Download The Applet Class

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UNIT II
Constructors and Methods
APPLETS
Applet Tag in HTML
Parameter Tag
Some Sample Applets
AWT
Button and Label
Button Class
Label Class
Applet with Buttons and Labels
Button in Action
Check Box and List
Check Box
Check Box Group
List
Menu
Text Components
Text Component Class
Text Field Class
Text Area Class
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Applets
An applet is a program that can be referenced by the HTML source code of a webpage. It is
dynamically downloaded from a webserver to a browser. The applet then executes within
the environment provided by the browser. Alternatively, we can use a tool such as an
appletviewer to run it.
Loading Applets in a Web Page
The applet will be executed by a Java-enabled web browser when it encounters the APPLET
tag within the HTML file. To view and test an applet more conveniently, simply include a
comment at the head of your Java source code file that contains the APPLET tag. This way,
your code is documented with the necessary HTML statements needed by your applet, and
you can test the compiled applet by starting the applet viewer with your Java source code
file specified as the target. Here is an example of such a comment:
/*
<applet code="MyApplet.class" width=200 height=60>
</applet>
*/
This comment contains an APPLET tag that will run an applet called MyApplet in a window
that is 200 pixels wide and 60 pixels high. Since the inclusion of an APPLET command
makes testing applets easier.
appletviewer MyApplet.html
Once you know your applet runs within the appletviewer, it is important to test your applet
running in a web browser by loading the applet's web page into the browser window. The
browser can retrieve the class files either from the internet or from the local working
directory used during development. If you make changes to your applet's code while it is
loaded in the browser, then you must recompile the applet and press the "Shift + Reload"
button in the browser to load the new version.
Applets vs. Applications
Applet
Application
1. Downloaded from web and run on a
browser
2. Applets have many restrictions:
a. cannot read/write to local filesystems
b. cannot communicate with any server
other than from where it is loaded
c. cannot run any program on user’s
system
3. They use init(), start(), stop() lifecycle
methods.
1. stand-alone program
2. They do not have restrictions that
applets have. They are typically
executed by JVM
3. They use only main() function
When to write Applets vs. Applications
In the early days of Java, one of the critical advantages that Java applets had over Java
applications was that applets could be easily deployed over the web while Java applications
required a more cumbersome installation process. Additionally, since applets are
downloaded from the internet, by default they have to run in a restricted security
environment, called the "sandbox", to ensure they don't perform any destructive operations
on the user's computer, such as reading/writing to the file system.
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The Applet Class
The Applet class defines the methods shown in Table 19-1. Applet provides all necessary
support for applet execution, such as starting and stopping. It also provides methods that
load and display images, and methods that load and play audio clips. Applet extends the
AWT class Panel. In turn, Panel extends Container, which extends Component. These
classes provide support for Java's window-based, graphical interface. Thus, Applet provides
all of the necessary support for window-based activities.
Table 19-1 The Methods Defined by Applet
Method
void destroy( )
Description
Called by the browser just before an
applet is terminated. Your applet will
override this method if it needs to perform
any cleanup prior to its destruction.
AppletContext getAppletContext( )
Returns the context associated with the
applet.
String getAppletInfo( )
Returns a string that describes the applet.
AudioClip getAudioClip(URL url)
Returns an AudioClip object that
encapsulates the audio clip found at the
location specified by url.
AudioClip getAudioClip(URL url, String clipName) Returns an AudioClip object that
encapsulates the audio clip found at the
location specified by url and having the
name specified by clipName.
URL getCodeBase( )
Returns the URL associated with the
invoking applet.
URL getDocumentBase( )
Returns the URL of the HTML document
that invokes the applet.
Image getImage(URL url)
Returns an Image object that
encapsulates the image found at the
location specified by url.
Image getImage(URL url, String imageName)
Returns an Image object that
encapsulates the image found at the
location specified by url and having the
name specified by imageName.
Locale getLocale( )
Returns a Locale object that is used by
various locale-sensitive classes and
methods.
String getParameter(String paramName)
Returns the parameter associated with
paramName .null is returned if the
specified parameter is not found.
String[ ] [ ] getParameterInfo( )
Returns a String table that describes the
parameters recognized by the applet.
Each entry in the table must consist of
three strings that contain the name of the
parameter, a description of its type and/or
range, and an explanation of its purpose.
void init( )
Called when an applet begins execution. It
is the first method called for any applet.
