j2ee

1.    What makes J2EE suitable for distributed multitiered Applications?
- The J2EE platform uses a multitiered distributed application model. Application logic is divided into components according to function, and the various application components that make up a J2EE application are installed on different machines depending on the tier in the multitiered J2EE environment to which the application component belongs. The J2EE application parts are:

§                  Client-tier components run on the client machine.

§                  Web-tier components run on the J2EE server.

§                  Business-tier components run on the J2EE server.

§                  Enterprise information system (EIS)-tier software runs on the EIS server.

2.    What is J2EE? - J2EE is an environment for developing and deploying enterprise applications. The J2EE platform consists of a set of services, application programming interfaces (APIs), and protocols that provide the functionality for developing multitiered, web-based applications.

3.    What are the components of J2EE application?
- A J2EE component is a self-contained functional software unit that is assembled into a J2EE application with its related classes and files and communicates with other components. The J2EE specification defines the following J2EE components:

A.  Application clients and applets are client components.

B.   Java Servlet and JavaServer Pages technology components are web components.

C.   Enterprise JavaBeans components (enterprise beans) are business components.

D.   Resource adapter components provided by EIS and tool vendors.

4.    What do Enterprise JavaBeans components contain? - Enterprise JavaBeans components contains Business code, which is logic
that solves or meets the needs of a particular business domain such as banking, retail, or finance, is handled by enterprise beans running in the business tier. All the business code is contained inside an Enterprise Bean which receives data from client programs, processes it (if necessary), and sends it to the enterprise information system tier for storage. An enterprise bean also retrieves data from storage, processes it (if necessary), and sends it back to the client program.

5.    Is J2EE application only a web-based? - No, It depends on type of application that client wants. A J2EE application can be web-based or non-web-based. if an application client executes on the client machine, it is a non-web-based J2EE application. The J2EE application can provide a way for users to handle tasks such as J2EE system or application administration. It typically has a graphical user interface created from Swing or AWT APIs, or a command-line interface. When user request, it can open an HTTP connection to establish communication with a servlet running in the web tier.

6.    Are JavaBeans J2EE components? - No. JavaBeans components are not considered J2EE components by the J2EE specification. They are written to manage the data flow between an application client or applet and components running on the J2EE server or between server components and a database. JavaBeans components written for the J2EE platform have instance variables and get and set methods for accessing the data in the instance variables. JavaBeans components used in this way are typically simple in design and implementation, but should conform to the naming and design conventions outlined in the JavaBeans component architecture.

7.    Is HTML page a web component? - No. Static HTML pages and applets are bundled with web components during application assembly, but are not considered web components by the J2EE specification. Even the server-side utility classes are not considered web components, either.

8.    What can be considered as a web component? - J2EE Web components can be either servlets or JSP pages. Servlets are Java programming language classes that dynamically process requests and construct responses. JSP pages are text-based documents that execute as servlets but allow a more natural approach to creating static content.

9.    What is the container? - Containers are the interface between a component and the low-level platform specific functionality that supports the component. Before a Web, enterprise bean, or application client component can be executed, it must be assembled into a J2EE application and deployed into its container.

10. What are container services? - A container is a runtime support of a system-level entity. Containers provide components with services such as lifecycle management, security, deployment, and threading.

11. What is the web container? - Servlet and JSP containers are collectively referred to as Web containers. It manages the execution of JSP page and servlet components for J2EE applications. Web components and their container run on the J2EE server.

12. What is Enterprise JavaBeans (EJB) container? - It manages the execution of enterprise beans for J2EE applications.
Enterprise beans and their container run on the J2EE server.

13. What is Applet container? - IManages the execution of applets. Consists of a Web browser and Java Plugin running on the client together.

14. How do we package J2EE components? - J2EE components are packaged separately and bundled into a J2EE application for deployment. Each component, its related files such as GIF and HTML files or server-side utility classes, and a deployment descriptor are assembled into a module and added to the J2EE application. A J2EE application is composed of one or more enterprise bean,Web, or application client component modules. The final enterprise solution can use one J2EE application or be made up of two or more J2EE applications, depending on design requirements. A J2EE application and each of its modules has its own deployment descriptor. A deployment descriptor is an XML document with an .xml extension that describes a component’s deployment settings.

