Java Interview Questions

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Java Interview Questions

  1. 1. 1. ::::::::J2EE Interview Questions::::::: 2. 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/. 3. 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. 4. 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: 5. Application clients and applets are components that run on the client. 6. Java servlet and JavaServer Pages (JSP) technology components are Web components that run on the server. 7. Enterprise JavaBeans (EJB) components (enterprise beans) are business components that run on the server. 8. 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 quot;standardquot; 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. 9. 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 10. 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. 11. 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. 12. 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:
  2. 2. 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 13. 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. 14. 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 15. 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 16. J2EE Containers 17. Normally, thin-client multitiered 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. 18. Container Services 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. The assembly process involves specifying container settings for each component in the J2EE application and for the J2EE application itself. Container settings customize the underlying support provided by the J2EE server, which includes services such as security, transaction management, Java Naming and Directory Interface (JNDI) lookups, and remote connectivity. Here are some of the highlights: The J2EE security model lets you configure a Web component or enterprise bean so that system resources are accessed only by authorized users. The J2EE transaction model lets you specify relationships among methods that make up a single transaction so that all methods in one transaction are treated as a single unit. JNDI lookup services provide a unified interface to multiple naming and directory services in the enterprise so that application components can access naming and directory services. The J2EE remote connectivity model manages low-level communications between clients and enterprise beans. After an enterprise bean is created, a client invokes methods on it as if it were in the same virtual machine.
  3. 3. The fact that the J2EE architecture provides configurable services means that application components within the same J2EE application can behave differently based on where they are deployed. For example, an enterprise bean can have security settings that allow it a certain level of access to database data in one production environment and another level of database access in another production environment. The container also manages nonconfigurable services such as enterprise bean and servlet life cycles, database connection resource pooling, data persistence, and access to the J2EE platform APIs described in the section J2EE APIs. Although data persistence is a nonconfigurable service, the J2EE architecture lets you override container-managed persistence by including the appropriate code in your enterprise bean implementation when you want more control than the default container-managed persistence provides. For example, you might use bean-managed persistence to implement your own finder (search) methods or to create a customized database cache. 19. Container Types The deployment process installs J2EE application components in the J2EE containers illustrated in Figure 1-5. 20. Figure 1-5 J2EE Server and Containers J2EE server The runtime portion of a J2EE product. A J2EE server provides EJB and Web containers. Enterprise JavaBeans (EJB) container Manages the execution of enterprise beans for J2EE applications. Enterprise beans and their container run on the J2EE server. Web container Manages the execution of JSP page and servlet components for J2EE applications. Web components and their container run on the J2EE server. Application client container Manages the execution of application client components. Application clients and their container run on the client. Applet container Manages the execution of applets. Consists of a Web browser and Java Plug-in running on the client together. 21. Question: What is Deployment Descriptor? Answer: A deployment descriptor is simply an XML(Extensible Markup Language) file with the extension of .xml. Deployment descriptor describes the component deployment settings.
  4. 4. Application servers reads the deployment descriptor to deploy the components contained in the deployment unit. For example ejb-jar.xml file is used to describe the setting of the EJBs. 22. Question: What do you understand by JTA and JTS? Answer: JTA stands for Java Transaction API and JTS stands for Java Transaction Service. JTA provides a standard interface which allows the developers to demarcate transactions in a manner that is independent of the transaction manager implementation. The J2EE SDK uses the JTA transaction manager to implement the transaction. The code developed by developers does not calls the JTS methods directly, but only invokes the JTA methods. Then JTA internally invokes the JTS routines. JTA is a high level transaction interface used by the application code to control the transaction. 23. Question: What is JAXP? Answer: The Java API for XML Processing (JAXP) enables applications to parse and transform XML documents independent of a particular XML processing implementation. JAXP or Java API for XML Parsing is an optional API provided by Javasoft. It provides basic functionality for reading, manipulating, and generating XML documents through pure Java APIs. It is a thin and lightweight API that provides a standard way to seamlessly integrate any XML-compliant parser with a Java application. More at http://java.sun.com/xml/ 24. Question: What is J2EE Connector architecture? Answer: J2EE Connector Architecture (JCA) is a Java-based technology solution for connecting application servers and enterprise information systems (EIS) as part of enterprise application integration (EAI) solutions. While JDBC is specifically used to connect Java EE applications to databases, JCA is a more generic architecture for connection to legacy systems (including databases). JCA was developed under the Java Community Process as JSR 16 (JCA 1.0) and JSR 112 (JCA 1.5). As of 2006, the current version of JCA is version 1.5. The J2EE Connector API is used by J2EE tools developers and system integrators to create resource adapters. Home page for J2EE Connector architecture http://java.sun.com/j2ee/connector/. 25. Question: What is difference between Java Bean and Enterprise Java Bean? Answer: Java Bean as is a plain java class with member variables and getter setter methods. Java Beans are defined under JavaBeans specification as Java-Based software component model which includes the features like introspection, customization, events, properties and persistence. Enterprise JavaBeans or EJBs for short are Java-based software components that comply with Java's EJB specification. EJBs are delpoyed on the EJB container and executes in the EJB container. EJB is not that simple, it is used for building distributed applications. Examples of EJB are Session Bean, Entity Bean and Message Driven Bean. EJB is used for server side programming whereas java bean is a client side. Bean is only development but the EJB is developed and then deploy on EJB Container. 