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boolean isActive( )
static final AudioClip newAudioClip(URL url)
void play(URL url)
void play(URL url, String clipName)
void resize(Dimension dim)
void resize(int width, int height)
final void setStub(AppletStub stubObj)
void showStatus(String str)
void start( )
void stop( )
Returns true if the applet has been
started. It returns false if the applet has
been stopped.
Returns an AudioClip object that
encapsulates the audio clip found at the
location specified by url. This method is
similar to getAudioClip() except that it is
static and can be executed without the
need for an Applet object.
If an audio clip is found at the location
specified by url, the clip is played.
If an audio clip is found at the location
specified by url with the name specified by
clipName, the clip is played.
Resizes the applet according to the
dimensions specified by dim. Dimension is
a class stored inside java.awt. It contains
two integer fields: width and height.
Resizes the applet according to the
dimensions specified by width and height.
Makes stubObj the stub for the applet.
This method is used by the run-time
system and is not usually called by your
applet. A stub is a small piece of code that
provides the linkage between your applet
and the browser
Displays str in the status window of the
browser or applet viewer. If the browser
does not support a status window, then no
action takes place.
Called by the browser when an applet
should start (or resume) execution. It is
automatically called after init() when an
applet first begins.
Called by the browser to suspend
execution of the applet. Once stopped, an
applet is restarted when the browser calls
start()
The Lifecycle of an Applet
A java application begins execution at the main() methods of a class. An applet executes
within an environment provided by a web browser or a tool such as the appletviewer.
Therefore it does not have a main() method. Instead there are four methods that are called
during the life cycle of an applet: init(), start(), stop() and destroy()
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They are defined by java.applet.Applet class and therefore are inherited by every applet.
When an applet begins, the AWT calls the following methods, in this sequence:
init()
start()
paint()
When an applet is terminated, the following sequence of method calls takes place:
stop()
destroy()
Let's look more closely at these methods.
init() - The init() method is the first method to be called. This is where you should
initialize variables. This method is called only once during the run time of your applet.
start() - The start() method is called after init(). It is also called to restart an applet
after it has been stopped. Whereas init() is called once—the first time an applet is loaded—
start() is called each time an applet's HTML document is displayed onscreen. So, if a user
leaves a web page and comes back, the applet resumes execution at start().
paint() - The paint() method is called each time your applet's output must be redrawn.
This situation can occur for several reasons. For example, the window in which the applet is
running may be overwritten by another window and then uncovered. Or the applet window
may be minimized and then restored. paint() is also called when the applet begins
execution. Whatever the cause, whenever the applet must redraw its output, paint() is
called. The paint() method has one parameter of type Graphics. This parameter will
contain the graphics context, which describes the graphics environment in which the applet
is running. This context is used whenever output to the applet is required.
stop() - The stop() method is called when a web browser leaves the HTML document
containing the applet—when it goes to another page, for example. When stop() is called,
the applet is probably running. You should use stop() to suspend threads that don't need to
run when the applet is not visible. You can restart them when start() is called if the user
returns to the page.
destroy() - The destroy() method is called when the environment determines that your
applet needs to be removed completely from memory. At this point, we free up any
resources the applet may be using. The stop() method is always called before destroy().
Simple Applet Display Methods
Applets are displayed in a window and they use the AWT to perform input and output.
Although we will examine the methods, procedures, and techniques necessary to fully
handle the AWT windowed. To output a string to an applet, use drawString(), which is a
member of the Graphics class. It is called from within either update() or paint(). It has
the following general form:
void drawString(String message, int x, int y)
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Here, message is the string to be output beginning at x, y. In a Java window, the upper left
corner is location 0, 0. The drawString() method will not recognize newline characters. If
you want to start a line of text on another line, you must do so manually, specifying the
precise X, Y location where you want the line to begin.
