Wednesday, 27 June 2012

J2EE interview questions and answers

  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:
    1. Application clients and applets are client components.
    2. Java Servlet and JavaServer Pages technology components are web components.
    3. Enterprise JavaBeans components (enterprise beans) are business components.
    4. 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.

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:
     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.
  37. What is a transient variable? - transient variable is a variable that may not be serialized.
  38. 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.
  39. 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.
  40. What state does a thread enter when it terminates its processing? - When a thread terminates its processing, it enters the dead state.
  41. 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.
  42. 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.
  43. Is sizeof a keyword? - The sizeof operator is not a keyword.
  44. 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
  45. 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.
  46. 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.
  47. Can a for statement loop indefinitely? - Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;
  48. To what value is a variable of the String type automatically initialized? - The default value of an String type is null.
  49. 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.
  50. What is the range of the short type? - The range of the short type is -(2^15) to 2^15 - 1.
  51. 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.
  52. 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.
  53. What is Downcasting ? - Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy
  54. 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
  55. 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
  56. 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.
  57. 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
  58. 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.
  59. 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
  60. 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.
  61. 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 lead to faster execution of Swing applications.

JDBC and JSP interview questions and answers

  1. What is the query used to display all tables names in SQL Server (Query analyzer)?
     select * from information_schema.tables
  2. How many types of JDBC Drivers are present and what are they?- There are 4 types of JDBC Drivers
    • JDBC-ODBC Bridge Driver
    • Native API Partly Java Driver
    • Network protocol Driver
    • JDBC Net pure Java Driver
  3. Can we implement an interface in a JSP?- No
  4. What is the difference between ServletContext and PageContext?- ServletContext: Gives the information about the container. PageContext: Gives the information about the Request
  5. What is the difference in using request.getRequestDispatcher() and context.getRequestDispatcher()?- request.getRequestDispatcher(path): In order to create it we need to give the relative path of the resource, context.getRequestDispatcher(path): In order to create it we need to give the absolutepath of the resource.
  6. How to pass information from JSP to included JSP?- Using <%jsp:param> tag.
  7. What is the difference between directive include and jsp include?- <%@ include>: Used to include static resources during translation time. JSP include: Used to include dynamic content or static content during runtime.
  8. What is the difference between RequestDispatcher and sendRedirect?- RequestDispatcher: server-side redirect with request and response objects. sendRedirect : Client-side redirect with new request and response objects.
  9. How does JSP handle runtime exceptions?- Using errorPage attribute of page directive and also we need to specify isErrorPage=true if the current page is intended to URL redirecting of a JSP.
  10. How do you delete a Cookie within a JSP?
     Cookie mycook = new Cookie("name","value");
     Cookie killmycook = new Cookie("mycook","value");
  11. How do I mix JSP and SSI #include?- If you’re just including raw HTML, use the #include directive as usual inside your .jsp file.
     <!--#include file=""-->
    But it’s a little trickier if you want the server to evaluate any JSP code that’s inside the included file. If your file contains jsp code you will have to use
     <%@ vinclude="" %>
    The <!–#include file=""–> is used for including non-JSP files.
  12. I made my class Cloneable but I still get Can’t access protected method clone. Why?- Some of the Java books imply that all you have to do in order to have your class support clone() is implement the Cloneable interface. Not so. Perhaps that was the intent at some point, but that’s not the way it works currently. As it stands, you have to implement your own public clone() method, even if it doesn’t do anything special and just calls super.clone().
  13. Why is XML such an important development?- It removes two constraints which were holding back Web developments: dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for; the complexity of full SGML, whose syntax allows many powerful but hard-to-program options. XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.
  14. What is the fastest type of JDBC driver?- JDBC driver performance will depend on a number of issues:
    • the quality of the driver code,
    • the size of the driver code,
    • the database server and its load,
    • network topology,
    • the number of times your request is translated to a different API.
    In general, all things being equal, you can assume that the more your request and response change hands, the slower it will be. This means that Type 1 and Type 3 drivers will be slower than Type 2 drivers (the database calls are make at least three translations versus two), and Type 4 drivers are the fastest (only one translation).
  15. How do I find whether a parameter exists in the request object?
    boolean hasFoo = !(request.getParameter("foo") == null
     || request.getParameter("foo").equals(""));
    boolean hasParameter =
     request.getParameterMap().contains(theParameter); //(which works in Servlet 2.3+)
  16. How can I send user authentication information while makingURLConnection?- You’ll want to use HttpURLConnection.setRequestProperty and set all the appropriate headers to HTTP authorization.