26. Question: What is the difference between JTS and JTA? Answer: In any J2EE application transaction management is one of the most crucial requirements of the application. Given the complexity of today's business requirements, transaction processing occupies one of the most complex segments of enterprise level distributed applications to build, deploy and maintain. JTS specifies the implementation of a Java transaction manager. JTS specifies the implementation of a Transaction Manager which supports the Java Transaction API (JTA) 1.0 This transaction manager supports the JTA, using which application servers can be built to support transactional Java applications. Internally the JTS implements the Java mapping of the OMG OTS 1.1 specifications. The Java mapping is specified in two packages: org.omg.CosTransactions and org.omg.CosTSPortability. The JTS thus provides a new architecture for transactional application servers and applications, while complying to the OMG OTS 1.1 interfaces internally. This allows the JTA compliant applications to interoperate with other OTS 1.1 complaint applications through the standard IIOP. Java-based applications and Java- based application servers access transaction management functionality via the JTA interfaces. The JTA interacts with a transaction management implementation via JTS. Similarly, the JTS can access resources via the JTA XA interfaces or can access OTS-enabled non-XA resources. JTS implementations can interoperate via CORBA OTS interfaces. The JTA specifies an architecture for building transactional application servers and defines a set of interfaces for various components of this architecture. The components are: the application, resource managers, and the application server. The JTA specifies standard interfaces for Java- based applications and application servers to interact with transactions, transaction managers, and resource managers JTA transaction management provides a set of interfaces utilized by an application server to manage the beginning and completion of transactions. Transaction synchronization and propagation services are also provided under the domain of transaction management. In the Java transaction model, the Java application components can conduct transactional operations on JTA compliant resources via the JTS. The JTS acts as a layer over the OTS. The applications can therefore initiate global transactions to include other OTS transaction managers,
  5. 5. or participate in global transactions initiated by other OTS compliant transaction managers. 27. Question: Can Entity Beans have no create() methods? Answer: Entity Beans can have no create() methods. Entity Beans have no create() method, when entity bean is not used to store the data in the database. In this case entity bean is used to retrieve the data from database. 28. Question: What are the call back methods in Session bean? Answer: Callback methods are called by the container to notify the important events to the beans in its life cycle. The callback methods are defined in the javax.ejb.EntityBean interface.The callback methods example are ejbCreate(), ejbPassivate(), and ejbActivate(). 29. Question: What is bean managed transaction? Answer: In EJB transactions can be maintained by the container or developer can write own code to maintain the transaction. If a developer doesn’t want a Container to manage transactions, developer can write own code to maintain the database transaction. 30. Question: What are transaction isolation levels in EJB? Answer: Thre are four levels of transaction isolation are: * Uncommitted Read * Committed Read * Repeatable Read * Serializable The four transaction isolation levels and the corresponding behaviors are described below: Isolation Level Dirty Read Non-Repeatable Read Phantom Read Read Uncommitted Possible Possible Possible Read Committed Not Possible Possible possible Repeatable Read Not Not possible Possible possible Serializable Not Not possible Not possible possible :::::::::::::::::::::::::::::::::::::::Java faqs page1::::::::::::::::::::::: 1. Question: Can a private method of a superclass be declared within a subclass? Answer: Sure. A private field or method or inner class belongs to its declared class and hides from its subclasses. There is no way for private stuff to have a runtime overloading or overriding (polymorphism) features. 2. Question: Why Java does not support multiple inheritence ? Answer: Java DOES support multiple inheritance via interface implementation. 3. Question:What is the difference between final, finally and finalize? Answer: o final - declare constant o finally - handles exception o finalize - helps in garbage collection 4. Question: Where and how can you use a private constructor. Answer: Private constructor can be used if you do not want any other class to instanstiate the object , the instantiation is done from a static public method, this method is used when dealing with the factory method pattern when the designer wants only one controller (fatory method ) to create the object. 5. Question: In System.out.println(),what is System,out and println,pls explain? Answer: System is a predefined final class,out is a PrintStream object and println is a built-in overloaded method in the out object. 6. Question: What is meant by quot;Abstract Interfacequot;? Answer: First, an interface is abstract. That means you cannot have any
  6. 6. implementation in an interface. All the methods declared in an interface are abstract methods or signatures of the methods. 7. Question: Can you make an instance of an abstract class? For example - java.util.Calender is an abstract class with a method getInstance() which returns an instance of the Calender class. Answer: No! You cannot make an instance of an abstract class. An abstract class has to be sub-classed. If you have an abstract class and you want to use a method which has been implemented, you may need to subclass that abstract class, instantiate your subclass and then call that method. 8. Question: What is the output of x<y? a:b = p*q when x=1,y=2,p=3,q=4? Answer: When this kind of question has been asked, find the problems you think is necessary to ask before you give an answer. Ask if variables a and b have been declared or initialized. If the answer is yes. You can say that the syntax is wrong. If the statement is rewritten as: x<y? a:(b=p*q); the return value would be variable a because the x is 1 and less than y = 2; the x < y statement return true and variable a is returned. 9. Question: What is the difference between Swing and AWT components? Answer: AWT components are heavy-weight, whereas Swing components are lightweight. Heavy weight components depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button. 10. Question: Why Java does not support pointers? Answer: Because pointers are unsafe. Java uses reference types to hide pointers and programmers feel easier to deal with reference types without pointers. This is why Java and C# shine. 11. Question: Parsers? DOM vs SAX parser Answer: parsers are fundamental xml components, a bridge between XML documents and applications that process that XML. The parser is responsible for handling xml syntax, checking the contents of the document against constraints established in a DTD or Schema. 12. Question: What is a platform? Answer: A platform is the hardware or software environment in which a program runs. Most platforms can be described as a combination of the operating system and hardware, like Windows 2000/XP, Linux, Solaris, and MacOS. 13. Question: What is the main difference between Java platform and other platforms? Answer: The Java platform differs from most other platforms in that it's a software- only platform that runs on top of other hardware-based platforms. The Java platform has two components: 1. The Java Virtual Machine (Java VM) 14. 2. The Java Application Programming Interface (Java API) 15. Question: What is the Java Virtual Machine? Answer: The Java Virtual Machine is a software that can be ported onto various hardware-based platforms. 16. Question: What is the Java API? Answer: The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets. 17. Question: What is the package? Answer: The package is a Java namespace or part of Java libraries. The Java API is grouped into libraries of related classes and interfaces; these libraries are known as packages.