To set the background color of an applet's window, use setBackground(). To set the
foreground color, use setForeground(). These methods are defined by Component, and
they have the following general forms:
void setBackground(Color newColor)
void setForeground(Color newColor)
Here, newColor specifies the new color. The class Color defines the constants shown here
that can be used to specify colors:
Color.BLACK
Color.MAGENTA
Color.BLUE
Color.ORANGE
Color.CYAN
Color.PINK
Color.DARKGRAY
Color.RED
Color.GRAY
Color.WHITE
Color.GREEN
Color.YELLOW
Color.LIGHTGRAY
For example, this sets the background color to green and the text color to red:
setBackground(Color.GREEN);
setForeground(Color.RED);
A good place to set the foreground and background colors is in the init() method. You can
change these colors as often as necessary during the execution of your applet. The default
foreground color is black. The default background color is light gray. You can obtain the
current settings for the background and foreground colors by calling getBackground() and
getForeground(), respectively. They are also defined by Component and are shown here:
Color getBackground()
Color getForeground()
Here is a very simple applet that sets the background color to cyan, the foreground color to
red, and displays a message that illustrates the order in which the init(), start(), and
paint() methods are called when an applet starts up:
/* A simple applet that sets the foreground and background colors and
outputs a string. */
import java.awt.*;
import java.applet.*;
/*
<applet code="Sample.class" width=300 height=50>
</applet>
*/
public class Sample extends Applet{
String str=” “;
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// set the foreground and background colors.
public void init() {
setBackground(Color.CYAN);
setForeground(Color.RED);
str += "Inside init( ) —";
}
// Initialize the string to be displayed.
public void start() {
str += " Inside start( ) —";
}
// Display str in applet window.
public void paint(Graphics g) {
str += " Inside paint( ).";
g.drawString(str, 10, 30);
}
}
This applet generates the window shown here:
The HTML APPLET Tag
The APPLET tag is used to start an applet from both an HTML document and from an applet
viewer. An applet viewer will execute each APPLET tag that it finds in a separate window,
while web browsers like Netscape Navigator, Internet Explorer, and HotJava will allow many
applets on a single page. So far, we have been using only a simplified form of the APPLET
tag. Now it is time to take a closer look at it. The syntax for the standard APPLET tag is
shown here. Bracketed items are optional.
<APPLET
[CODEBASE = codebaseURL]
CODE = appletFile
[ALT = alternateText]
[NAME = appletInstanceName]
WIDTH = pixels HEIGHT = pixels
[ALIGN = alignment]
[VSPACE = pixels] [HSPACE = pixels]
>
[< PARAM NAME = AttributeName VALUE = AttributeValue>]
[< PARAM NAME = AttributeName2 VALUE = AttributeValue>]
. . .
[HTML Displayed in the absence of Java]
</APPLET>
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Let's take a look at each part now.
CODEBASE
CODEBASE is an optional attribute that specifies the base URL of the applet code which is
the directory that will be searched for the applet's executable class file. The HTML
document's URL directory is used as the CODEBASE if this attribute is not specified. The
CODEBASE does not have to be on the host from which the HTML document was read.
CODE
CODE is a required attribute that gives the name of the file containing your applet's
compiled .class file. This file is relative to the code base URL of the applet, which is the
directory that the HTML file was in or the directory indicated by CODEBASE if set.
ALT
The ALT tag is an optional attribute used to specify a short text message that should be
displayed if the browser understands the APPLET tag but can't currently run Java applets.
This is distinct from the alternate HTML you provide for browsers that don't support applets.
NAME
NAME is an optional attribute used to specify a name for the applet instance. Applets must
be named in order for other applets on the same page to find them by name and
communicate with them. To obtain an applet by name, use getApplet(), which is defined
by the AppletContext interface.
WIDTH and HEIGHT
WIDTH and HEIGHT are required attributes that give the size of the applet display area.
ALIGN
ALIGN is an optional attribute that specifies the alignment of the applet. This attribute is
treated the same as the HTML IMG tag with these possible values: LEFT, RIGHT, TOP,
BOTTOM, MIDDLE, BASELINE, TEXTTOP, ABSMIDDLE, and ABSBOTTOM.
VSPACE and HSPACE
These attributes are optional. VSPACE specifies the space, in pixels, above and below the
applet. HSPACE specifies the space, in pixels, on each side of the applet. They're treated the
same as the IMG tag's VSPACE and HSPACE attributes.
PARAM NAME and VALUE
The PARAM tag allows you to specify applet-specific arguments in an HTML page. Applets
access their attributes with the getParameter() method.
HANDLING OLDER BROWSERS
Some older web browsers can't execute applets and don't recognize the APPLET tag.
Although these browsers are now nearly extinct (having been replaced by Java compatible
ones), you may need to deal with them for a while longer. The best way to design your
HTML page to deal with such browsers is to include HTML text and markup within your
<applet> </applet> tags. If the applet tags are not recognized by your browser, you will
see the alternate markup. If Java is available, it will consume the entire markup between
the <applet> </applet> tags and disregard the alternate markup.
