a Menu are objects of MenuItem

A MenuItem can be added to a Menu using the method add

…

MenuItem menuitem1 = new MenuItem(“Open”);

MenuItem menuitem2 = new MenuItem(“New”);

…

…

m1.add(mi1);

m1.add(mi2);

…



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Panel

Panel is a Container and can contain other Components

Generally we do not use Panel as the main Container since it is not a

Window

Panel can be used to logically group some Components together

We can add three Panels to a Frame, each Panel can have a different Layout

and Components can be added to the Panels



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Dialog

Dialog is a Window that is used to display some message and get a

response from the user

For example, a small window that asks the user “Are you sure?” can be

implemented using Dialog

The Dialog can have two Buttons also that says “Yes” and “No”

The method dispose() helps to dispose the dialogbox



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Events

Events correspond to :

– Physical actions (E.g.: mouse button down, Key press/release)

– Logical events (E.g.: gotfocus - receiving focus on a component)

Event is an encapsulation of some input delivered asynchronously to the

application

The java.awt.event package defines classes to represent different type of

events.



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Event Handling Mechanisms

Delegation event model – The new approach

Concept :

– Source generates the events and sends them to one or more listeners

– Listener waits until it receives an event

– Once received, the listener processes the event and then returns

Event handling is totally separated from UI component

A UI is able to "delegate" the event handling procedure to a separate piece of

code



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Hierarchy of Event Classes in java.awt.event

EventObject

AWTEvent

Action Event AdjustmentEvent ComponentEvent ItemEvent TextEvent

ContainerEvent FocusEvent InputEvent PaintEvent WindowEvent

KeyEvent Mouse Event



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Main Event Classes in java.awt.event (1 of 2)

EVENT CLASS DESCRIPTION LISTENER

ActionEvent Generated when a button is pressed. ActionListener

AdjustmentEvent Generated when a scroll bar is used. AdjustmentListener

ComponentEventGenerated when a component is moved, resized,

shown or hidden.ComponentListener

ContainerEventGenerated when a component is added or removed

from the container.ContainerListener

FocusEventGenerated when a component gains or loses

keyboard focusFocusListener

InputEventAbstract superclass for all component input event

classes.



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Main Event Classes in java.awt.event (2 of 2)

EVENT CLASS DESCRIPTION LISTENER

ItemEvent Generated when a check box or list item is clicked ItemListener

KeyEvent Generated when input is received from the keyboard. KeyListener

MouseEventGenerated when the mouse is moved, dragged,

clicked, or released.MouseListener

TextEventGenerated when the value of a text area or text field is

changed. TextListener

WindowEventGenerated when a window is activated, closed,

deactivated, deiconified, iconified, opened.WindowListener



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Using the delegation event model

Create a Listener

object which

implements the

listener interface.

Associate a Listener

with the source

public class MyApplication{

...

Button button = new Button("I'm a button!");

button.addActionListener(new MyHandler());

}

public class MyHandler implements ActionListener{

public void actionPerformed(ActionEvent e){

numClicks++;

}

}

Method in the interface (ActionListener)



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Adapter Classes (1 of 2)

While implementing an interface, we must implement all the methods listed in

the interface

E.g., the MouseListener interface contains five methods: mousePressed,

mouseReleased, mouseEntered, mouseExited, and mouseClicked

If you don’t want to implement some methods, you must have empty bodies of

code for those methods

Difficult to read the code, and maintain it !



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Adapter Classes (2 of 2)

Adapter classes help you avoid implementing empty method bodies

The Java API includes an adapter class for each listener interface with more

than one method.

For example, the MouseAdapter class implements the MouseListener

interface.

An adapter class implements empty versions of all its interface's methods.

Instead of implementing the listener interface, one can extend the

corresponding adapter class and implement only the necessary method(s)



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Anonymous inner class

Sometimes, anonymous inner classes are used for event handling

The general syntax for an anonymous inner class is:

…

b.addActionListener( new ActionListener(){

public void actionPerformed(ActionEvent ae){

System.out.println(“Button Clicked”);

}

});

…

Declaring a class without a name which implements the

ActionListener interface. The class declaration and

instantiation is happening in the same step

new <BaseClassName/InterfaceName>(){

//method implementations

}



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Applets

A Java class that can be embedded within a HTML page and downloaded

and executed by a web browser

An applet can be used to enhance the features of a Web Page

The applet runs on JVM embedded in the browser



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Applets

Applet is a container class that is available in java.applet package

We can create our own Applets by extending the Applet class



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Lifecycle of an Applet (1 of 3)

Applet Working

Applet Displayed

Idle State

Applet Destroyed

start( )

paint( )

stop( )

destroy( )

Draw/Redraw Applet

Destroy Applet

init( )Start State



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The browser calls the init method of the Applet, followed by the start method

If the users leaves the web page, the browser will call the stop method of the

Applet



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If the user comes back to the page the browser will call the start method of the

Applet again

The destroy method is called just before the applet is finally unloaded from

memory



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Execution of an applet (1 of 2)

Compile the applet program, say MyApplet.java, to get the .class file,

MyApplet.class

Embed the <applet></applet> tags in a html file as follows

– Code, width and height are mandatory attributes for the applet tag

Use any java compatible browser to run the html file

<applet code=“MyApplet" width=200 height=200>

</applet>



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Execution of an applet (2 of 2)

For testing an Applet, we can use the appletviewer tool which is in

<javahome>\bin directory

Type the applet tag alone in a file, say applet.txt, and type the following

command

– appletviewer applet.txt

Instead of creating a separate file, the applet tag can be included as a

comment in MyApplet.java file itself. The command will now be as follows

– appletviewer MyApplet.java

The appletviewer tool will open a window to display the Applet



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Parameter passing to an applet

Parameters are passed into an applet as name-value pair using the param

tag within applet tag

– <applet code="ParameterApplet" width=200 height=200>

<param name=“name” value=“value">

</applet>

The values are passed as String values

getParameter (String parameterName) returns the value



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Summary

Abstract Window Toolkit basics

Abstract Window Toolkit controls

Layout Managers

Event Handling Mechanisms

Event Model

APIs to handle events

Applets



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Thank You!

ppt la10 java programming_05

Documents

frame container

ercorpcrsla10003 version

infosys technologies

frame myframe

frame class

container object

frame f

pure container