  7. 7. 18. Question: What is native code? Answer: The native code is code that after you compile it, the compiled code runs on a specific hardware platform. 19. Question: Is Java code slower than native code? Answer: Not really. As a platform-independent environment, the Java platform can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring performance close to that of native code without threatening portability. 20. Question: What is the serialization? Answer: The serialization is a kind of mechanism that makes a class or a bean persistence by having its properties or fields and state information saved and restored to and from storage. 21. Question: How to make a class or a bean serializable? Answer: By implementing either the java.io.Serializable interface, or the java.io.Externalizable interface. As long as one class in a class's inheritance hierarchy implements Serializable or Externalizable, that class is serializable 22. Question: How many methods in the Serializable interface? Answer:There is no method in the Serializable interface. The Serializable interface acts as a marker, telling the object serialization tools that your class is serializable. 23. Question: . How many methods in the Externalizable interface? Answer: There are two methods in the Externalizable interface. You have to implement these two methods in order to make your class externalizable. These two methods are readExternal() and writeExternal(). 24. Question: What is the difference between Serializalble and Externalizable interface? Answer: When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject() two methods to control more complex object serailization process. When you use Externalizable interface, you have a complete control over your class's serialization process. 25. Question: What is a transient variable? Answer: A transient variable is a variable that may not be serialized. If you don't want some field to be serialized, you can mark that field transient or static. 26. Question: Which containers use a border layout as their default layout? Answer: The Window, Frame and Dialog classes use a border layout as their default layout. 27. Question: . How are Observer and Observable used? Answer: 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. 28. Question: What is synchronization and why is it important? Answer: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often causes dirty data and leads to significant errors. 29. Question: What are synchronized methods and synchronized statements? Answer: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
  8. 8. 30. Question: How are Observer and Observable used? Answer: 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. 31. Question: What is synchronization and why is it important? Answer: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often causes dirty data and leads to significant errors. 32. Question: What are synchronized methods and synchronized statements? Answer: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement. 33. Question: What are three ways in which a thread can enter the waiting state? Answer: A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method. 34. Question: Can a lock be acquired on a class? Answer: Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object. 35. Question: What's new with the stop(), suspend() and resume() methods in JDK 1.2? Answer: The stop(), suspend() and resume() methods have been deprecated in JDK 1.2. 36. Question: What is the preferred size of a component? Answer: The preferred size of a component is the minimum component size that will allow the component to display normally. 37. Question: What method is used to specify a container's layout? Answer: The setLayout() method is used to specify a container's layout. 38. Question: Which containers use a FlowLayout as their default layout? Answer: The Panel and Applet classes use the FlowLayout as their default layout. 39. Question: What is thread? Answer: A thread is an independent path of execution in a system. 40. Question: What is multithreading? Answer: Multithreading means various threads that run in a system. 41. Question: How does multithreading take place on a computer with a single CPU? Answer: The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially. 42. Question: How to create multithread in a program? Answer: You have two ways to do so. First, making your class quot;extendsquot; Thread class. Second, making your class quot;implementsquot; Runnable interface. Put jobs in a run() method and call start() method to start the thread. 43. Question: Can Java object be locked down for exclusive use by a given thread? Answer: Yes. You can lock an object by putting it in a quot;synchronizedquot; block. The locked object is inaccessible to any thread other than the one that explicitly claimed it
  9. 9. 44. Question: Can each Java object keep track of all the threads that want to exclusively access to it? Answer: Yes 45. Question: What state does a thread enter when it terminates its processing? Answer: When a thread terminates its processing, it enters the dead state. 46. Question: What invokes a thread's run() method? Answer: After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed. 47. Question: What is the purpose of the wait(), notify(), and notifyAll() methods? Answer: The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate each other. 48. Question: What are the high-level thread states? Answer: The high-level thread states are ready, running, waiting, and dead. 49. Question: What is the Collections API? Answer: The Collections API is a set of classes and interfaces that support operations on collections of objects. 50. Question: What is the List interface? Answer: The List interface provides support for ordered collections of objects. 51. Question: How does Java handle integer overflows and underflows? Answer: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation. 52. Question: What is the Vector class? Answer: The Vector class provides the capability to implement a growable array of objects What modifiers may be used with an inner class that is a member of an outer class? A (non-local) inner class may be declared as public, protected, private, static, final, or abstract. 53. Question: If a method is declared as protected, where may the method be accessed? Answer: 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. 54. Question: What is an Iterator interface? Answer: The Iterator interface is used to step through the elements of a Collection. 55. Question: How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters? Answer: Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns. 56. Question: What is the difference between yielding and sleeping? Answer: When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state. 57. Question: Is sizeof a keyword? Answer: The sizeof operator is not a keyword. 58. Question: What are wrapped classes? Answer: Wrapped classes are classes that allow primitive types to be accessed as objects. 59. Question: Does garbage collection guarantee that a program will not run out of memory? Answer: No, it doesn't. 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 60. Question: What is the difference between preemptive scheduling and time slicing? Answer: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters
  10. 10. the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors. 61. Question: Name Component subclasses that support painting. Answer: The Canvas, Frame, Panel, and Applet classes support painting. 62. Question: What is a native method? Answer: A native method is a method that is implemented in a language other than Java. 63. Question: How can you write a loop indefinitely? Answer: for(;;)--for loop; while(true)--always true, etc. 64. Question: . Can an anonymous class be declared as implementing an interface and extending a class? Answer: An anonymous class may implement an interface or extend a superclass, but may not be declared to do both. 65. Question: What is the purpose of finalization? Answer: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected. 66. Question: Which class is the superclass for every class. Answer: Object. 67. Question: What is the difference between the Boolean & operator and the && operator? Answer: 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. Operator & has no chance to skip both sides evaluation and && operator does. If asked why, give details as above. 68. Question: What is the GregorianCalendar class? Answer: The GregorianCalendar provides support for traditional Western calendars. 69. Question: What is the SimpleTimeZone class? Answer: The SimpleTimeZone class provides support for a Gregorian calendar. 70. Question: Which Container method is used to cause a container to be laid out and redisplayed? Answer: validate() 71. Question: What is the Properties class? Answer: The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used. 72. Question: What is the purpose of the Runtime class? Answer: The purpose of the Runtime class is to provide access to the Java runtime system. 73. Question: What is the purpose of the System class? Answer:The purpose of the System class is to provide access to system resources. 74. Question: What is the purpose of the finally clause of a try-catch-finally statement? Answer: The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught. 75. Question: What is the Locale class? Answer: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
  11. 11. 76. Question: What must a class do to implement an interface? Answer: It must provide all of the methods in the interface and identify the interface in its implements clause. 77. Question: What is an abstract method? Answer: An abstract method is a method whose implementation is deferred to a subclass. Or, a method that has no implementation (an interface of a method). 78. Question: What is a static method? Answer: A static method is a method that belongs to the class rather than any object of the class and doesn't apply to an object or even require that any objects of the class have been instantiated. 79. Question: What is a protected method? Answer: A protected method is a method that can be accessed by any method in its package and inherited by any subclass of its class. 80. Question: What is the difference between a static and a non-static inner class? Answer: 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. 81. Question: What is an object's lock and which object's have locks? Answer: 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. 82. Question: When can an object reference be cast to an interface reference? Answer: An object reference be cast to an interface reference when the object implements the referenced interface. 83. Question: What is the difference between a Window and a Frame? Answer: The Frame class extends Window to define a main application window that can have a menu bar. 84. Question: What do heavy weight components mean? Answer: Heavy weight components like Abstract Window Toolkit (AWT), depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button. In this relationship, the Motif button is called the peer to the java.awt.Button. If you create two Buttons, two peers and hence two Motif Buttons are also created. The Java platform communicates with the Motif Buttons using the Java Native Interface. For each and every component added to the application, there is an additional overhead tied to the local windowing system, which is why these components are called heavy weight. 85. Question: Which package has light weight components? Answer: javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components. 86. Question: What are peerless components? Answer: The peerless components are called light weight components. 87. Question: What is the difference between the Font and FontMetrics classes? Answer: The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object. 88. Question: What happens when a thread cannot acquire a lock on an object? Answer: If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available. 89. Question: What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy? Answer: The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.