Here's the HTML to start an applet called SampleApplet in Java and to display a message
in older browsers:
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<applet code="SampleApplet" width=200 height=40>
If you were driving a Java powered Navigator, you'd see
&quote;A Sample Applet&quote; here.<p>
</applet>
Passing Parameters to Applets
As just discussed, the APPLET tag in HTML allows you to pass parameters to your applet. To
retrieve a parameter, use the getParameter() method. It returns the value of the specified
parameter in the form of a String object. Thus, for numeric and Boolean values, you will
need to convert their string representations into their internal formats.
Here is an example that demonstrates passing parameters:
// Use Parameters
import java.awt.*;
import java.applet.*;
/*
<applet code="ParamDemo.class" width=300 height=80>
<param name=fontName value=Courier>
<param name=fontSize value=14>
<param name=leading value=2>
<param name=accountEnabled value=true>
</applet>
*/
public class ParamDemo extends Applet {
String fontName;
int fontSize;
float leading;
boolean active;
public void start() {
String param;
fontName = getParameter("fontName");
if(fontName == null)
fontName = "Not Found";
param = getParameter("fontSize");
try {
if(param != null) // if not found
fontSize = Integer.parseInt(param);
else
fontSize = 0;
} catch(NumberFormatException e) {
fontSize = -1;
}
param = getParameter("leading");
try {
if(param != null) // if not found
leading = Float.valueOf(param).floatValue();
else
leading = 0;
} catch(NumberFormatException e) {
leading = -1;
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}
param = getParameter("accountEnabled");
if(param != null)
active = Boolean.valueOf(param).booleanValue();
}
// Display parameters.
public void paint(Graphics g) {
g.drawString("Font name: " + fontName, 0, 10);
g.drawString("Font size: " + fontSize, 0, 26);
g.drawString("Leading: " + leading, 0, 42);
g.drawString("Account Active: " + active, 0, 58);
}
}
Sample output from this program is shown here:
As the program shows, you should test the return values from getParameter(). If a
parameter isn't available, getParameter() will return null.
Abstract Windowing Toolkit
Introduction: The Abstract Windowing Toolkit (AWT) is a library of java packages that forms
part of the java API. It includes over 25 packages that define hundreds of classes used for
GUI. The AWT classes contained in the java.awt packages. The following diagram shows the
component hierarchy:
Button - A Button component is a simple pushbutton that displays a string label and
responds to mouse presses from the user.
Canvas - A Canvas is a blank area suitable for drawing in. You can use a Graphics object to
put polygons or images on the Canvas.
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Checkbox - A Checkbox is a small box with two states: either it's checked or unchecked.
Clicking the mouse over an enabled Checkbox toggles its state. It can have a string label.
CheckboxGroup - You put Checkboxes in a CheckboxGroup in order to make them exhibit
"radio button" behavior: when one is checked, the others become unchecked. Only one
Checkbox from the group can be checked at any time.
Choice - A Choice component allows the user to specify one of a short list of choices, which
appear on a little popup menu next to the current choice.
Label - A Label component displays a line of text. The text can be aligned to the left, right,
or center of the Label. The user isn't allowed to edit the text in a Label.
List - A List presents a scrollable list of items, identified by string names. Use this instead of
a Choice when multiple selections are meaningful or when there may be too many items to
conveniently display on a single popup menu.
Scrollbar - Most Scrollbar components are automatically generated when required by List
or TextArea components. If you want to create your own scrollbars, you can do so. The
orientation is specified by the constants Scrollbar.HORIZONTAL and Scrollbar.VERTICAL.
TextField - A TextField component holds a single line of text in a little window. The text is
allowed to be longer than the window, in which case only part of it shows.
TextArea
A TextArea is a pane containing lines of text. Like a TextField, it can be editable or not.
Layout Managers
When we add a component to an applet or a container, the container uses its layout
manager to decide where to put the component. Different LayoutManager classes use
different rules to place components. java.awt.LayoutManager is an interface. Classes in
the java.awt package implement it: GridLayout, BorderLayout & FlowLayout
GridLayout
A GridLayout divides an applet into a specified number of rows and columns which form a
grid of cells, each equally sized and spaced. As components are added to the layout, they
are placed in the cells, starting at the upper left hand corner and moving to the right and
down the page. Each component is sized to fit into its cell.