15. What is a thin client? - A thin client is a lightweight interface to the application that does not have such operations like query databases, execute complex business rules, or connect to legacy applications.

16. What are types of J2EE clients? - Following are the types of J2EE clients:

§                  Applets

§                  Application clients

§                  Java Web Start-enabled rich clients, powered by Java Web Start technology.

§                  Wireless clients, based on Mobile Information Device Profile (MIDP) technology.

17. What is deployment descriptor? - A deployment descriptor is an Extensible Markup Language (XML) text-based file with an .xml extension that describes a component’s deployment settings. A J2EE application and each of its modules has its own deployment descriptor. For example, an enterprise bean module deployment descriptor declares transaction attributes and security authorizations
for an enterprise bean. Because deployment descriptor information is declarative, it can be changed without modifying the bean source code. At run time, the J2EE server reads the deployment descriptor and acts upon the component accordingly.

18. What is the EAR file? - An EAR file is a standard JAR file with an .ear extension, named from Enterprise ARchive file. A J2EE application with all of its modules is delivered in EAR file.

19. What is JTA and JTS? - JTA is the abbreviation for the Java Transaction API. JTS is the abbreviation for the Jave Transaction Service. JTA provides a standard interface and allows you to demarcate transactions in a manner that is independent of the transaction manager implementation. The J2EE SDK implements the transaction manager with JTS. But your code doesn’t call the JTS methods directly. Instead, it invokes the JTA methods, which then call the lower-level JTS routines. Therefore, JTA is a high level transaction interface that your application uses to control transaction. and JTS is a low level transaction interface and ejb uses behind the scenes (client code doesn’t directly interact with JTS. It is based on object transaction service(OTS) which is part of CORBA.

20. What is JAXP? - JAXP stands for Java API for XML. XML is a language for representing and describing text-based data which can be read and handled by any program or tool that uses XML APIs. It provides standard services to determine the type of an arbitrary piece of data, encapsulate access to it, discover the operations available on it, and create the appropriate JavaBeans component to perform those operations.

21. What is J2EE Connector? - The J2EE Connector API is used by J2EE tools vendors and system integrators to create resource adapters that support access to enterprise information systems that can be plugged into any J2EE product. Each type of database or EIS has a different resource adapter. Note: A resource adapter is a software component that allows J2EE application components to access and interact with the underlying resource manager. Because a resource adapter is specific to its resource manager, there is typically a different resource adapter for each type of database or enterprise information system.

22. What is JAAP? - The Java Authentication and Authorization Service (JAAS) provides a way for a J2EE application to authenticate and authorize a specific user or group of users to run it. It is a standard Pluggable Authentication Module (PAM) framework that extends the Java 2 platform security architecture to support user-based authorization.

23. What is Java Naming and Directory Service? - The JNDI provides naming and directory functionality. It provides applications with methods for performing standard directory operations, such as associating attributes with objects and searching for objects using their attributes. Using JNDI, a J2EE application can store and retrieve any type of named Java object. Because JNDI is independent of any specific implementations, applications can use JNDI to access multiple naming and directory services, including existing naming and
directory services such as LDAP, NDS, DNS, and NIS.

24. What is Struts? - A Web page development framework. Struts combines Java Servlets, Java Server Pages, custom tags, and message resources into a unified framework. It is a cooperative, synergistic platform, suitable for development teams, independent developers, and everyone between.

25. How is the MVC design pattern used in Struts framework? - In the MVC design pattern, application flow is mediated by a central Controller. The Controller delegates requests to an appropriate handler. The handlers are tied to a Model, and each handler acts as an adapter between the request and the Model. The Model represents, or encapsulates, an application’s business logic or state. Control is usually then forwarded back through the Controller to the appropriate View. The forwarding can be determined by consulting a set of mappings, usually loaded from a database or configuration file. This provides a loose coupling between the View and Model, which can make an application significantly easier to create and maintain. Controller: Servlet controller which supplied by Struts itself; View: what you can see on the screen, a JSP page and presentation components; Model: System state and a business logic JavaBeans.