  12. 12. 90. Question: What classes of exceptions may be caught by a catch clause? Answer: A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types. 91. Question:What is the difference between throw and throws keywords? Answer: The throw keyword denotes a statement that causes an exception to be initiated. It takes the Exception object to be thrown as argument. The exception will be caught by an immediately encompassing try-catch construction or propagated further up the calling hierarchy. The throws keyword is a modifier of a method that designates that exceptions may come out of the mehtod, either by virtue of the method throwing the exception itself or because it fails to catch such exceptions that a method it calls may throw. 92. Question: If a class is declared without any access modifiers, where may the class be accessed? Answer: A class that is declared without any access modifiers is said to have package or friendly access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package. 93. Question: What is the Map interface? Answer: The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values. 94. Question: Does a class inherit the constructors of its superclass? Answer: A class does not inherit constructors from any of its superclasses. 95. Question: Name primitive Java types. Answer: The primitive types are byte, char, short, int, long, float, double, and boolean. 96. Question: Which class should you use to obtain design information about an object? Answer: The Class class is used to obtain information about an object's design. 97. Question: How can a GUI component handle its own events? Answer: A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener. 98. Question: How are the elements of a GridBagLayout organized? Answer:The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes. 99. Question: What advantage do Java's layout managers provide over traditional windowing systems? Answer: Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems. 100.Question: What are the problems faced by Java programmers who don't use layout managers? Answer: Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system. 101.Question: What is the difference between static and non-static variables? Answer: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance. 102.Question: What is the difference between the paint() and repaint() methods? Answer: The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.
  13. 13. 103.Question: What is the purpose of the File class? Answer: The File class is used to create objects that provide access to the files and directories of a local file system. 104.Question: What restrictions are placed on method overloading? Answer: Two methods may not have the same name and argument list but different return types. 105.Question: What restrictions are placed on method overriding? Answer: Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method. 106.Question: What is casting? Answer: 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. 107.Question: Name Container classes. Answer: Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane 108.Question: What class allows you to read objects directly from a stream? Answer: The ObjectInputStream class supports the reading of objects from input streams. 109.Question: How are this() and super() used with constructors? Answer: this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor. 110.Question: How is it possible for two String objects with identical values not to be equal under the == operator? Answer: The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory. 111.Question: What an I/O filter? Answer: 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. 112.Question: What is the Set interface? Answer: The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements. 113.Question: What is the List interface? Answer: The List interface provides support for ordered collections of objects. 114.Question: What is the purpose of the enableEvents() method? Answer: The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods. 115.Question: What is the difference between the File and RandomAccessFile classes? Answer: 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. 116.Question: What interface must an object implement before it can be written to a stream as an object? Answer: An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object. 117.Question: What is the ResourceBundle class? Answer: The ResourceBundle class is used to store locale-specific resources that
  14. 14. can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run. 118.Question: What is the difference between a Scrollbar and a ScrollPane? Answer: A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling. 119.Question: What is a Java package and how is it used? Answer: A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces. 120.Question: What are the Object and Class classes used for? Answer: The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program. 121.Question: What is Serialization and deserialization? Answer: Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects. 122.Question: what is tunnelling? Answer: Tunnelling is a route to somewhere. For example, RMI tunnelling is a way to make RMI application get through firewall. In CS world, tunnelling means a way to transfer data. 123.Question: Does the code in finally block get executed if there is an exception and a return statement in a catch block? Answer: If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(1) statement is executed earlier or the system shut down earlier or the memory is used up earlier before the thread goes to finally block. 124.Question: How you restrict a user to cut and paste from the html page? Answer: Using javaScript to lock keyboard keys. It is one of solutions. 125.Question: Is Java a super set of JavaScript? Answer: No. They are completely different. Some syntax may be similar. 126.Question: What is a Container in a GUI? Answer: A Container contains and arranges other components (including other containers) through the use of layout managers, which use specific layout policies to determine where components should go as a function of the size of the container. 127.Question: How the object oriented approach helps us keep complexity of software development under control? Answer: 128.We can discuss such issue from the following aspects: • Objects allow procedures to be encapsulated with their data to reduce potential interference. • Inheritance allows well-tested procedures to be reused and enables changes to make once and have effect in all relevant places. • The well-defined separations of interface and implementation allows constraints to be imposed on inheriting classes while still allowing the flexibility of overriding and overloading. 129.Question: What is polymorphism? Answer: Polymorphism allows methods to be written that needn't be concerned about the specifics of the objects they will be applied to. That is, the method can be specified at a higher level of abstraction and can be counted on to work even on objects of yet unconceived classes. 130.Question: What is design by contract? Answer: The design by contract specifies the obligations of a method to any other methods that may use its services and also theirs to it. For example, the
  15. 15. preconditions specify what the method required to be true when the method is called. Hence making sure that preconditions are. Similarly, postconditions specify what must be true when the method is finished, thus the called method has the responsibility of satisfying the post conditions. In Java, the exception handling facilities support the use of design by contract, especially in the case of checked exceptions. The assert keyword can be used to make such contracts. 131.Question: What are use cases? Answer: A use case describes a situation that a program might encounter and what behavior the program should exhibit in that circumstance. It is part of the analysis of a program. The collection of use cases should, ideally, anticipate all the standard circumstances and many of the extraordinary circumstances possible so that the program will be robust. 132.Question: What is the difference between interface and abstract class? Answer: • interface contains methods that must be abstract; abstract class may contain concrete methods. • interface contains variables that must be static and final; abstract class may contain non-final and final variables. • members in an interface are public by default, abstract class may contain non-public members. • interface is used to quot;implementsquot;; whereas abstract class is used to quot;extendsquot;. • interface can be used to achieve multiple inheritance; abstract class can be used as a single inheritance. • interface can quot;extendsquot; another interface, abstract class can quot;extendsquot; another class and quot;implementsquot; multiple interfaces. • interface is absolutely abstract; abstract class can be invoked if a main() exists. • interface is more flexible than abstract class because one class can only quot;extendsquot; one super class, but quot;implementsquot; multiple interfaces. • If given a choice, use interface instead of abstract class. :::::::::::::::::::::::::::::::::::::::::;;jsp faqs::::::::::::::::::::::::::::::::::::::::::::::::: 1. Question: What do you understand by JSP Actions? Answer: JSP actions are XML tags that direct the server to use existing components or control the behavior of the JSP engine. JSP Actions consist of a typical (XML-based) prefix of quot;jspquot; followed by a colon, followed by the action name followed by one or more attribute parameters. There are six JSP Actions: • <jsp:include/> • <jsp:forward/> • <jsp:plugin/> • <jsp:usebean/> • <jsp:setProperty/> • <jsp:getProperty/>