Advanced EJB interview questions

  1. Are enterprise beans allowed to use Thread.sleep()? - Enterprise beans make use of the services provided by the EJB container, such as life-cycle management. To avoid conflicts with these services, enterprise beans are restricted from performing certain operations: Managing or synchronizing threads
  2. Is it possible to write two EJB’s that share the same Remote and Home interfaces, and have different bean classes? if so, what are the advantages/disadvantages? - It’s certainly possible. In fact, there’s an example that ships with the Inprise Application Server of an Account interface with separate implementations for CheckingAccount and SavingsAccount, one of which was CMP and one of which was BMP.
  3. Is it possible to specify multiple JNDI names when deploying an EJB? - No. To achieve this you have to deploy your EJB multiple times each specifying a different JNDI name.
  4. Is there any way to force an Entity Bean to store itself to the db? I don’t wanna wait for the container to update the db, I want to do it NOW! Is it possible? - Specify the transaction attribute of the bean as RequiresNew. Then as per section of the EJB v 1.1 spec EJB container automatically starts a new transaction before the method call. The container also performs the commit protocol before the method result is sent to the client.
  5. I am developing a BMP Entity bean. I have noticed that whenever the create method is invoked, the ejbLoad() and the ejbStore() methods are also invoked. I feel that once my database insert is done, having to do a select and update SQL queries is major overhead. is this behavior typical of all EJB containers? Is there any way to suppress these invocations? - This is the default behaviour for EJB. The specification states that ejbLoad() will be called before every transaction and ejbStore() after every transaction. Each Vendor has optimizations, which are proprietary for this scenario.
  6. Can an EJB send asynchronous notifications to its clients? - Asynchronous notification is a known hole in the first versions of the EJB spec. The recommended solution to this is to use JMS, which is becoming available in J2EE-compliant servers. The other option, of course, is to use client-side threads and polling. This is not an ideal solution, but it’s workable for many scenarios.
  7. How can I access EJB from ASP? - You can use the Java 2 Platform, Enterprise Edition Client Access Services (J2EETM CAS) COM Bridge 1.0, currently downloadable from Sun
  8. Is there a guarantee of uniqueness for entity beans? - There is no such guarantee. The server (or servers) can instantiate as many instances of the same underlying Entity Bean (with the same PK) as it wants. However, each instance is guaranteed to have up-to-date data values, and be transactionally consistent, so uniqueness is not required. This allows the server to scale the system to support multiple threads, multiple concurrent requests, and multiple hosts.
  9. How do the six transaction attributes map to isolation levels like “dirty read”? Will an attribute like “Required” lock out other readers until I’m finished updating? - The Transaction Attributes in EJB do not map to the Transaction Isolation levels used in JDBC. This is a common misconception. Transaction Attributes specify to the container when a Transaction should be started, suspended(paused) and committed between method invocations on Enterprise JavaBeans. For more details and a summary of Transaction Attributes refer to section 11.6 of the EJB 1.1 specification.
  10. I have created a remote reference to an EJB in FirstServlet. Can I put the reference in a servlet session and use that in SecondServlet? - Yes. The EJB client (in this case your servlet) acquires a remote reference to an EJB from the Home Interface; that reference is serializable and can be passed from servlet to servlet. If it is a session bean, then the EJB server will consider your web client’s servlet session to correspond to a single EJB session, which is usually (but not always) what you want.
  11. Can the primary key in the entity bean be a Java primitive type such as int?- The primary key can’t be a primitive type–use the primitive wrapper classes, instead. For example, you can use java.lang.Integer as the primary key class, but not int (it has to be a class, not a primitive)
  12. What’s new in the EJB 2.0 specification? - Following are the main features supported in EJB 2.0: Integration of EJB with JMS, Message Driven Beans, Implement additional Business methods in Home interface which are not specific for bean instance, EJB QL.
  13. How many types of protocol implementations does RMI have? - RMI has at least three protocol implementations: Java Remote Method Protocol(JRMP), Internet Inter ORB Protocol(IIOP), and Jini Extensible Remote Invocation(JERI). These are alternatives, not part of the same thing, All three are indeed layer 6 protocols for those who are still speaking OSI reference model.
  14. What is the need of Remote and Home interfaces. Why can’t there be one? - In a few words, I would say that the main reason is because there is a clear division of roles and responsabilities between the two interfaces. The home interface is your way to communicate with the container, that is who is responsable of creating, locating even removing one or more beans. The remote interface is your link to the bean, that will allow you to remotely access to all its methods and members. As you can see there are two distinct elements (the container and the beans) and you need two different interfaces for accessing to both of them.
  15. What is the difference between Java Beans and EJB? - Java Beans are client-side objects and EJBs are server side object, and they have completely different development, lifecycle, purpose.
  16. Question With regard to Entity Beans, what happens if both my EJB Server and Database crash, what will happen to unsaved changes? Is there any transactional log file used? - Actually, if your EJB server crashes, you will not even be able to make a connection to the server to perform a bean lookup, as the server will no longer be listening on the port for incoming JNDI lookup requests. You will lose any data that wasn’t committed prior to the crash. This is where you should start looking into clustering your EJB server. Any unsaved and uncommited changes are lost the moment your EJB Server crashes. If your database also crashes, then all the saved changes are also lost unless you have some backup or some recovery mechanism to retrieve the data. So consider database replication and EJB Clustering for such scenarios, though the occurence of such a thing is very very rare. Thx, Uma All databse have the concept of log files(for exampe oracle have redo log files concept). So if data bases crashes then on starting up they fill look up the log files to perform all pending jobs. But is EJB crashes, It depend upon the container how frequenlty it passivates or how frequesntly it refreshes the data with Database.