Question: What is J2EE?
Answer: J2EE Stands for Java 2 Enterprise Edition. J2EE is an environment for developing and deploying enterprise applications. J2EE specification is defined by Sun Microsystems Inc. The J2EE platform is one of the best platform for the development and deployment of enterprise applications. The J2EE platform is consists of a set of services, application programming interfaces (APIs), and protocols, which provides the functionality necessary for developing multi-tiered, web-based applications. You can download the J2EE SDK and development tools from http://java.sun.com/.

Question: What do you understand by a J2EE module?
Answer: A J2EE module is a software unit that consists of one or more J2EE components of the same container type along with one deployment descriptor of that type. J2EE specification defines four types of modules:
a) EJB
b) Web
c) application client and
d) resource adapter
  
In the J2EE applications modules can be deployed as stand-alone units. Modules can also be assembled into J2EE applications.
   

Question: Tell me something about J2EE component?
Answer: J2EE component is a self-contained functional software unit supported by a container and configurable at deployment time. The J2EE specification defines the following J2EE components:

• Application clients and applets are components that run on the client.
• Java servlet and JavaServer Pages (JSP) technology components are Web components that run on the server.
• Enterprise JavaBeans (EJB) components (enterprise beans) are business components that run on the server. J2EE components are written in the Java programming language and are compiled in the same way as any program in the language. The difference between J2EE components and "standard" Java classes is that J2EE components are assembled into a J2EE application, verified to be well formed and in compliance with the J2EE specification, and deployed to production, where they are run and managed by the J2EE server or client container.

Source: J2EE v1.4 Glossary
    

Question: 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 descriptors
  

Question: 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.
  

Question: 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.
    

Question: 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
    

Question: 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.
  

Question: 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
   

Question: What are types of J2EE clients?
Answer: J2EE clients are the software that access the services components installed on the J2EE container. Following are the J2EE clients:
a) Applets
b) Java-Web Start clients
c) Wireless clients
d) Web applications

1. What do you understand by the term J2EE?
J2EE is a specification. provides middleware services.
EJB JDBC JNDI JMS JavaMail JTA Servlets JSPs Connectors JAAS

EJB : is Server Side component architecture for developing distributed applications.
Developers need to concentrate only on business logic.
The middleware services are provided by appserver.
(transaction management,persistence, threading,concurreny )


Session Beans : Model processes
Entity Beans : Model data
Message Driven beans : Similar to session beans. Invoked only by sending messages.

2. Diff between RMI and EJB

EJB uses RMI technology.
We have to declare the middleware services needed by the EJB in a descriptor.
The Appserver will generate stubs for the same.

RMI : one has to write the middleware services. (txn mgmt, persitence.)

EJB cannot be invoked directly by remote clients because there are not network enabled.
Location transparency.


3. How many tiers does an J2EE application have? Explain multi-tier applications?

4. What is a web component?
A web componenet is a software entity which runs on a web server, handles requests and generates dynamic web content.

5. What is a web container?
Runtime environment for servlets and jsp
Manages lifecycle of instances
Supports HTTP

6. Should web container always be a part of web server?
No.

7.What are servlets?
8. What are JSP’s?
9.What are web services?
10.Explain SOA?
11. Explain EJB containers?
12.What is the purpose of init() in servlet class?

13. Can servlets have constructors? Can they be used for initializing servlets instead of init()
Yes. But we shouldn’t. Reason being init has access to servlet config and servlet context object , constructors won’t.

14.Name JSP scripting elements?
declarations, scriptlets, expressions.

15.What is SingleThreadModel ?
16. How to implement a thread-safe JSP page? What directive should be added?

‹ %@ page isThreadSafe="false" %

17 .How can we include static files within a JSP page?
Answer: using the JSP include directive.

18.Distinguish between servletconfig and servlet context?
19.What is the difference in between doPost and doGet methods ?
20.Why does EJB needs two interface( Home and Remote Interface) ?
21.Explain deployment descriptors?
22.Why cant we invoke methods on ejbs directly ?
23.Explain clustering and fault tolerance
24.How would you go about analyzing performance of the application

Application performance testing

When performance testing a Web application, several requirements must be
determined either through interpretation of data from an existing application that
performs similar work. Those requirements are:

User base
What is the expected number of users that will access this application? This is
generally expressed in hits per month, day, hour, or minute depending on
volumes.

Total concurrent users

During a peak interval, what is the maximum possible number of users
accessing the application at the same time.

Peak request rate
How many pages will need to be served per second?

Good questions asked during Java interview

1.  Is “abc” a primitive value? - The String literal “abc” is not a primitive value. It is a String object.

2. What restrictions are placed on the values of each case of a switch statement? - During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.

3. What modifiers may be used with an interface declaration? - An interface may be declared as public or abstract.

4. Is a class a subclass of itself? - A class is a subclass of itself.

5.  What is the difference between a while statement and a do statement? - A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

6. What modifiers can be used with a local inner class? - A local inner class may be final or abstract.

7. What is the purpose of the File class? - The File class is used to create objects that provide access to the files and directories of a local file system.

8. Can an exception be rethrown? - Yes, an exception can be rethrown.

9. When does the compiler supply a default constructor for a class? - The compiler supplies a default constructor for a class if no other constructors are provided.

10.   If a method is declared as protected, where may the method be accessed? - A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

11.   Which non-Unicode letter characters may be used as the first character of an identifier? - The non-Unicode letter characters $ and _ may appear as the first character of an identifier

12.   What restrictions are placed on method overloading? - Two methods may not have the same name and argument list but different return types.

13.   What is casting? - There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

14.   What is the return type of a program’s main() method? - A program’s main() method has a void return type.

15.   What class of exceptions are generated by the Java run-time system? - The Java runtime system generates RuntimeException and Error exceptions.

16.   What class allows you to read objects directly from a stream? - The ObjectInputStream class supports the reading of objects from input streams.

17.   What is the difference between a field variable and a local variable? - A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.

18.   How are this() and super() used with constructors? - this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

19.   What is the relationship between a method’s throws clause and the exceptions that can be thrown during the method’s execution? - A method’s throws clause must declare any checked exceptions that are not caught within the body of the method.

20.  Why are the methods of the Math class static? - So they can be invoked as if they are a mathematical code library.

21.   What are the legal operands of the instanceof operator? - The left operand is an object reference or null value and the right operand is a class, interface, or array type.

22.  What an I/O filter? - An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

23.  If an object is garbage collected, can it become reachable again? - Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.

24.  What are E and PI? - E is the base of the natural logarithm and PI is mathematical value pi.

25.   Are true and false keywords? - The values true and false are not keywords.

26.  What is the difference between the File and RandomAccessFile classes? - The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

27.  What happens when you add a double value to a String? - The result is a String object.

28.  What is your platform’s default character encoding? - If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1.

29.  Which package is always imported by default? - The java.lang package is always imported by default.

30.  What interface must an object implement before it can be written to a stream as an object? - An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

31.   How can my application get to know when a HttpSession is removed? - Define a Class HttpSessionNotifier which implements HttpSessionBindingListener and implement the functionality what you need in valueUnbound() method. Create an instance of that class and put that instance in HttpSession.

32.  Whats the difference between notify() and notifyAll()? - notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them. Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the change (for example, when freeing a buffer back into a pool). notifyAll() is necessary (for correctness) if multiple threads should resume (for example, when releasing a “writer” lock on a file might permit all “readers” to resume).

33.  Why can’t I say just abs() or sin() instead of Math.abs() and Math.sin()? - The import statement does not bring methods into your local name space. It lets you abbreviate class names, but not get rid of them altogether. That’s just the way it works, you’ll get used to it. It’s really a lot safer this way.
However, there is actually a little trick you can use in some cases that gets you what you want. If your top-level class doesn’t need to inherit from anything else, make it inherit from java.lang.Math. That *does* bring all the methods into your local name space. But you can’t use this trick in an applet, because you have to inherit from java.awt.Applet. And actually, you can’t use it on java.lang.Math at all, because Math is a “final” class which means it can’t be extended.

34.  Why are there no global variables in Java? - Global variables are considered bad form for a variety of reasons: Adding state variables breaks referential transparency (you no longer can understand a statement or expression on its own: you need to understand it in the context of the settings of the global variables), State variables lessen the cohesion of a program: you need to know more to understand how something works. A major point of Object-Oriented programming is to break up global state into more easily understood collections of local state, When you add one variable, you limit the use of your program to one instance. What you thought was global, someone else might think of as local: they may want to run two copies of your program at once. For these reasons, Java decided to ban global variables.

35.   What does it mean that a class or member is final? - A final class can no longer be subclassed. Mostly this is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. Methods may be declared final as well. This means they may not be overridden in a subclass. Fields can be declared final, too. However, this has a completely different meaning. A final field cannot be changed after it’s initialized, and it must include an initializer statement where it’s declared. For example, public final double c = 2.998; It’s also possible to make a static field final to get the effect of C++’s const statement or some uses of C’s #define, e.g. public static final double c = 2.998;

36.  What does it mean that a method or class is abstract? - An abstract class cannot be instantiated. Only its subclasses can be instantiated. You indicate that a class is abstract with the abstract keyword like this:

37.            public abstract class Container extends Component {

Abstract classes may contain abstract methods. A method declared abstract is not actually implemented in the current class. It exists only to be overridden in subclasses. It has no body. For example,

public abstract float price();

Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do. Each subclass of an abstract class must override the abstract methods of its superclasses or itself be declared abstract.

38.  What is a transient variable? - transient variable is a variable that may not be serialized.

39.  How are Observer and Observable used? - Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

40.  Can a lock be acquired on a class? - Yes, a lock can be acquired on a class. This lock is acquired on the class’s Class object.

41.   What state does a thread enter when it terminates its processing? - When a thread terminates its processing, it enters the dead state.

42.  How does Java handle integer overflows and underflows? - It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

43.  What is the difference between the >> and >>> operators? - The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.

44.  Is sizeof a keyword? - The sizeof operator is not a keyword.

45.   Does garbage collection guarantee that a program will not run out of memory? - Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection

46.  Can an object’s finalize() method be invoked while it is reachable? - An object’s finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object’s finalize() method may be invoked by other objects.

47.  What value does readLine() return when it has reached the end of a file? - The readLine() method returns null when it has reached the end of a file.

48.  Can a for statement loop indefinitely? - Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;

49.  To what value is a variable of the String type automatically initialized? - The default value of an String type is null.

50.   What is a task’s priority and how is it used in scheduling? - A task’s priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.

51.   What is the range of the short type? - The range of the short type is -(2^15) to 2^15 - 1.

52.   What is the purpose of garbage collection? - The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.

53.   What do you understand by private, protected and public? - These are accessibility modifiers. Private is the most restrictive, while public is the least restrictive. There is no real difference between protected and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package.

54.   What is Downcasting ? - Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy

55.   Can a method be overloaded based on different return type but same argument type ? - No, because the methods can be called without using their return type in which case there is ambiquity for the compiler

56.   What happens to a static var that is defined within a method of a class ? - Can’t do it. You’ll get a compilation error

57.   How many static init can you have ? - As many as you want, but the static initializers and class variable initializers are executed in textual order and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope.

58.   What is the difference amongst JVM Spec, JVM Implementation, JVM Runtime ? - The JVM spec is the blueprint for the JVM generated and owned by Sun. The JVM implementation is the actual implementation of the spec by a vendor and the JVM runtime is the actual running instance of a JVM implementation

59.   Describe what happens when an object is created in Java? - Several things happen in a particular order to ensure the object is constructed properly: Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data. The instance variables of the objects are initialized to their default values. The constructor for the most derived class is invoked. The first thing a constructor does is call the consctructor for its superclasses. This process continues until the constrcutor for java.lang.Object is called, as java.lang.Object is the base class for all objects in java. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last.

60.  What does the “final” keyword mean in front of a variable? A method? A class? - FINAL for a variable: value is constant. FINAL for a method: cannot be overridden. FINAL for a class: cannot be derived

61.   What is the difference between instanceof and isInstance? - instanceof is used to check to see if an object can be cast into a specified type without throwing a cast class exception. isInstance() Determines if the specified Object is assignment-compatible with the object represented by this Class. This method is the dynamic equivalent of the Java language instanceof operator. The method returns true if the specified Object argument is non-null and can be cast to the reference type represented by this Class object without raising a ClassCastException. It returns false otherwise.

62.  Why does it take so much time to access an Applet having Swing Components the first time? - Because behind every swing component are many Java objects and resources. This takes time to create them in memory. JDK 1.3 from Sun has some improvements which may

Java interview questions and answers

1. What is garbage collection? What is the process that is responsible for doing that in java? - Reclaiming the unused memory by the invalid objects. Garbage collector is responsible for this process

2. What kind of thread is the Garbage collector thread? - It is a daemon thread.

3. What is a daemon thread? - These are the threads which can run without user intervention. The JVM can exit when there are daemon thread by killing them abruptly.