  16. 16. 2. Question: What is the difference between <jsp:include page = ... > and <%@ include file = ... >?. Answer: Both the tag includes the information from one page in another. The differences are as follows: <jsp:include page = ... >: This is like a function call from one jsp to another jsp. It is executed ( the included page is executed and the generated html content is included in the content of calling jsp) each time the client page is accessed by the client. This approach is useful to for modularizing the web application. If the included file changed then the new content will be included in the output. <%@ include file = ... >: In this case the content of the included file is textually embedded in the page that have <%@ include file=quot;..quot;> directive. In this case in the included file changes, the changed content will not included in the output. This approach is used when the code from one jsp file required to include in multiple jsp files. 3. Question: What is the difference between <jsp:forward page = ... > and response.sendRedirect(url),?. Answer: The <jsp:forward> element forwards the request object containing the client request information from one JSP file to another file. The target file can be an HTML file, another JSP file, or a servlet, as long as it is in the same application context as the forwarding JSP file. sendRedirect sends HTTP temporary redirect response to the browser, and browser creates a new request to go the redirected page. The response.sendRedirect kills the session variables. 4. Question: Identify the advantages of JSP over Servlet. a) Embedding of Java code in HTML pages b) Platform independence c) Creation of database-driven Web applications d) Server-side programming capabilities Answer :- Embedding of Java code in HTML pages 5. Write the following code for a JSP page: <%@ page language = quot;javaquot; %> <HTML> <HEAD><TITLE>RESULT PAGE</TITLE></HEAD> <BODY> <% PrintWriter print = request.getWriter(); print.println(quot;Welcomequot;); %> </BODY> </HTML> Suppose you access this JSP file, Find out your answer. a) A blank page will be displayed. b) A page with the text Welcome is displayed c) An exception will be thrown because the implicit out object is not used
  17. 17. d) An exception will be thrown because PrintWriter can be used in servlets only Answer :- A page with the text Welcome is displayed 6. Question: What are implicit Objects available to the JSP Page? Answer: Implicit objects are the objects available to the JSP page. These objects are created by Web container and contain information related to a particular request, page, or application. The JSP implicit objects are: Variable Class Description The context for the JSP page's servlet and any Web application javax.servlet.ServletContext components contained in the same application. config javax.servlet.ServletConfig Initialization information for the JSP page's servlet. exception java.lang.Throwable Accessible only from an error page. out javax.servlet.jsp.JspWriter The output stream. The instance of the JSP page's servlet processing the current page java.lang.Object request. Not typically used by JSP page authors. The context for the JSP page. Provides a single API to pageContext javax.servlet.jsp.PageContext manage the various scoped attributes. Subtype of request The request triggering the execution of the JSP page. javax.servlet.ServletRequest Subtype of The response to be returned to the client. Not typically used response javax.servlet.ServletResponse by JSP page authors. session javax.servlet.http.HttpSession The session object for the client. 7. 8. Question: What are all the different scope values for the <jsp:useBean> tag? Answer:<jsp:useBean> tag is used to use any java object in the jsp page. Here are the scope values for <jsp:useBean> tag: a) page b) request c) session and d) application 9. Question: What is JSP Output Comments? Answer: JSP Output Comments are the comments that can be viewed in the HTML source file. Example: <!-- This file displays the user login screen --> and <!-- This page was loaded on <%= (new java.util.Date()).toLocaleString() %> --> 10. Question: What is expression in JSP? Answer: Expression tag is used to insert Java values directly into the output. Syntax for the Expression tag is: <%= expression %> An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. The following expression tag displays time on the output: <%=new java.util.Date()%> 11. Question: What types of comments are available in the JSP? Answer: There are two types of comments are allowed in the JSP. These are
  18. 18. hidden and output comments. A hidden comments does not appear in the generated output in the html, while output comments appear in the generated output. Example of hidden comment: <%-- This is hidden comment --%> Example of output comment: <!-- This is output comment --> 12. Question: What is JSP declaration? Answer: JSP Decleratives are the JSP tag used to declare variables. Declaratives are enclosed in the <%! %> tag and ends in semi-colon. You declare variables and functions in the declaration tag and can use anywhere in the JSP. Here is the example of declaratives: <%@page contentType=quot;text/htmlquot; %> <html> <body> <%! int cnt=0; private int getCount(){ //increment cnt and return the value cnt++; return cnt; } %> <p>Values of Cnt are:</p> <p><%=getCount()%></p> </body> </html> 13. Question: What is JSP Scriptlet? Answer: JSP Scriptlet is jsp tag which is used to enclose java code in the JSP pages. Scriptlets begins with <% tag and ends with %> tag. Java code written inside scriptlet executes every time the JSP is invoked. Example: <% //java codes String userName=null; userName=request.getParameter(quot;userNamequot;); %> 14. Question: What are the life-cycle methods of JSP? Answer: Life-cycle methods of the JSP are: a) jspInit(): The container calls the jspInit() to initialize the servlet instance. It is called before any other method, and is called only once for a servlet instance. b)_jspService(): The container calls the _jspservice() for each request and it passes the request and the response objects. _jspService() method cann't be overridden. c) jspDestroy(): The container calls this when its instance is about to destroyed. The jspInit() and jspDestroy() methods can be overridden within a JSP page.
  19. 19. :::::::::::::::::::::::::::::::::::::::::::Corejava faqs::::::::::::::::::::::::::::::::::: 1. Question: It is said that the code in a finally clause will never fail to execute, Is there any example where it fails to execute? Answer: Here is the example code where the finally clause code will not execute. 2. public class testFinally{ a. public static void main(String[] args){ i. System.out.println(quot;Executing the programquot;); 1. try { a. System.out.println(quot;In the try blockquot;); b. System.exit(1); c. } finally { d. System.out.println(quot;In the finally.quot;); e. } ii. } 3. } 4. Download the code 5. Question: Why there are no global variables in Java? Answer: Global variables are globally accessible. Java does not support globally accessible variables due to following reasons: * The global variables breaks the referential transparency * Global variables creates collisions in namespace. 6. Question: What platforms is the Java-technology software available on? Answer: Sun provides ports of the Java 2 Platform for Windows 95, Windows 98, Windows NT, Windows 2000, Solaris-SPARC, Solaris-Intel, and Linux. 7. Question: Where can I download latest version of Java? Answer: Latest version of JDK can be downloaded from Sun web site http://www.java.sun.com. 8. Question: Do I need to know C++ to learn Java? Answer: No, you don't need to know C or C++ to learn Java. Java is much simpler that C++. 9. Question: What is the difference between Java and Java Script? Answer: In Java and Java Script only the quot;Javaquot; word is common. Java is programming language from Sun. JavaScript is a programming language from Netscape, which runs in their browsers. 10. Question: Differentiate between applet and application. Answer: Java applications runs as stand-alone application whereas applet runs in web browser. Application is a Java class that has a main() method. Applet class extends java.applet.Applet class. 11. Question: How to convert String to Number in java program? Answer: The valueOf() function of Integer class is is used to convert string to Number. Here is the code example: String strId = quot;10quot;;
  20. 20. 12. int id=Integer.valueOf(strId); 13. Question: What is interface? Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface. Example of Interface: public interface sampleInterface { public void functionOne(); public long CONSTANT_ONE = 1000; } 14. Question: How you can force the garbage collection? Answer: Garbage collection automatic process and can't be forced. 15. Question: What is transient variable? Answer: Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null. 16. Question: Name the containers which uses Border Layout as their default layout? Answer: Containers which uses Border Layout as their default are: window, Frame and Dialog classes. 17. Question: What do you understand by Synchronization? Answer: Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption. E.g. Synchronizing a function: public synchronized void Method1 () { // Appropriate method-related code. } E.g. Synchronizing a block of code inside a function: public myFunction (){ synchronized (this) { // Synchronized code here. } } 18. Question: What is Collection API? Answer: The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces. Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
