Top Java Applet Interview Questions

1. What is an Applet? Should applets have constructors?

Applets are small programs transferred through Internet, automatically installed and run as part of web-browser. Applets implements functionality of a client.

Applet is a dynamic and interactive program that runs inside a Web page displayed by a Java-capable browser. We don’t have the concept of Constructors in Applets. Applets can be invoked either through browser or through Appletviewer utility provided by JDK.

2. How will you initialize an Applet?

Write my initialization code in the applets init method or applet constructor.

3. How to insert your applets into FrontPage?

1. Place the .class file in the directory containing the HTML document into which you want to insert the applet.
2. Copy the … tag from your applet implementation or examples to the clipboard.
3. In FrontPage select the “HTML” tab from the lower left hand corner.
4. Paste the … tag in an appropriate place between the and tags. You’ll find a gray box with the aqua letter “J” in the “Normal” view indicating the the applet tag has been inserted.
5. To see the applet appearance select the “Preview” tab.

4. In our URLs and in the text of the buttons we have comma. Its causing an error. Is there a way to change the delimiting character for the menu arguments?

Since 2.00 version our applets support an user-defined delimiter for the menu arguments. To modify the default delimiter add the following parameter (you can use any character as a delimiter): and use it within “menuItems”:

5. What is the order of method invocation in an Applet?

? public void init() : Initialization method called once by browser.
? public void start() : Method called after init() and contains code to start processing. If the user leaves the page and returns without killing the current browser session, the start () method is called without being preceded by init ().
? public void stop() : Stops all processing started by start (). Done if user moves off page.
? public void destroy() : Called if current browser session is being terminated. Frees all resources used by applet.

6. What are the Applets Life Cycle methods? Explain them?

methods in the life cycle of an Applet:

? init() method – called when an applet is first loaded. This method is called only once in the entire cycle of an applet. This method usually intialize the variables to be used in the applet.
? start( ) method – called each time an applet is started.
? paint() method – called when the applet is minimized or refreshed. This method is used for drawing different strings, figures, and images on the applet window.
? stop( ) method – called when the browser moves off the applet’s page.
? destroy( ) method – called when the browser is finished with the applet.

7. What is the sequence for calling the methods by AWT for applets?

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()

8. How do Applets differ from Applications?

Following are the main differences:

Application: Stand Alone, doesn’t need

web-browser. Applet: Needs no explicit installation on local machine. Can be transferred through Internet on to the local machine and may run as part of web-browser. Application: Execution starts with main() method. Doesn’t work if main is not there. Applet: Execution starts with init() method. Application: May or may not be a GUI. Applet: Must run within a GUI (Using AWT). This is essential feature of applets.

9. Can we pass parameters to an applet from HTML page to an applet? How?

We can pass parameters to an applet using tag in the following way:

? ? Access those parameters inside the applet is done by calling getParameter() method inside the applet. Note that getParameter() method returns String value corresponding to the parameter name.

10. How do we read number information from my applets parameters, given that Applets getParameter() method returns a string?

Use the parseInt() method in the Integer Class, the Float(String) constructor or parseFloat() method in the Class Float, or the
Double(String) constructor or parseDoulbl() method in the class Double.

11. How can I arrange for different applets on a web page to communicate with each other?

Name your applets inside the Applet tag and invoke AppletContext’s getApplet() method in your applet code to obtain references to the other applets on the page.

12. How do I select a URL from my Applet and send the browser to that page?

Ask the applet for its applet context and invoke showDocument() on that context object.
URL targetURL;
String URLString
AppletContext context = getAppletContext();
try
{
targetURL = new URL(URLString);
}
catch (MalformedURLException e)
{
// Code for recover from the exception
}
context. showDocument (targetURL);

13. Can applets on different pages communicate with each other?

Use the getSize() method, which the Applet class inherits from the Component class in the Java.awt package. The getSize() method returns the size of the applet as a Dimension object, from which you extract separate width, height fields.

The following code snippet explains this:

Dimension dim = getSize();
int appletwidth = dim.width();
int appletheight = dim.height();

14. Which classes and interfaces does Applet class consist?

Applet class consists of a single class, the Applet class and three interfaces: AppletContext, AppletStub, and AudioClip.

15. What is AppletStub Interface?

The applet stub interface provides the means by which an applet and the browser communicate. Your code will not typically implement this interface.

16. What tags are mandatory when creating HTML to display an applet?

1. name, height, width
2. code, name
3. codebase, height, width
4. code, height, width
Correct answer is d.

17. What are the Applets information methods?

The following are the Applet’s information methods: getAppletInfo() method: Returns a string describing the applet, its author, copyright information, etc. getParameterInfo( ) method: Returns an array of string describing the applet’s parameters.

18. What are the steps involved in Applet development?

Following are the steps involved in Applet development:

? Create/Edit a Java source file. This file must contain a class which extends Applet class.
? Compile your program using javac
? Execute the appletviewer, specifying the name of your applet’s source file or html file. In case the applet information is stored in html file then Applet can be invoked using java enabled web browser.

19. Which method is used to output a string to an applet? Which function is this method included in?

drawString( ) method is used to output a string to an applet. This method is included in the paint method of the Applet.

20. When is update method called?

Whenever a screen needs redrawing (e.g., upon creation, resizing, validating) the update method is called. By default, the update method clears the screen and then calls the paint method, which normally contains all the drawing code.

21. How will you communicate between two Applets?

The simplest method is to use the static variables of a shared class since there’s only one instance of the class and hence only one copy of its static variables. A slightly more reliable method relies on the fact that all the applets on a given page share the same AppletContext.

We obtain this applet context as follows:

AppletContext ac = getAppletContext();
AppletContext provides applets with methods such as getApplet(name), getApplets(),getAudioClip, getImage, showDocument and showStatus().

22. How do you communicate in between Applets and Servlets?

We can use the java.net.URLConnection and java.net.URL classes to open a standard HTTP connection and “tunnel” to the web server. The server then passes this information to the servlet in the normal way. Basically, the applet pretends to be a web browser, and the servlet doesn’t know the difference. As far as the servlet is concerned, the applet is just another HTTP client.

23. What is the base class for all swing components?

JComponent (except top-level containers)

24. What is Difference between AWT and Swing?

Swing provides a richer set of components than AWT. They are 100% Java-based. AWT on the other hand was developed with the mind set that if a component or capability of a component werent available on one platform, it wouldnt be available on any platform. Due to the peer-based nature of AWT, what might work on one implementation might not work on another, as the peer-integration might not be as robust.

There are a few other advantages to Swing over AWT:

? Swing provides both additional components and added functionality to AWT-replacement components
? Swing components can change their appearance based on the current “look and feel” library that’s being used.
? Swing components follow the Model-View-Controller (MVC) paradigm, and thus can provide a much more flexible UI.
? Swing provides “extras” for components, such as:
? Icons on many components
? Decorative borders for components
? Tool tips for components
? Swing components are lightweight (less resource intensive than AWT)
? Swing provides built-in double buffering
? Swing provides paint debugging support for when you build your own components

Swing also has a few disadvantages:

? It requires Java 2 or a separate JAR file
? If you’re not very careful when programming, it can be slower than AWT (all components are drawn)
? Swing components that look like native components might not act exactly like native components

25. Why do you Canvas?

The Canvas class of java.awt is used to provide custom drawing and event handling. It provides a general GUI component for drawing images and text on the screen. It does not support any drawing methods of its own, but provides access to a Graphics object through its paint() method. The paint() method is invoked upon the creation and update of a canvas so that the Graphics object associated with a Canvas object can be updated.