4. How will you invoke any external process in Java? - Runtime.getRuntime().exec(….)

5. What is the finalize method do? - Before the invalid objects get garbage collected, the JVM give the user a chance to clean up some resources before it got garbage collected.

6. What is mutable object and immutable object? - If a object value is changeable then we can call it as Mutable object. (Ex., StringBuffer, …) If you are not allowed to change the value of an object, it is immutable object. (Ex., String, Integer, Float, …)

7. What is the basic difference between string and stringbuffer object? - String is an immutable object. StringBuffer is a mutable object.

8. What is the purpose of Void class? - The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the primitive Java type void.

9. What is reflection? - Reflection allows programmatic access to information about the fields, methods and constructors of loaded classes, and the use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions.

10. What is the base class for Error and Exception? - Throwable

11.  What is the byte range? -128 to 127

12.  What is the implementation of destroy method in java.. is it native or java code? - This method is not implemented.

13.  What is a package? - To group set of classes into a single unit is known as packaging. Packages provides wide namespace ability.

14.  What are the approaches that you will follow for making a program very efficient? - By avoiding too much of static methods avoiding the excessive and unnecessary use of synchronized methods Selection of related classes based on the application (meaning synchronized classes for multiuser and non-synchronized classes for single user) Usage of appropriate design patterns Using cache methodologies for remote invocations Avoiding creation of variables within a loop and lot more.

15.  What is a DatabaseMetaData? - Comprehensive information about the database as a whole.

16.  What is Locale? - A Locale object represents a specific geographical, political, or cultural region

17.  How will you load a specific locale? - Using ResourceBundle.getBundle(…);

18.  What is JIT and its use? - Really, just a very fast compiler… In this incarnation, pretty much a one-pass compiler — no offline computations. So you can’t look at the whole method, rank the expressions according to which ones are re-used the most, and then generate code. In theory terms, it’s an on-line problem.

19.  Is JVM a compiler or an interpreter? - Interpreter

20. When you think about optimization, what is the best way to findout the time/memory consuming process? - Using profiler

21.  What is the purpose of assert keyword used in JDK1.4.x? - In order to validate certain expressions. It effectively replaces the if block and automatically throws the AssertionError on failure. This keyword should be used for the critical arguments. Meaning, without that the method does nothing.

22. How will you get the platform dependent values like line separator, path separator, etc., ? - Using Sytem.getProperty(…) (line.separator, path.separator, …)

23. What is skeleton and stub? what is the purpose of those? - Stub is a client side representation of the server, which takes care of communicating with the remote server. Skeleton is the server side representation. But that is no more in use… it is deprecated long before in JDK.

24. What is the final keyword denotes? - final keyword denotes that it is the final implementation for that method or variable or class. You can’t override that method/variable/class any more.

25. What is the significance of ListIterator? - You can iterate back and forth.

26. What is the major difference between LinkedList and ArrayList? - LinkedList are meant for sequential accessing. ArrayList are meant for random accessing.

27.  What is nested class? - If all the methods of a inner class is static then it is a nested class.

28. What is inner class? - If the methods of the inner class can only be accessed via the instance of the inner class, then it is called inner class.

29. What is composition? - Holding the reference of the other class within some other class is known as composition.

30. What is aggregation? - It is a special type of composition. If you expose all the methods of a composite class and route the method call to the composite method through its reference, then it is called aggregation.

31.  What are the methods in Object? - clone, equals, wait, finalize, getClass, hashCode, notify, notifyAll, toString

32. Can you instantiate the Math class? - You can’t instantiate the math class. All the methods in this class are static. And the constructor is not public.

33. What is singleton? - It is one of the design pattern. This falls in the creational pattern of the design pattern. There will be only one instance for that entire JVM. You can achieve this by having the private constructor in the class. For eg., public class Singleton { private static final Singleton s = new Singleton(); private Singleton() { } public static Singleton getInstance() { return s; } // all non static methods … }

34. What is DriverManager? - The basic service to manage set of JDBC drivers.

35. What is Class.forName() does and how it is useful? - It loads the class into the ClassLoader. It returns the Class. Using that you can get the instance ( “class-instance”.newInstance() ).