  21. 21. Example of interfaces: Collection, Set, List and Map. 19. Question: Is Iterator a Class or Interface? What is its use? Answer: Iterator is an interface which is used to step through the elements of a Collection. 20. Question: What is similarities/difference between an Abstract class and Interface? Answer: Differences are as follows: a. Interfaces provide a form of multiple inheritance. A class can extend only one other class. b. Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc. c. A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class. d. Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast. 21. Similarities: a. Neither Abstract classes or Interface can be instantiated. 22. Question: How to define an Abstract class? Answer: A class containing abstract method is called Abstract class. An Abstract class can't be instantiated. Example of Abstract class: abstract class testAbstractClass { protected String myString; public String getMyString() { return myString; } public abstract string anyAbstractFunction(); } 23. Question: How to define an Interface? Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface. Emaple of Interface: public interface sampleInterface { public void functionOne(); public long CONSTANT_ONE = 1000; } 24. Question: Explain the user defined Exceptions? Answer: User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions. Example: class myCustomException extends Exception { // The class simply has to exist to be an exception }
  22. 22. 25. Question: Explain the new Features of JDBC 2.0 Core API? Answer: The JDBC 2.0 API includes the complete JDBC API, which includes both core and Optional Package API, and provides inductrial-strength database computing capabilities. New Features in JDBC 2.0 Core API: a. Scrollable result sets- using new methods in the ResultSet interface allows programmatically move the to particular row or to a position relative to its current position b. JDBC 2.0 Core API provides the Batch Updates functionality to the java applications. c. Java applications can now use the ResultSet.updateXXX methods. d. New data types - interfaces mapping the SQL3 data types e. Custom mapping of user-defined types (UTDs) f. Miscellaneous features, including performance hints, the use of character streams, full precision for java.math.BigDecimal values, additional security, and support for time zones in date, time, and timestamp values. 26. Question: Explain garbage collection? Answer: Garbage collection is one of the most important feature of Java. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected. 27. Question: How you can force the garbage collection? Answer: Garbage collection automatic process and can't be forced. 28. Question: What is OOPS? Answer: OOP is the common abbreviation for Object-Oriented Programming. 29. Question: Describe the principles of OOPS. Answer: There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation. 30. Question: Explain the Encapsulation principle. Answer: Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper. 31. Question: Explain the Inheritance principle. Answer: Inheritance is the process by which one object acquires the properties of another object.
  23. 23. 32. Question: Explain the Polymorphism principle. Answer: The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as quot;one interface, multiple methodsquot;. 33. Question: Explain the different forms of Polymorphism. Answer: From a practical programming viewpoint, polymorphism exists in three distinct forms in Java: a. Method overloading b. Method overriding through inheritance c. Method overriding through the Java interface d. 34. Question: What are Access Specifiers available in Java? Answer: Access specifiers are keywords that determines the type of access to the member of a class. These are: a. Public b. Protected c. Private d. Defaults e. 35. Question: Describe the wrapper classes in Java. Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type. 36. Following table lists the primitive types and the corresponding wrapper classes: 37. Primitive 38. Wrapper 39. boolean 40. java.lang.Boolean 41. byte 42. java.lang.Byte 43. char 44. java.lang.Character 45. double 46. java.lang.Double 47. float 48. java.lang.Float 49. int 50. java.lang.Integer 51. long 52. java.lang.Long 53. short 54. java.lang.Short 55. void 56. java.lang.Void a. 57. Question: Read the following program: 58. public class test { public static void main(String [] args) { int x = 3; int y = 1; if (x = y) System.out.println(quot;Not equalquot;); else System.out.println(quot;Equalquot;);
  24. 24. } } 59. What is the result? A. The output is “Equal” B. The output in “Not Equal” C. An error at quot; if (x = y)quot; causes compilation to fall. D. The program executes but no output is show on console. Answer: C :::::::::::::::::::::::::::::::::::::Jakarta Struts Interview Questions:::::::::::::::::::::::::::: 1. Q: What is Jakarta Struts Framework? A: Jakarta Struts is open source implementation of MVC (Model-View-Controller) pattern for the development of web based applications. Jakarta Struts is robust architecture and can be used for the development of application of any size. Struts framework makes it much easier to design scalable, reliable Web applications with Java. 2. Q: What is ActionServlet? A: The class org.apache.struts.action.ActionServlet is the called the ActionServlet. In the the Jakarta Struts Framework this class plays the role of controller. All the requests to the server goes through the controller. Controller is responsible for handling all the requests. 3. Q: How you will make available any Message Resources Definitions file to the Struts Framework Environment? A: Message Resources Definitions file are simple .properties files and these files contains the messages that can be used in the struts project. Message Resources Definitions files can be added to the struts-config.xml file through <message- resources /> tag. Example: <message-resources parameter=quot;MessageResourcesquot; /> 4. Q: What is Action Class? A: The Action is part of the controller. The purpose of Action Class is to translate the HttpServletRequest to the business logic. To use the Action, we need to Subclass and overwrite the execute() method. The ActionServlet (commad) passes the parameterized class to Action Form using the execute() method. There should be no database interactions in the action. The action should receive the request, call business objects (which then handle database, or interface with J2EE, etc) and then determine where to go next. Even better, the business objects could be handed to the action at runtime (IoC style) thus removing any dependencies on the model. The return type of the execute method is ActionForward which is used by the Struts Framework to forward the request to the file as per the value of the returned ActionForward object. 5. Q: Write code of any Action Class? A: Here is the code of Action Class that returns the ActionForward object. TestAction.java
  25. 25. 1. package roseindia.net; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.apache.struts.action.Action; import org.apache.struts.action.ActionForm; import org.apache.struts.action.ActionForward; import org.apache.struts.action.ActionMapping; public class TestAction extends Action { public ActionForward execute( ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws Exception{ return mapping.findForward(quot;testActionquot;); } } 6. Q: What is ActionForm? A: An ActionForm is a JavaBean that extends org.apache.struts.action.ActionForm. ActionForm maintains the session state for web application and the ActionForm object is automatically populated on the server side with data entered from a form on the client side. 7. Q: What is Struts Validator Framework? A: Struts Framework provides the functionality to validate the form data. It can be use to validate the data on the users browser as well as on the server side. Struts Framework emits the java scripts and it can be used validate the form data on the client browser. Server side validation of form can be accomplished by sub classing your From Bean with DynaValidatorForm class. 8. The Validator framework was developed by David Winterfeldt as third-party add- on to Struts. Now the Validator framework is a part of Jakarta Commons project and it can be used with or without Struts. The Validator framework comes integrated with the Struts Framework and can be used without doing any extra settings. 9. Q. Give the Details of XML files used in Validator Framework? A: The Validator Framework uses two XML configuration files validator- rules.xml and validation.xml. The validator-rules.xml defines the standard validation routines, these are reusable and used in validation.xml. to define the form specific validations. The validation.xml defines the validations applied to a form bean. 10. Q. How you will display validation fail errors on jsp page? A: Following tag displays all the errors: <html:errors/> 11. Q. How you will enable front-end validation based on the xml in validation.xml? A: The <html:javascript> tag to allow front-end validation based on the xml in validation.xml. For example the code: <html:javascript formName=quot;logonFormquot; dynamicJavascript=quot;truequot; staticJavascript=quot;truequot; /> generates the client side java script for the form quot;logonFormquot; as defined in the validation.xml file. The <html:javascript> when added in the jsp file generates the client site validation script.