//Program to demonstrate GridLayout
import java.applet.*;
import java.awt.*;
/*
<applet code ="GridLayoutApplet.class" width=200 height=200>
</applet>
*/
public class GridLayoutApplet extends Applet{
public void init(){
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setLayout (new GridLayout(3,2));
Button b1 = new Button("A");
Button b2 = new Button("B");
Button b3 = new Button ("C");
Button b4 = new Button("D");
Button b5 = new Button ("E");
Button b6 = new Button("F");
add(b1);add(b2);add(b3);
add(b4);add(b5);add(b6);
}
}
BorderLayout
A BorderLayout organizes an applet into North, South, East, West and Center sections.
North, South, East and West are the rectangular edges of the applet. They're continually
resized to fit the sizes of the widgets included in them. Center is whatever's left over in the
middle.
//Program to demonstrate BorderLayout
import java.applet.*;
import java.awt.*;
/*
<applet code="BorderLayoutApplet.class" width=300 height=300>
</applet>
*/
public class BorderLayoutApplet extends Applet{
public void init(){
setLayout (new BorderLayout(5,5));
Button b1 = new Button("North");
Button b2 = new Button ("South");
Button b3 = new Button ("East");
Button b4 = new Button ("West");
Button b5 = new Button ("Center");
add(b1,"North");add(b2,"South");
add(b3,"East");add(b4,"West");
add(b5,"Center");
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}
}
FlowLayout
A FlowLayout arranges widgets from left to right until there's no more space left. Then it
begins a row lower and moves from left to right again. Each component in a FlowLayout
gets as much space as it needs and no more. A FlowLayout is useful for laying out buttons
but not for much else. This is the default LayoutManager for applets and panels
//Program to demonstrate FlowLayout
import java.applet.*;
import java.awt.*;
/*
<applet code="FlowLayoutApplet.class" width=300 height=200>
</applet>
*/
public class FlowLayoutApplet extends Applet{
public void init(){
setLayout (new FlowLayout (FlowLayout.LEFT,5,5));
for (int i=0;i<20;i++){
add (new Button ("Button"+i));
}
}
}
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Buttons and Labels
The Button Class
A Button is a component that simulates the appearance of a push button on an electronic
instrument. With user’s mouse click inside its boundaries, the button changes its
appearance to provide feedback for the user. The major resources of the Button class are
given in Table 2.9.
Table 2.9 Major resources of the Button class.
The first, default, constructor will create a Button without a label. The first two methods
allow the Button's label to be changed or obtained. The remaining methods are concerned
with the Button's listener and the command string sent to it.
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//Program To Demonstrate Button
import java.awt.*;
import java.applet.Applet;
/*
<applet code="ButtonApplet.class" width=200 height=200>
</applet>
*/
public class ButtonApplet extends Applet{
public void init(){
Button b1,b2,b3;
b1 = new Button("Apple");
b2 = new Button("Banana");
b3 = new Button("Grapes");
add(b1);
add(b2);add(b3);
}
}
The Label Class
A Label is a string that appears on a GUI. Labels are very frequently used in programs.
Often placed before text fields to inform the user about the input that is expected. This is a
relatively simple class and provides a convenient point to start the tour. Table 2.1 lists the
major methods and other resources of the class.
Table 2.1 Major resources of the Label class.
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The data attributes of a Label instance are the text which it will display and the alignment of
the text. The possible alignments for the text are enumerated by the three Label class wide
constants CENTER, LEFT and RIGHT, with a default of LEFT when it is not explicitly specified
otherwise.
Follow these steps to use a Label in an Applet:
1. create a Label object
2. invoke the add() method to add the Label object to the Applet
//Program To Demonstrate Label
import java.awt.*;
import java.applet.Applet;
/*
<applet code="LabelApplet.class" width=200 height=200>
</applet>
*/
public class LabelApplet extends Applet
{
public void init()
{
setLayout(new GridLayout(3,1));
setBackground(Color.YELLOW);
Label l1 = new Label("This Text is Left Aligned",
Label.LEFT);
Label l2 = new Label("This Text is Center Aligned",
Label.CENTER);
Label l3 = new Label("This Text is Right Aligned",
Label.RIGHT);
add(l1);
add(l2);
add(l3);
}
}
Checkbox and List
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The Checkbox Class
The Checkbox class provides a text label and an associated on/ off state indicator. It is used
to implement check buttons and, in conjunction with the CheckboxGroup class, radio
buttons. The major resources of this class are presented in Table 2.11.
A Checkbox instance has three data attributes, the label which it will display, the
CheckboxGroup to which it might belong and the state (on or off) of its indicator. If a
Checkbox instance is not a member of a CheckboxGroup it will behave as a check button
and its state can be set or unset independently of all other Checkbox buttons. If a Checkbox
instance is a member of a CheckboxGroup it will behave as a radio button, where only one
member of the group can be checked at any one instant and checking one of the buttons in
the group will automatically unset all other buttons.
The first two constructors are comparable to the first two constructors of the Label class
given above. The third constructor specifies its initial state, the last the initial state and the
CheckboxGroup to which it belongs.
Table 2.11 Major resources of the Checkbox class.
//Program To Demonstrate Checkbox
import java.applet.Applet;
import java.awt.*;
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public class Check extends Applet{
public void init(){
setLayout(new GridLayout(3,1));
Checkbox c1=new Checkbox("Tamil", true);
Checkbox c2=new Checkbox("English", false);
Checkbox c3=new Checkbox("Telugu", false);
add(c1);
add(c2);
add(c3);
}
}
/*
<applet code="Check.class" width=200 height=200>
</applet>
*/
The CheckboxGroup Class
In order to configure Checkbox instances as radio buttons an instance of the
CheckboxGroup class has to be created and specified as the group attribute of the set of
buttons. The major resources of the CheckboxGroup class are given in Table 2.13.
Table 2.13 Major resources of the CheckboxGroup class.
The CheckboxGroup class encapsulates the identities of the Checkboxes which comprise its
group and maintains knowledge of which one is currently selected. It has a single default
constructor and an enquiry and a modifier method for the currently selected component.
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//Program To Demonstrate CheckBoxGroup
import java.applet.Applet;
import java.awt.*;
public class checkbg extends Applet{
public void init(){
setLayout(new GridLayout(3,1));
CheckboxGroup cbg=new CheckboxGroup();
Checkbox c1=new Checkbox("Tamil",cbg,true);
Checkbox c2=new Checkbox("English",cbg,false);
Checkbox c3=new Checkbox("Telugu",cbg,false);
add(c1);
add(c2);
add(c3);
}
}
/*
<applet code="checkbg.class" width=200 height=200>
</applet>
*/
The List Class
The List class provides a scrolling, list box which allows the user to select one or a number
of options, from a list of choices. The major resources of this class are given in Table 2.15.
Table 2.15 Major resources of the List class.
An instance of the List class encapsulates a list of Strings, known as items, which represent
the choices to be presented to the user. It also knows if it is available for single or multiple
selections and the number of items in the list (rows) which are to be visible.
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The third constructor requires the number of rows to be specified and a boolean value to
indicate if multiple selections are to be allowed.

The two add() methods allow an item to be added to end or at a specific location within list.

The two remove() methods allow a single item or a continuous range of items to be
deleted and removeAll() clears the list.

The getItemCount() method indicates the number of items in the list.

The two methods, setMultipleMode() and isMultipleMode(), allow the multiple selection
state of the list to be set or queried.

The select() and deSelect() methods are supplied to allow items within the list to be
selected & the isSelected() boolean method indicates the state of an individual item.

The getRows() and getVisibleIndex() methods indicate the number of visible items and
the first item which is visible.

The makeVisible() method specifies item is visible somewhere within the list's window.

The getSelectedIndex() should only be used if the List is in single selection mode and
returns the index of the currently selected item or -1 if none are selected.

If the list is in multiple selection mode the getSelectedIndexes() method will return an
array of the indexes of the currently selected items.

The getSelectedItem() and getSelectedItems() methods return a String
//Program To Demonstrate List
import java.awt.*;
import java.applet.Applet;
public class MyList extends Applet{
public void init(){
List icecream = new List(6,true);
icecream.add("Chocolate");
icecream.add("Vanilla");
icecream.add("Strawberry");
icecream.add("Casata");
icecream.add("Rumraisin");
icecream.add("ButterScotch");
icecream.add("Mintchip");
add(icecream);
}
}
/*
<applet code = "MyList.class" width="200" height="200">
</applet>
*/
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Table 2.15 Major resources of the List class
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The Menu Class
Java provides two types of menu: pull down and popup menu.
Pull down menus are accessed via menu bar. It may contain many menus.