36. Inq adds a question: Expain the reason for each keyword of

public static void main(String args[])

J2EE interview questions and answers

  What makes J2EE suitable for distributed multitiered Applications?
- The J2EE platform uses a multitiered distributed application model. Application logic is divided into components according to function, and the various application components that make up a J2EE application are installed on different machines depending on the tier in the multitiered J2EE environment to which the application component belongs. The J2EE application parts are:

§                Client-tier components run on the client machine.

§                Web-tier components run on the J2EE server.

§                Business-tier components run on the J2EE server.

§                Enterprise information system (EIS)-tier software runs on the EIS server.

2. What is J2EE? - J2EE is an environment for developing and deploying enterprise applications. The J2EE platform consists of a set of services, application programming interfaces (APIs), and protocols that provide the functionality for developing multitiered, web-based applications.

3. What are the components of J2EE application?
- A J2EE component is a self-contained functional software unit that is assembled into a J2EE application with its related classes and files and communicates with other components. The J2EE specification defines the following J2EE components:

A.     Application clients and applets are client components.

B.     Java Servlet and JavaServer Pages technology components are web components.

C.     Enterprise JavaBeans components (enterprise beans) are business components.

D.     Resource adapter components provided by EIS and tool vendors.

4. What do Enterprise JavaBeans components contain? - Enterprise JavaBeans components contains Business code, which is logic
that solves or meets the needs of a particular business domain such as banking, retail, or finance, is handled by enterprise beans running in the business tier. All the business code is contained inside an Enterprise Bean which receives data from client programs, processes it (if necessary), and sends it to the enterprise information system tier for storage. An enterprise bean also retrieves data from storage, processes it (if necessary), and sends it back to the client program.

5. Is J2EE application only a web-based? - No, It depends on type of application that client wants. A J2EE application can be web-based or non-web-based. if an application client executes on the client machine, it is a non-web-based J2EE application. The J2EE application can provide a way for users to handle tasks such as J2EE system or application administration. It typically has a graphical user interface created from Swing or AWT APIs, or a command-line interface. When user request, it can open an HTTP connection to establish communication with a servlet running in the web tier.

6. Are JavaBeans J2EE components? - No. JavaBeans components are not considered J2EE components by the J2EE specification. They are written to manage the data flow between an application client or applet and components running on the J2EE server or between server components and a database. JavaBeans components written for the J2EE platform have instance variables and get and set methods for accessing the data in the instance variables. JavaBeans components used in this way are typically simple in design and implementation, but should conform to the naming and design conventions outlined in the JavaBeans component architecture.

7. Is HTML page a web component? - No. Static HTML pages and applets are bundled with web components during application assembly, but are not considered web components by the J2EE specification. Even the server-side utility classes are not considered web components, either.

8. What can be considered as a web component? - J2EE Web components can be either servlets or JSP pages. Servlets are Java programming language classes that dynamically process requests and construct responses. JSP pages are text-based documents that execute as servlets but allow a more natural approach to creating static content.

9. What is the container? - Containers are the interface between a component and the low-level platform specific functionality that supports the component. Before a Web, enterprise bean, or application client component can be executed, it must be assembled into a J2EE application and deployed into its container.

10. What are container services? - A container is a runtime support of a system-level entity. Containers provide components with services such as lifecycle management, security, deployment, and threading.

11.  What is the web container? - Servlet and JSP containers are collectively referred to as Web containers. It manages the execution of JSP page and servlet components for J2EE applications. Web components and their container run on the J2EE server.

12.  What is Enterprise JavaBeans (EJB) container? - It manages the execution of enterprise beans for J2EE applications.
Enterprise beans and their container run on the J2EE server.

13.  What is Applet container? - IManages the execution of applets. Consists of a Web browser and Java Plugin running on the client together.

14.  How do we package J2EE components? - J2EE components are packaged separately and bundled into a J2EE application for deployment. Each component, its related files such as GIF and HTML files or server-side utility classes, and a deployment descriptor are assembled into a module and added to the J2EE application. A J2EE application is composed of one or more enterprise bean,Web, or application client component modules. The final enterprise solution can use one J2EE application or be made up of two or more J2EE applications, depending on design requirements. A J2EE application and each of its modules has its own deployment descriptor. A deployment descriptor is an XML document with an .xml extension that describes a component’s deployment settings.