  26. 26. 12. Question: What is RequestProcessor and RequestDispatcher? Answer: The controller is responsible for intercepting and translating user input into actions to be performed by the model. The controller is responsible for selecting the next view based on user input and the outcome of model operations. The Controller receives the request from the browser, invoke a business operation and coordinating the view to return to the client. The controller is implemented by a java servlet, this servlet is centralized point of control for the web application. In struts framework the controller responsibilities are implemented by several different components like The ActionServlet Class The RequestProcessor Class The Action Class The ActionServlet extends the javax.servlet.http.httpServlet class. The ActionServlet class is not abstract and therefore can be used as a concrete controller by your application. The controller is implemented by the ActionServlet class. All incoming requests are mapped to the central controller in the deployment descriptor as follows. <servlet> <servlet-name>action</servlet-name> <servlet-class>org.apache.struts.action.ActionServlet</servlet-class> </servlet> All request URIs with the pattern *.do are mapped to this servlet in the deployment descriptor as follows. <servlet-mapping> <servlet-name>action</servlet-name> <url-pattern>*.do</url-pattern> <url-pattern>*.do</url-pattern> A request URI that matches this pattern will have the following form. http://www.my_site_name.com/mycontext/actionName.do The preceding mapping is called extension mapping, however, you can also specify path mapping where a pattern ends with /* as shown below. <servlet-mapping> <servlet-name>action</servlet-name> <url-pattern>/do/*</url-pattern> <url-pattern>*.do</url-pattern> A request URI that matches this pattern will have the following form. http://www.my_site_name.com/mycontext/do/action_Name The class org.apache.struts.action.requestProcessor process the request from the controller. You can sublass the RequestProcessor with your own version and modify how the request is processed. Once the controller receives a client request, it delegates the handling of the request to a helper class. This helper knows how to execute the business operation associated with the requested action. In the Struts framework this helper class is descended of org.apache.struts.action.Action class. It acts as a bridge between a client-side user action and business operation. The Action class decouples the client request from the business model. This decoupling allows for more than one-to-one mapping between the user request and an action. The Action class also can perform
  27. 27. other functions such as authorization, logging before invoking business operation. the Struts Action class contains several methods, but most important method is the execute() method. public ActionForward execute(ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws Exception; The execute() method is called by the controller when a request is received from a client. The controller creates an instance of the Action class if one doesn’t already exist. The strut framework will create only a single instance of each Action class in your application. Action are mapped in the struts configuration file and this configuration is loaded into memory at startup and made available to the framework at runtime. Each Action element is represented in memory by an instance of the org.apache.struts.action.ActionMapping class . The ActionMapping object contains a path attribute that is matched against a portion of the URI of the incoming request. <action> path= quot;/somerequestquot; type=quot;com.somepackage.someActionquot; scope=quot;requestquot; name=quot;someFormquot; validate=quot;truequot; input=quot;somejsp.jspquot; <forward name=quot;Successquot; path=quot;/action/xysquot; redirect=quot;truequot;/> <forward name=quot;Failurequot; path=quot;/somejsp.jspquot; redirect=quot;truequot;/> </action> Once this is done the controller should determine which view to return to the client. The execute method signature in Action class has a return type org.apache.struts.action.ActionForward class. The ActionForward class represents a destination to which the controller may send control once an action has completed. Instead of specifying an actual JSP page in the code, you can declaratively associate as action forward through out the application. The action forward are specified in the configuration file. <action> path= quot;/somerequestquot; type=quot;com.somepackage.someActionquot; scope=quot;requestquot; name=quot;someFormquot; validate=quot;truequot; input=quot;somejsp.jspquot; <forward name=quot;Successquot; path=quot;/action/xysquot; redirect=quot;truequot;/> <forward name=quot;Failurequot; path=quot;/somejsp.jspquot; redirect=quot;truequot;/> </action> The action forward mappings also can be specified in a global section, independent of any specific action mapping. <global-forwards> <forward name=quot;Successquot; path=quot;/action/somejsp.jspquot; /> <forward name=quot;Failurequot; path=quot;/someotherjsp.jspquot; /> </global-forwards> public interface RequestDispatcher Defines an object that receives requests from the client and sends them to any
  28. 28. resource (such as a servlet, HTML file, or JSP file) on the server. The servlet container creates the RequestDispatcher object, which is used as a wrapper around a server resource located at a particular path or given by a particular name. This interface is intended to wrap servlets, but a servlet container can create RequestDispatcher objects to wrap any type of resource. getRequestDispatcher public RequestDispatcher getRequestDispatcher(java.lang.String path) Returns a RequestDispatcher object that acts as a wrapper for the resource located at the given path. A RequestDispatcher object can be used to forward a request to the resource or to include the resource in a response. The resource can be dynamic or static. The pathname must begin with a quot;/quot; and is interpreted as relative to the current context root. Use getContext to obtain a RequestDispatcher for resources in foreign contexts. This method returns null if the ServletContext cannot return a RequestDispatcher. Parameters: path - a String specifying the pathname to the resource Returns: a RequestDispatcher object that acts as a wrapper for the resource at the specified path See Also: RequestDispatcher, getContext(java.lang.String) getNamedDispatcher public RequestDispatcher getNamedDispatcher(java.lang.String name) Returns a RequestDispatcher object that acts as a wrapper for the named servlet. Servlets (and JSP pages also) may be given names via server administration or via a web application deployment descriptor. A servlet instance can determine its name using ServletConfig.getServletName(). This method returns null if the ServletContext cannot return a RequestDispatcher for any reason. Parameters: name - a String specifying the name of a servlet to wrap Returns: a RequestDispatcher object that acts as a wrapper for the named servlet See Also: RequestDispatcher, getContext(java.lang.String), ServletConfig.getServletName() 13. Question: Why cant we overide create method in StatelessSessionBean? Answer: From the EJB Spec : - A Session bean's home interface defines one or morecreate(...) methods. Each create method must be named create and must match one of the ejbCreate methods defined in the enterprise Bean class. The return type of a create method must be the enterprise Bean's remote interface type. The home interface of a stateless session bean must have one create method that takes no arguments.
