Saturday, December 12

Java Fundamental-III


1. When should the method invokeLater()be used?
This method is used to ensure that Swing components are updated through the event-dispatching thread.


2. How can a subclass call a method or a constructor defined in a superclass?
Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass’s constructor.


3. What’s the difference between a queue and a stack?
Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule.


4. You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?
Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.


5. What comes to mind when you hear about a young generation in Java?
Garbage collection.


6. What comes to mind when someone mentions a shallow copy in Java?
Object cloning.


7. If you’re overriding the method equals() of an object, which other method you might also consider?
hashCode()


8. You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList?
ArrayList


9. How would you make a copy of an entire Java object with its state?
Have this class implement Cloneable interface and call its method clone().




10. How can you minimize the need of garbage collection and make the memory use more effective?
Use object pooling and weak object references.


11. There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?
If these classes are threads I’d consider notify() or notifyAll(). For regular classes you can use the Observer interface.


12. What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?
You do not need to specify any access level, and Java will use a default package access level


13. What are the contents of web module?
Answer: A web module may contain:
a) JSP files
b) Java classes
c) gif and html files and
d) web component deployment descriptor


14. Differentiate between .ear, .jar and .war files.
Answer: These files are simply zipped file using java jar tool. These files are created for different purposes. Here is the description of these files:
.jar files: These files are with the .jar extenstion. The .jar files contains the libraries, resources and accessories files like property files.
.war files: These files are with the .war extension. The war file contains the web application that can be deployed on the any servlet/jsp container. The .war file contains jsp, html, javascript and other files for necessary for the development of web applications.
.ear files: The .ear file contains the EJB modules of the application.


15. What is the difference between Session Bean and Entity Bean?
Answer:
Session Bean: Session is one of the EJBs and it represents a single client inside the Application Server. Stateless session is easy to develop and its efficient. As compare to entity beans session beans require few server resources.


A session bean is similar to an interactive session and is not shared; it can have only one client, in the same way that an interactive session can have only one user. A session bean is not persistent and it is destroyed once the session terminates.


Entity Bean: An entity bean represents persistent global data from the database. Entity beans data are stored into database.


16. Why J2EE is suitable for the development distributed multi-tiered enterprise applications?
Answer: The J2EE platform consists of multi-tiered distributed application model. J2EE applications allows the developers to design and implement the business logic into components according to business requirement. J2EE architecture allows the development of multi-tired applications and the developed applications can be installed on different machines depending on the tier in the multi-tiered J2EE environment . The J2EE application parts are:


a) Client-tier components run on the client machine.
b) Web-tier components run on the J2EE server.
c) Business-tier components run on the J2EE server and the
d) Enterprise information system (EIS)-tier software runs on the EIS servers


17. Why do understand by a container?
Answer: Normally, thin-client multi-tiered applications are hard to write because they involve many lines of intricate code to handle transaction and state management, multithreading, resource pooling, and other complex low-level details. The component-based and platform-independent J2EE architecture makes J2EE applications easy to write because business logic is organized into reusable components. In addition, the J2EE server provides underlying services in the form of a container for every component type. Because you do not have to develop these services yourself, you are free to concentrate on solving the business problem at hand (Source: http://java.sun.com/j2ee/1.3/docs/tutorial/doc/Overview4.html ).


In short containers are the interface between a component and the low-level platform specific functionality that supports the component. The application like Web, enterprise bean, or application client component must be assembled and deployed on the J2EE container before executing.


18. What are the services provided by a container?
Answer: The services provided by container are as follows:
a) Transaction management for the bean
b) Security for the bean
c) Persistence of the bean
d) Remote access to the bean
e) Lifecycle management of the bean
f) Database-connection pooling
g) Instance pooling for the bean

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