15.  What is a thin client? - A thin client is a lightweight interface to the application that does not have such operations like query databases, execute complex business rules, or connect to legacy applications.

16.  What are types of J2EE clients? - Following are the types of J2EE clients:

§                Applets

§                Application clients

§                Java Web Start-enabled rich clients, powered by Java Web Start technology.

§                Wireless clients, based on Mobile Information Device Profile (MIDP) technology.

17.  What is deployment descriptor? - A deployment descriptor is an Extensible Markup Language (XML) text-based file with an .xml extension that describes a component’s deployment settings. A J2EE application and each of its modules has its own deployment descriptor. For example, an enterprise bean module deployment descriptor declares transaction attributes and security authorizations
for an enterprise bean. Because deployment descriptor information is declarative, it can be changed without modifying the bean source code. At run time, the J2EE server reads the deployment descriptor and acts upon the component accordingly.

18.  What is the EAR file? - An EAR file is a standard JAR file with an .ear extension, named from Enterprise ARchive file. A J2EE application with all of its modules is delivered in EAR file.

19.  What is JTA and JTS? - JTA is the abbreviation for the Java Transaction API. JTS is the abbreviation for the Jave Transaction Service. JTA provides a standard interface and allows you to demarcate transactions in a manner that is independent of the transaction manager implementation. The J2EE SDK implements the transaction manager with JTS. But your code doesn’t call the JTS methods directly. Instead, it invokes the JTA methods, which then call the lower-level JTS routines. Therefore, JTA is a high level transaction interface that your application uses to control transaction. and JTS is a low level transaction interface and ejb uses behind the scenes (client code doesn’t directly interact with JTS. It is based on object transaction service(OTS) which is part of CORBA.

20. What is JAXP? - JAXP stands for Java API for XML. XML is a language for representing and describing text-based data which can be read and handled by any program or tool that uses XML APIs. It provides standard services to determine the type of an arbitrary piece of data, encapsulate access to it, discover the operations available on it, and create the appropriate JavaBeans component to perform those operations.

21.  What is J2EE Connector? - The J2EE Connector API is used by J2EE tools vendors and system integrators to create resource adapters that support access to enterprise information systems that can be plugged into any J2EE product. Each type of database or EIS has a different resource adapter. Note: A resource adapter is a software component that allows J2EE application components to access and interact with the underlying resource manager. Because a resource adapter is specific to its resource manager, there is typically a different resource adapter for each type of database or enterprise information system.

22. What is JAAP? - The Java Authentication and Authorization Service (JAAS) provides a way for a J2EE application to authenticate and authorize a specific user or group of users to run it. It is a standard Pluggable Authentication Module (PAM) framework that extends the Java 2 platform security architecture to support user-based authorization.

23. What is Java Naming and Directory Service? - The JNDI provides naming and directory functionality. It provides applications with methods for performing standard directory operations, such as associating attributes with objects and searching for objects using their attributes. Using JNDI, a J2EE application can store and retrieve any type of named Java object. Because JNDI is independent of any specific implementations, applications can use JNDI to access multiple naming and directory services, including existing naming and
directory services such as LDAP, NDS, DNS, and NIS.

24. What is Struts? - A Web page development framework. Struts combines Java Servlets, Java Server Pages, custom tags, and message resources into a unified framework. It is a cooperative, synergistic platform, suitable for development teams, independent developers, and everyone between.

25. How is the MVC design pattern used in Struts framework? - In the MVC design pattern, application flow is mediated by a central Controller. The Controller delegates requests to an appropriate handler. The handlers are tied to a Model, and each handler acts as an adapter between the request and the Model. The Model represents, or encapsulates, an application’s business logic or state. Control is usually then forwarded back through the Controller to the appropriate View. The forwarding can be determined by consulting a set of mappings, usually loaded from a database or configuration file. This provides a loose coupling between the View and Model, which can make an application significantly easier to create and maintain. Controller: Servlet controller which supplied by Struts itself; View: what you can see on the screen, a JSP page and presentation components; Model: System state and a business logic JavaBeans.