  29. 29. 14. Question: Is struts threadsafe?Give an example? Answer: Struts is not only thread-safe but thread-dependant. The response to a request is handled by a light-weight Action object, rather than an individual servlet. Struts instantiates each Action class once, and allows other requests to be threaded through the original object. This core strategy conserves resources and provides the best possible throughput. A properly-designed application will exploit this further by routing related operations through a single Action. 15. Question: Can we Serialize static variable? Answer: Serialization is the process of converting a set of object instances that contain references to each other into a linear stream of bytes, which can then be sent through a socket, stored to a file, or simply manipulated as a stream of data. Serialization is the mechanism used by RMI to pass objects between JVMs, either as arguments in a method invocation from a client to a server or as return values from a method invocation. In the first section of this book, There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are 1. Serialization ignores static fields, because they are not part of any particular object's state. 2. Base class fields are only handled if the base class itself is serializable. 3. Transient fields. There are four basic things you must do when you are making a class serializable. They are: a. Implement the Serializable interface. b. Make sure that instance-level, locally defined state is serialized properly. c. Make sure that superclass state is serialized properly. d. Override equals( )and hashCode( ). it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process .... (Source: http://www.oreilly.com/catalog/javarmi/chapter/ch10.html) 16. Question: What are the uses of tiles-def.xml file, resourcebundle.properties file, validation.xml file? Answer: tiles-def.xml is is an xml file used to configure tiles with the struts application. You can define the layout / header / footer / body content for your View. See more at http://www.roseindia.net/struts/using-tiles-defs-xml.shtml. The resourcebundle.properties file is used to configure the message (error/ other messages) for the struts applications. The file validation.xml is used to declare sets of validations that should be applied to Form Beans. Fpr more information please visit http://www.roseindia.net/struts/address_struts_validator.shtml. 17. Question: What is the difference between perform() and execute() methods? Answer: Perform method is the method which was deprecated in the Struts Version 1.1. In Struts 1.x, Action.perform() is the method called by the ActionServlet. This is typically where your business logic resides, or at least the flow control to your JavaBeans and EJBs that handle your business logic. As we already mentioned, to support declarative exception handling, the method signature changed in perform. Now execute just throws Exception. Action.perform() is now deprecated; however, the Struts v1.1 ActionServlet is smart enough to know whether or not it should call perform or execute in the Action, depending on which
  30. 30. one is available. 18. Question: What are the various Struts tag libraries? Answer: Struts is very rich framework and it provides very good and user friendly way to develop web application forms. Struts provide many tag libraries to ease the development of web applications. These tag libraries are: * Bean tag library - Tags for accessing JavaBeans and their properties. * HTML tag library - Tags to output standard HTML, including forms, text boxes, checkboxes, radio buttons etc.. * Logic tag library - Tags for generating conditional output, iteration capabilities and flow management * Tiles or Template tag library - For the application using tiles * Nested tag library - For using the nested beans in the application 19. Question: What do you understand by DispatchAction? Answer: DispatchAction is an action that comes with Struts 1.1 or later, that lets you combine Struts actions into one class, each with their own method. The org.apache.struts.action.DispatchAction class allows multiple operation to mapped to the different functions in the same Action class. For example: A package might include separate RegCreate, RegSave, and RegDelete Actions, which just perform different operations on the same RegBean object. Since all of these operations are usually handled by the same JSP page, it would be handy to also have them handled by the same Struts Action. 20. A very simple way to do this is to have the submit button modify a field in the form which indicates which operation to perform. i. <html:hidden property=quot;dispatchquot; value=quot;errorquot;/ > <SCRIPT>function set(target) {document.forms[0].dispatch.value=target;}</SCR IPT> <html:submit onclick=quot;set('save');quot;>SAVE</html:submit> <html:submit onclick=quot;set('create');quot;>SAVE AS NEW</html:submitl> <html:submit onclick=quot;set('delete);quot;>DELETE</html:submit> 21. Then, in the Action you can setup different methods to handle the different operations, and branch to one or the other depending on which value is passed in the dispatch field. i. String dispatch = myForm.getDispatch(); if (quot;createquot;.equals(dispatch)) { ... if (quot;savequot;.equals(dispatch)) { ... 22. The Struts Dispatch Action [org.apache.struts.actions] is designed to do exactly the same thing, but without messy branching logic. The base perform method will check a dispatch field for you, and invoke the indicated method. The only catch is that the dispatch methods must use the same signature as perform. This is a very modest requirement, since in practice you usually end up doing that anyway. 23. To convert an Action that was switching on a dispatch field to a DispatchAction, you simply need to create methods like this i. public ActionForward create( ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { ...
  31. 31. ii. public ActionForward save( ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { ... 24. Cool. But do you have to use a property named dispatch? No, you don't. The only other step is to specify the name of of the dispatch property as the quot;parameterquot; property of the action-mapping. So a mapping for our example might look like this: i. <action path=quot;/reg/dispatchquot; type=quot;app.reg.RegDispatchquot; name=quot;regFormquot; scope=quot;requestquot; validate=quot;truequot; parameter=quot;dispatchquot;/> 25. If you wanted to use the property quot;oquot; instead, as in o=create, you would change the mapping to i. <action path=quot;/reg/dispatchquot; type=quot;app.reg.RegDispatchquot; name=quot;regFormquot; scope=quot;requestquot; validate=quot;truequot; parameter=quot;oquot;/> 26. Again, very cool. But why use a JavaScript button in the first place? Why not use several buttons named quot;dispatchquot; and use a different value for each? 27. You can, but the value of the button is also its label. This means if the page designers want to label the button something different, they have to coordinate the Action programmer. Localization becomes virtually impossible. (Source: http://husted.com/struts/tips/002.html). 28. Question: How Struts relates to J2EE? Answer: Struts framework is built on J2EE technologies (JSP, Servlet, Taglibs), but it is itself not part of the J2EE standard. 29. Question: What is Struts actions and action mappings? Answer: A Struts action is an instance of a subclass of an Action class, which implements a portion of a Web application and whose perform or execute method returns a forward. An action can perform tasks such as validating a user name and password. An action mapping is a configuration file entry that, in general, associates an action name with an action. An action mapping can contain a reference to a form bean that the action can use, and can additionally define a list of local forwards that is visible only to this action. An action servlet is a servlet that is started by the servlet container of a Web server to process a request that invokes an action. The servlet receives a forward from the action and asks the servlet container to pass the request to the forward's URL. An action servlet must be an instance of an org.apache.struts.action.ActionServlet class or of a subclass of that class. An action servlet is the primary component of the controller.

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