AWT Event Handling Basics

AWT Event Handling

• The term event describes the change in state of an object.

• Events are triggered by user interaction with GUI components.

• User interaction includes clicking a button, moving the mouse, keyboard input, selecting list items, or scrolling.

• Events can be classified into two categories:

  Foreground Events

  • Occur due to direct user interaction with GUI components
  • Examples include clicking a button, moving the mouse, keyboard input, selecting list items, or scrolling.

  Background Events

  • Occur without direct user interaction.
  • Examples include operating system interrupts, hardware/software failures, timer expirations, or operation completions.

• Event Handling is the mechanism that controls events and determines the appropriate response to them.

• The event handling mechanism uses event handlers, which are code segments executed when an event occurs.

• Java employs the Delegation Event Model for event handling, which defines a standard way to generate and manage events.

• Key participants in the Delegation Event Model:

  Source

  • The object where the event occurs.
  • Provides information about the event to its handler.
  • Java provides classes for creating source objects.

  Listener

  • Also known as the event handler.
  • Responsible for generating a response to an event.
  • Implemented as an object.
  • Waits for event notifications and then triggers a response.
  • Responds to events by displaying results to the user.

• GUI is based on events.

• Examples of GUI events include clicking a button, closing a window, opening a window, or typing in a text area.

• GUI aids user interaction and application design, making applications user-friendly.

Basic Terminologies

• Component - A GUI object with a visual representation on the screen that enables user interaction.
• Container - A component that can hold other components.
• Panel - A container that provides space for attaching other components.
• Window - A rectangular area displayed on the screen, allowing for separate program execution and data display.
• Frame - Similar to a Window, but includes a title bar, menu bar, borders, and resizing corners.
• Canvas - A blank rectangular area on the screen used for drawing and capturing user input.

Examples of GUI Based Applications

• Automated Teller Machine (ATM)
• Airline Ticketing System
• Information Kiosks at railway stations
• Mobile Applications
• Navigation Systems

Advantages of GUI over CUI

• GUI offers visual icons for interaction, while CUI relies on text-based interfaces.
• GUI makes applications more engaging, while CUI lacks visual appeal.
• GUI provides a click-and-execute environment, while CUI requires commands for each task.
• GUI is easier for new users with visual indicators compared to CUI's text-based commands.
• GUI offers more file system and operating system control than CUI, which relies on commands.
• GUI windows allow multitasking with multiple applications, while CUI manages one task at a time.
• GUI provides a multitasking environment, but with greater ease and efficiency than CUI.
• GUI simplifies operating system navigation and control, while CUI relies heavily on commands.
• GUI can be easily customized.

Java AWT Hierarchy

• The Container class is a component in AWT capable of holding other components, such as buttons, text fields, and labels.
• Container subclasses include Frame, Dialog, and Panel, all of which are containers.
• Window is a container without borders or menu bars and requires a Frame, Dialog, or another Window as an owner when created.
• Panel is a container without a title bar or menu bar, but can include other components like buttons or text fields.

Java AWT TextField Example

• The example demonstrates using a TextField object for text input.
• The code creates a Frame window with two TextField objects.
• The setBounds method sets the position and size of each TextField.
• The add method adds the TextFields to the Frame.
• The setSize method sets the Frame's size.
• The setLayout method sets the layout for the Frame, in this case, null.
• The setVisible method makes the Frame visible.
• The output is a window with two text fields.

Java AWT Choice

• The Choice class creates a popup menu with a selection list.
• Users can choose from the available options in the popup menu
• The Choice object is a subclass of Component.
• The example code creates a Frame with a Choice object.
• The add method adds items (Strings) to the Choice list.
• The setBounds method sets the position and size of the Choice object.
• The add method adds the Choice object to the Frame.
• The setSize method sets the Frame's size.
• The setLayout method sets the layout for the Frame, in this case, null.
• The setVisible method makes the Frame visible.
• The output is a window with a popup menu containing the added list items.

Java AWT List

• The List class represents a list of text items, allowing users to select one or multiple items.
• The List object is a subclass of Component.
• The example creates a Frame with a List object.
• The add method adds items (Strings) to the List.
• The setBounds method sets the position and size of the List object.
• The add method adds the List object to the Frame.
• The setSize method sets the Frame's size.
• The setLayout method sets the layout for the Frame, in this case, null.
• The setVisible method makes the Frame visible.
• The output is a window with a list containing the added items.

Java AWT Scrollbar

• The Scrollbar class creates a horizontal or vertical scrollbar.
• Scrollbars enable viewing content that exceeds the visible area.
• The example code creates a Frame window with one Scrollbar.
• The setBounds method sets the position and size of the Scrollbar.
• The add method adds the Scrollbar to the Frame.
• The setSize method sets the Frame's size.
• The setLayout method sets the layout for the Frame, in this case, null.
• The setVisible method makes the Frame visible.
• The output is a window with a scrollbar.

Java AWT TextArea

• The TextArea class represents a multi-line region that displays and allows editing of text.
• The next section of the text provides an example of how to implement a TextArea using Java AWT.