Applets

Applet

Applet is a small program which executes on browser or Java supported browser. There are two types of applets: Local and Remote.

1.      Local Applets are those which execute on local machine. There is no need of internet for local applet.

2.      Remote applets are those which exist on another or remote machine. We can download and execute them on local machine.

Following is the life cycle of an applet:

1.      Initialization (used to initialize methods, variables, etc.)

2.      Start (execution starts from this method)

3.      Paint (used to display output)

4.      Stop (used to stop execution)

5.      Destroy (used to de-allocate memory)

Format:

public void init()

{}

public void start()

{}

public void paint()

{}

public void stop()

{}

public void destroy()

{}

Compilation and Execution

Compilation à javac filename.java

Execution à appletviewer filename.java

Note: The class-name and file-name are same in case of applets.

e.g. (basic)

Note:

·         Browser only understands markup language. That’s why, we write the tags in comments section.

·         No need of main() in applets.

e.g. (using some methods)

Color codes:

Red = 255,0,0

Green = 0,255,0

Blue = 0,0,255

White = 0,0,0

Black = 255,255,255

e.g. (experimenting with colors)

Note:

Class Graphics and Color lie in awt.

e.g. (drawing shapes)

Controls in awt:

·         Label – static

·         Button

·         Text Field

·         Checkbox

·         Radio-button

·         List-box

·         Combo-box

·         Scrollbar

·         Text Area

Steps to use these:

1.      Initialize the class

Format: class_name object_name = new class_name();

2.      Add into the component class

Format: add(object_name)

Label is static control and has 3 constructors:

1.      Label();

2.      Label(String str);

3.      Label(String str, Allignment);

Button is a class used for event purpose. It has 2 constructors.

e.g. (adding Buttons)

Text Field is a class used for input purpose. It has 3 constructors:

1.      Default constructor

TextField();

2.      TextField(10);

3.      TextField(String str);

e.g. (including Label, text, button)

Checkbox is a class that provides true and false value. It has 3 constructors:

1.      Checkbox();

2.      Checkbox(String str);

3.      Checkbox(String str, Boolean);

Radio button

Format:

CheckboxGroup cbg = new ChackboxGroup();

Checkbox ch = new Checkbox(String str, cbg, boolean);

List is a group of items through which we can chose one or more than one items. It has 3 constructors:

1.      Default:                       List();

2.      Parametrized:              List(int num);

3.      List(int num, Boolean);

Combo-box – This is similar to List. This allows only one choice while list may allow more than one option.

Format:

Choice ch = new Choice();

To add:

ch.add();

ch.addItem();

Text Area is a combination of rows and columns. It has 3 constructors:

1.      TextArea();

2.      TextArea(int row, int column);

3.      TextArea(String str);

Event Delegation Model

Event is a change in state of an object. Change necessarily requires a source. Events are performed on static objects.

Requirements:

1.      Event class

2.      Sources

3.      Listener

Required Package for event àjava.awt.event.*

Required Interface à ActionListener (used with button)

3 necessary steps:

1.      Initialize the object

2.      Add into the component

3.      Register the objects

Format:            object_name.addInterfaceName(CurrentObject);

e.g.                  b1.addActionListener(this);

The ActionEvent class provides the method getActionCommand(). This returns 2 kinds of values – String.

e.g. (Registration of object)

e.g. (performing action)

Ques. WAP to display:

1.      First number label and textbox

2.      Second number label and textbox

3.      Result label and textbox

4.      Buttons – add, subtract, multiply, divide

(Basic calculator program)

Class               Interface                     Event Class                Method

Button             ActionListener                        ActionEvent                actionPerformed(ActionEvent ae)

TextField                     ”                                  ”                                  ”

List                              ”                                  ”                                  ”

Checkbox        ItemListener                ItemEvent                   itemStateChanged(ItemEvent)

Radio                          ”                                  ”                                  ”

Choice                         ”                                  ”                                  ”

TextArea         ActionListener                        ActionEvent                actionPerformed(ActionEvent ae)

Scrollbar          AdjustmentListener    AdjustmentEvent        adjustementValueChanged()

Swings

Swings are a part of Advanced Java.

Package required:        import java.swing.*;

Default layout of awt was “Flow Layout”. In swings, we need to set layout.

Swings are light weight components which provide extendibility to awt components. Swings is better than awt in look and feel. Swings also add further new components. e.g. JLabel, JTable, JScrollPane, JTree.

JLabel constructors:

1.      JLabel();

2.      JLabel(str, str);

3.      JLabel(str, image);

To add image:             ImageIcon ic = new ImageIcon(“Image_path”);

JButton constructors:

1.      JButton();

2.      JButton(String str)

3.      JButton(String str, image)

Demo

ImageIcon ic = new ImageIcon(“Image_path”);

JLabel l1 = new JLabel(“Name”);

JButton j = new JButton();

JButton j1 = new JButton(“Name”);

JButton j2 = new JButton(“Name”, ic);

cp.add(l1);

cp.add(j);

cp.add(j1);

cp.add(j2);

j1.addActionListener(this);

Checkbox constructors:

1.      JCheckbox();

2.      JCheckbox(str);

3.      JCheckbox(str, boolean);

4.      JCheckbox(str, image);

Method for Checkbox:                       isSelected();