Menu bars may only appear in a frame
Steps for Creating pull down Menus:
1. Create an object for the MenuBar class
2. Attach the MenuBar to the Frame
3. Create Menu class for each Menu you want on the MenuBar
The Menu class takes a single argument of the menu class that is caption of the menu.
4. Add Menu to MenuBar
5. Create an Object for MenuItem or CheckboxMenuItem class for each submenu item.
6. Add the MenuItem to Menu class
//Program To Demonstrate Menu
import java.awt.*;
public class MyMenu {
public static void main(String args[]) {
Frame f=new Frame(“This is test for Menu”);
MenuBar main=new MenuBar();
f.setMenuBar(mb);
Menu filemenu = new Menu("File");
Menu editmenu = new Menu("edit");
Menu helpmenu = new Menu("Help");
mb.add(filemenu);
mb.add(editmenu);
mb.add(helpmenu);
MenuItem
MenuItem
MenuItem
MenuItem
MenuItem
new1 = new MenuItem("New");
open = new MenuItem("open");
save = new MenuItem("save");
close = new MenuItem("close");
line = new MenuItem("-");
CheckboxMenuItem print = new CheckboxMenuItem("print");
MenuItem exit = new MenuItem("exit");
filemenu.add(new1);
filemenu.add(open);
filemenu.add(save);
filemenu.add(line);
filemenu.add(print);
filemenu.add(exit);
MenuItem cut = new MenuItem("cut");
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MenuItem copy = new MenuItem("copy");
MenuItem paste = new MenuItem("paste");
MenuItem undo= new MenuItem("undo");
editmenu.add(cut);
editmenu.add(copy);
editmenu.add(paste);
editmenu.add(undo);
undo.setEnabled(false);
Menu more= new Menu("More");
helpmenu.add(more);
more.add("commands");
more.add("about");
f.setSize(200,200);
f.setVisible(true);
}
}
OUTPUT:
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The TextComponent, TextArea and TextField Classes
These three classes supply facilitates for the input, and output, of text information. The
TextComponent class is not intended to be instantiated, although it is not declared
abstract, and provides common behavior for the single line TextField & multiline TextArea
classes. The major resources of the TextComponent class are presented in Table 2.19.
Table 2.19 Major resources of the TextComponent class.
There is no constructor for this class as it is only intended that instances of the TextField or
TextArea classes will be created.
TextArea
TextArea class handles the multilines of text. This is also derived from TextComponent.
Textarea components are useful for getting a large amount of data from a user, or for
displaying a large amount of data that is free to be modified. They take up more screen real
estate than textfields, but can conserve quite a lot of room because they have support for
scrollbars. The major resources added by the TextArea class are presented in Table 2.20.
//Program To Demonstrate TextArea
import java.applet.*;
import java.awt.*;
public class TextArea1 extends Applet{
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public void init(){
TextArea ta = new TextArea(5,8);
Label l1 = new Label(“Text Area”, Label.CENTER);
ta.setText(“This \n is the\n Test\n for Text Area \n with \n non editable \n
mode");
add(l1); add(ta);
ta.setEditable(false);//cannot edit
}
}
/*
<applet code=”TextArea1.class” height=200 width=200>
</applet>
*/
TextField
TextField components are useful for obtaining short pieces of information from users. They
are limited to single line input (for multiple lines, use a TextArea instead). Whenever a user
types data into a text field, and hits enter or changes focus, the textfield will generate an
event.
//Program To Demonstrate TextField
import java.applet.*;
import java.awt.*;
public class Text extends Applet{
public void init(){
Label l1 = new Label("Enter the Username", Label.CENTER);
TextField t1 = new TextField(10);
Label l2 = new Label("Enter Password", Label.CENTER);
TextField t2 = new TextField(10);
t2.setEchoChar('*');
add(l1); add(t1);
add(l2);add(t2);
}
}
/*
<applet code="Text.class" height=200 width=200>
</applet>
*/
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Table 2.20 Major resources of the TextField class.
The constructors allow various combinations of the String to be initially displayed and the
number of columns to be specified. The number of columns defaults to the number of
characters in the String for the second constructor and zero for the default constructor. The
String contained within the TextComponet can be longer than the number of columns in
which case it can be scrolled to the left and right by the user's actions. If the echoChar
attribute is set then the specified character will be used to give the user feedback as they
type in the field, as will be demonstrated below.
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