  17. Question Can you control when passivation occurs? - The developer, according to the specification, cannot directly control when passivation occurs. Although for Stateful Session Beans, the container cannot passivate an instance that is inside a transaction. So using transactions can be a a strategy to control passivation. The ejbPassivate() method is called during passivation, so the developer has control over what to do during this exercise and can implement the require optimized logic. Some EJB containers, such as BEA WebLogic, provide the ability to tune the container to minimize passivation calls. Taken from the WebLogic 6.0 DTD - “The passivation-strategy can be either “default” or “transaction”. With the default setting the container will attempt to keep a working set of beans in the cache. With the “transaction” setting, the container will passivate the bean after every transaction (or method call for a non-transactional invocation).”
  18. Does RMI-IIOP support dynamic downloading of classes? - No, RMI-IIOP doesn’t support dynamic downloading of the classes as it is done with CORBA in DII (Dynamic Interface Invocation).Actually RMI-IIOP combines the usability of Java Remote Method Invocation (RMI) with the interoperability of the Internet Inter-ORB Protocol (IIOP).So in order to attain this interoperability between RMI and CORBA,some of the features that are supported by RMI but not CORBA and vice versa are eliminated from the RMI-IIOP specification.
  19. Does EJB 1.1 support mandate the support for RMI-IIOP ? What is the meaning of “the client API must support the Java RMI-IIOP programming model for portability, but the underlying protocol can be anything” ? - EJB1.1 does mandate the support of RMI-IIOP. There are 2 types of implementations that an EJB Server might provide: CORBA-based EJB Servers and Proprietry EJB Servers. Both support the RMI-IIOP API but how that API is implemented is a different story. (NB: By API we mean the interface provided to the client by the stub or proxy). A CORBA-based EJB Server actually implements its EJB Objects as CORBA Objects (it therefore encorporates an ORB and this means that EJB’s can be contacted by CORBA clients (as well as RMI-IIOP clients) A proprietry EJB still implements the RMI-IIOP API (in the client’s stub) but the underlying protocol can be anything. Therefore your EJB’s CANNOT be contacted by CORBA clients. The difference is that in both cases, your clients see the same API (hence, your client portability) BUT how the stubs communicate with the server is different.
  20. The EJB specification says that we cannot use Bean Managed Transaction in Entity Beans. Why? - The short, practical answer is… because it makes your entity beans useless as a reusable component. Also, transaction management is best left to the application server - that’s what they’re there for. It’s all about atomic operations on your data. If an operation updates more than one entity then you want the whole thing to succeed or the whole thing to fail, nothing in between. If you put commits in the entity beans then it’s very difficult to rollback if an error occurs at some point late in the operation.
  21. Can I invoke Runtime.gc() in an EJB? - You shouldn’t. What will happen depends on the implementation, but the call will most likely be ignored. You should leave system level management like garbage collection for the container to deal with. After all, that’s part of the benefit of using EJBs, you don’t have to manage resources yourself.
  22. What is clustering? What are the different algorithms used for clustering? - Clustering is grouping machines together to transparantly provide enterprise services.The client does not now the difference between approaching one server or approaching a cluster of servers.Clusters provide two benefits: scalability and high availability. Further information can be found in the JavaWorld article J2EE Clustering.
  23. What is the advantage of using Entity bean for database operations, over directly using JDBC API to do database operations? When would I use one over the other? - Entity Beans actually represents the data in a database. It is not that Entity Beans replaces JDBC API. There are two types of Entity Beans Container Managed and Bean Mananged. In Container Managed Entity Bean - Whenever the instance of the bean is created the container automatically retrieves the data from the DB/Persistance storage and assigns to the object variables in bean for user to manipulate or use them. For this the developer needs to map the fields in the database to the variables in deployment descriptor files (which varies for each vendor). In the Bean Managed Entity Bean - The developer has to specifically make connection, retrive values, assign them to the objects in the ejbLoad() which will be called by the container when it instatiates a bean object. Similarly in the ejbStore() the container saves the object values back the the persistance storage. ejbLoad and ejbStore are callback methods and can be only invoked by the container. Apart from this, when you use Entity beans you dont need to worry about database transaction handling, database connection pooling etc. which are taken care by the ejb container. But in case of JDBC you have to explicitly do the above features. what suresh told is exactly perfect. ofcourse, this comes under the database transations, but i want to add this. the great thing about the entity beans of container managed, whenever the connection is failed during the transaction processing, the database consistancy is mantained automatically. the container writes the data stored at persistant storage of the entity beans to the database again to provide the database consistancy. where as in jdbc api, we, developers has to do manually.
  24. What is the role of serialization in EJB? - A big part of EJB is that it is a framework for underlying RMI: remote method invocation. You’re invoking methods remotely from JVM space ‘A’ on objects which are in JVM space ‘B’ — possibly running on another machine on the network. To make this happen, all arguments of each method call must have their current state plucked out of JVM ‘A’ memory, flattened into a byte stream which can be sent over a TCP/IP network connection, and then deserialized for reincarnation on the other end in JVM ‘B’ where the actual method call takes place. If the method has a return value, it is serialized up for streaming back to JVM A. Thus the requirement that all EJB methods arguments and return values must be serializable. The easiest way to do this is to make sure all your classes implement
  25. What is EJB QL? - EJB QL is a Query Language provided for navigation across a network of enterprise beans and dependent objects defined by means of container managed persistence. EJB QL is introduced in the EJB 2.0 specification. The EJB QL query language defines finder methods for entity beans with container managed persistenceand is portable across containers and persistence managers. EJB QL is used for queries of two types of finder methods: Finder methods that are defined in the home interface of an entity bean and which return entity objects. Select methods, which are not exposed to the client, but which are used by the Bean Provider to select persistent values that are maintained by the Persistence Manager or to select entity objects that are related to the entity bean on which the query is defined.

