• Applets have limited file access
• Applets have limited network access

The Basics

The Life of an Applet

• init() is the first method called by an applet once it has been loaded by the browser. It is called before the applet begins execution, and can be overridden to perform any initialization tasks such as loading images or fonts, establishing layout, or setting parameters.

• After an applet has been loaded by the browser and init() has been called, start() is automatically called to begin the execution of the applet. To provide startup behavior for your applet, override the start() method.

• stop() is called when the user moves off the page containing the applet. You implement this to stop any operations you started in the start() method.

• destroy() is called when an applet has completely finished executing, and resources allocated by the applet are returned to the system. Java takes care of garbage collection and memory allocation, so it is generally not necessary to override destroy() unless you have specific resources that need to be released.

The Abstract Windowing Toolkit (AWT)

1. Interface components encompass all of the standard widgets associated with a windowing system. These widgets include: buttons, canvases, checkboxes, labels, lists, scrollbars, text fields, and text areas.

2. Containers encompass areas in which components can be placed, allowing several components to be grouped together to form a more cohesive object for manipulation. Panel is the most commonly used container.

3. Windows are a special case of the Component class, and are not used very often in applets. In applets, the browser provides the main window and menu bar, so you don't need windows.

Layout managers in Java allow the user even greater control over the placement of components. The layout manager determines how portions of the screen will be sectioned and how components within that panel will be placed. There are four different layout managers available in Java. They are discussed in the table below.

Layout Name	Function
BorderLayout	Layout according to compass points
GridLayout	Layout on a grid
GridBagLayout	Layout on a grid where elements can be different sizes
CardLayout	Layout that contains a series of "cards" which can be flipped through
FlowLayout	Layout that arranges components left to right

So that is a basic description of Java's Abstract Windowing Toolkit. You will learn more about this as we look at applets and observe how different components and layouts are implemented to create a user interface.

Threads

There are four modifications you need to make to create an applet that uses threads (Taken from Java Unleashed):

• Change the signature of your applet class to include the words implement Runnable.

• Include an instance variable to hold this applet's thread.

• Modify your start() method to do nothing but spawn a thread and start it running.

• Create a run() method that contains the actual code that starts your applet running.

Handling User Input

Event Handling
The UI (User Interface) components defined in dataPanel need to handle user input. To intercept an action by any UI component, we must define an action() method in our applet. All basic UI components have different actions and arguments (taken from Teach Yourself Java In 21 Days by Laura Lemay and Charles L. Perkins):

• Buttons create actions when they are selected, and a button's argument is the label of the button.

• Checkboxes, both exclusive and nonexclusive, generate actions when a box is checked. The argument is always true.

• Choice Menus generate an action when a menu item is selected, and the argument is that item.

• Text fields create actions when the user presses Return inside that text field. Note that if the user tabs to a different text field or uses the mouse to change the input focus, an action is NOT generated. Only a Return triggers the action.

There are three necessary components to run an applet on the Internet: