Athipathy Applet

Introduction Java applets are one of three kinds of Java programs: ◦ An application is a standalone program that can be invoked from the command line. ◦ An applet is a program that runs in the context of a browser session. ◦ A servlet is a program that is invoked on a server program, and it runs in the context of a web server process.

Applications versus Applets 

an Applet is a Java program that runs in a web page, while an application is one that runs from the command line.

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An Applet is a Panel that is automatically inserted into a web page. The browser displaying the web page instantiates and adds the Applet to the proper part of the web page.

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The browser tells the Applet when to create its GUI (by calling the init() method of Applet) and when to start() and stop() any special processing.

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Applications run from a command prompt. When you execute an application from the command prompt, the interpreter starts by calling the application's main() method.

AWT The

Java programming language provides a class library called the Abstract Window Toolkit (AWT) that contains a number of common graphical widgets

EVENT handling mechanism Event Object Event Source • generates events • ex. Button

• describes an event • ex. ActionEvent holds state of Shift key Event Listener • any object can implement these interfaces • ex. ActionListener has method actionPerformed()

The hierarchy of class java.applet.Applet  The

following representation shows the hierarchy of Applet back to Object and the packages they are defined in..

Object                          java.lang

   +-Component                 java.awt         +--Container            java.awt              +--Window          java.awt              |    +--Frame      java.awt              +--Panel           java.awt                   +--

Window Fundamentals Component An abstract class that encapsulate all of the attributes of a visual component. All the user interface element that are displayed on the screen and that interact with the user are subclasses of Component Defines method for managing events,such as mouse and keyboard inputs ,positioning and sizing of window.. Responsible for remembering the foreground and background colors

Window Fundamentals Container ◦ Subclass of component. ◦ Allows other component objects to be nested within it

Panel ◦ Subclass of Container and superclass of applet ◦ Screen output directed to applet , it is drawn on the surface of a Panel object ◦ A panel is a window that does not contain a title bar, menu bar and border ◦ Other components can be added by add() method

Window Window

◦ Wont create window objects directly ◦ Instead we use a subclass of Window called Frame Frame

◦ Has a title bar, menu bar, borders and resizing corners ◦ Creating a frame object within an applet, it will contain a warning message(“A applet window”) to the user.

Window Fundamentals Canvas

◦ Encapsulates a blank window upon which we can draw.

Buttons A

Button has a single line label and may be "pushed" with a mouse click.

Type of event generated- Action Event

import java.awt.*; import java.applet.*; public class ButtonTest extends Applet { public void init() { Button button = new Button("OK"); add(button); } }

Checkbox A

Checkbox is a label with a small pushbutton. The state of a Checkbox is either true (button is checked) or false (button not checked). The default initial state is false. Clicking a Checkbox toggles its state. Checkbox m = new Checkbox("Label", true);

Type of event generated- Item Event (when the check box is selected or deselected)

import java.awt.*; import java.applet.*; public class CheckboxSimpleTest extends Applet { public void init() { Checkbox m = new Checkbox ("Allow Mixed Case");

add(m); } }

CheckboxGroup A

CheckboxGroup is used to control the behavior of a group of Checkbox objects (each of which has a true or false state). Exactly one of the Checkbox objects is allowed to be true at one time. Checkbox objects controlled with a CheckboxGroup are usually referred to as "radio buttons".

import java.awt.*; import java.applet.*; public class CheckboxGroupTest extends Applet { public void init() { // create button controller CheckboxGroup cbg = new CheckboxGroup(); Checkbox cb1 = new Checkbox("Show lowercase only", cbg, true); Checkbox cb2 = new Checkbox("Show uppercase only", cbg, false); add(cb1); add(cb2); } }

Type of event generated- Item Event (when the check box is selected or deselected)

Choice Choice

objects are drop-down lists. The visible label of the Choice object is the currently selected entry of the Choice.

Type of event generated- Item Event (when the choice is changed)

Example import java.awt.*; import java.applet.*; public class ChoiceSimpleTest extends Applet { public void init() { Choice rgb = new Choice(); rgb.add("Red"); rgb.add("Green"); rgb.add("Blue"); add(rgb); } }

Label A

Label is a displayed Label object. It is usually used to help indicate what other parts of the GUI do, such as the purpose of a neighboring text field.

import java.awt.*; import java.applet.*; public class LabelTest extends Applet { public void init() { add(new Label("A label")); //equivalent to Label a1=new Label(“A label”)); add(a1); // right justify next label add(new Label("Another label", Label.RIGHT));

List A

List is a scrolling list box that allows you to select one or more items. Multiple selections may be used by passing true as the second argument to the constructor.

import java.awt.*; import java.applet.*; public class ListSimpleTest extends Applet { public void init() { List list = new List(5, false); list.add("Seattle"); list.add("Washington"); list.add("New York"); list.add("Chicago"); list.add("Miami"); list.add("San Jose"); list.add("Denver"); add(list); }

Type

of event generatedItem Event (when the item is selected or deselected) Action Event(when an item is double clicked)

TextField A

TextField is a scrollable text display object with one row of characters. The preferred width of the field may be specified during construction and an initial string may be specified.

import java.awt.*; import java.applet.*; public class TextFieldSimpleTest extends Applet { public void init() { TextField f1 = new TextField("type something"); add(f1); } }

TextArea A

TextArea is a multi-row text field that displays a single string of characters, where newline ('\n' or '\n\r' or '\r', depending on platform) ends each row. The width and height of the field is set at construction, but the text can be scrolled up/down and left/right.

import java.awt.*; import java.applet.*; public class TextAreaSimpleTest extends Applet { TextArea disp; public void init() { disp = new TextArea("Code goes here", 10, 30); add(disp); }

Event Handling Low-level Events 

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    

Low-level events represent a low-level input or window operation, like a key press, mouse movement, or window opening. The following table displays the different lowlevel events, and the operations that generate each event (each operation corresponds to a method of the listener interface): ComponentEvent Hiding, moving, resizing, showing ContainerEvent Adding/removing component FocusEvent Getting/losing focus KeyEvent Pressing, releasing, or typing (both) a key MouseEvent Clicking, dragging, entering, exiting, moving, pressing, or releasing

Example For

instance, typing the letter 'A' on the keyboard generates three events, one for pressing, one for releasing, and one for typing. Depending upon your interests, you can do something for any of the three events.

Semantic Events Semantic

events represent interaction with a GUI component; for instance selecting a button, or changing the text of a text field.

ActionEvent

Do the command AdjustmentEvent Value adjusted ItemEvent State changed TextEvent

Text changed

Event Sources The

following table represents the different event sources. Keep in mind the object hierarchy. For instance, when Component is an event source for something, so are all its subclasses: Low-Level Events Component ComponentListener FocusListener KeyListener MouseListener MouseMotionListener

Container Window 

ContainerListener WindowListener

Semantic Events Button

List MenuItem TextField ActionListener Choice Checkbox MenuItem List ItemListener Scrollbar AdjustmentListener TextArea TextField TextListener

Canvas A

Canvas is a graphical component representing a region where you can draw things such as rectangles, circles, and text strings.  You subclass Canvas to override its default paint() method to define your own components.

import java.awt.*; class DrawingRegion extends Canvas { public DrawingRegion() { setSize(100, 50); } public void paint(Graphics g) { g.drawRect(0, 0, 99, 49); // draw border g.drawString("A Canvas", 20,20); } }

import java.applet.*; public class CanvasPaintTest extends Applet { public void init() { DrawingRegion region = new DrawingRegion(); add(region); } }