Junior Java programmer interview questions

  1. What is the purpose of finalization? - The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
  2. What is the difference between the Boolean & operator and the && operator?- If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.
  3. How many times may an object’s finalize() method be invoked by the garbage collector? - An object’s finalize() method may only be invoked once by the garbage collector.
  4. What is the purpose of the finally clause of a try-catch-finally statement? - The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.
  5. What is the argument type of a program’s main() method? - A program’s main() method takes an argument of the String[] type.
  6. Which Java operator is right associative? - The = operator is right associative.
  7. Can a double value be cast to a byte? - Yes, a double value can be cast to a byte.
  8. What is the difference between a break statement and a continue statement?- A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.
  9. What must a class do to implement an interface? - It must provide all of the methods in the interface and identify the interface in its implements clause.
  10. What is the advantage of the event-delegation model over the earlier event-inheritance model? - The event-delegation model has two advantages over the event-inheritance model. First, it enables event handling to be handled by objects other than the ones that generate the events (or their containers). This allows a clean separation between a component’s design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the event-inheritance model.
  11. How are commas used in the intialization and iteration parts of a for statement? - Commas are used to separate multiple statements within the initialization and iteration parts of a for statement.
  12. What is an abstract method? - An abstract method is a method whose implementation is deferred to a subclass.
  13. What value does read() return when it has reached the end of a file? - The read() method returns -1 when it has reached the end of a file.
  14. Can a Byte object be cast to a double value? - No, an object cannot be cast to a primitive value.
  15. What is the difference between a static and a non-static inner class? - A non-static inner class may have object instances that are associated with instances of the class’s outer class. A static inner class does not have any object instances.
  16. If a variable is declared as private, where may the variable be accessed? - A private variable may only be accessed within the class in which it is declared.
  17. What is an object’s lock and which object’s have locks? - An object’s lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object’s lock. All objects and classes have locks. A class’s lock is acquired on the class’s Class object.
  18. What is the % operator? - It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.
  19. When can an object reference be cast to an interface reference? - An object reference be cast to an interface reference when the object implements the referenced interface.
  20. Which class is extended by all other classes? - The Object class is extended by all other classes.
  21. Can an object be garbage collected while it is still reachable? - A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected.
  22. Is the ternary operator written x : y ? z or x ? y : z ? - It is written x ? y : z.
  23. How is rounding performed under integer division? - The fractional part of the result is truncated. This is known as rounding toward zero.
  24. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy? - The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.
  25. What classes of exceptions may be caught by a catch clause? - A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.
  26. If a class is declared without any access modifiers, where may the class be accessed? - A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.
  27. Does a class inherit the constructors of its superclass? - A class does not inherit constructors from any of its superclasses.
  28. What is the purpose of the System class? - The purpose of the System class is to provide access to system resources.
  29. Name the eight primitive Java types. - The eight primitive types are byte, char, short, int, long, float, double, and boolean.
  30. Which class should you use to obtain design information about an object? - The Class class is used to obtain information about an object’s design.