Java_XML_faqs

GENERAL

Q. What is XML?

A. XML, the Extensible Markup Language, is a universal syntax for describing and structuring data independent from the application logic. XML can be used to define unlimited languages for specific industries and applications.

Q. Who developed XML?

A. XML is an activity of the World Wide Web Consortium (W3C). The XML development effort started in 1996.

A diverse group of markup language experts, from industry to academia, developed a simplified version of SGML (Standard Generalized Markup Language) for the Web. In February 1998, XML 1.0 specification became a recommendation by the W3C.

Q. What are the key benefits of XML?

A. XML promises to simplify and lower the cost of data interchange and publishing in a Web environment. XML is a text-based syntax that is readable by both computer and humans. XML offers data portability and reusability across different platforms and devices. It is also flexible and extensible, allowing new tags to be added without breaking an existing document structure. Based on Unicode, XML provides global language support.

Q. What are the applications of XML?

A. XML is poised to play a prominent role as a data interchange format in B2B Web applications such as e-commerce, supply-chain management, workflow, and application integration. Another use of XML is for structured information management, including information from databases. XML also supports media-independent publishing, allowing documents to be written once and published in multiple media formats and devices. On the client, XML can be used to create customized views into data.

Q. What is the relationship between XML and Java technology?

A. XML and the Java technology are complementary. Java technology provides the portable, maintainable code to process portable, reusable XML data. In addition, XML and Java technology have a number of shared features that make them the ideal pair for Web computing, including being industry standards, platform-independence, extensible, reusable, Web-centric, and internationalized.

Q. What are the benefits of using Java technology with XML?

A. Java technology offers a substantial productivity boost for software developers compared to programming languages such as C or C++. In addition, developers using the Java platform can create sophisticated programs that are reusable and maintainable compared to programs written with scripting languages. Using XML and Java together, developers can build sophisticated, interoperable Web applications more quickly and at a lower cost.

Q. What XML-related activities is Sun participating in?

A. Sun is actively participating in W3C working groups for XML Stylesheet/Transformation Language (XSL/T), XML Schema, Xlink, and XML Query. Sun is also participating in a number of other industry consortia including Oasis, XML.org, and Apache.

Q. Where can I find additional documentation?

A. The Java Technology & XML Documentation page has a comprehensive list of all documentation related to Java Technology and XML available on this website.

Q. Where can I send comments and suggestions?

A. For feedback on the project, please send email to xml-feedback@sun.com.

Q. Are there other Sun hosted XML mailing lists I can subscribe to?

A. For general discussion about topics related to XML technologies in the Java platform, subscribe to xml-interest@java.sun.com.




Java API for XML-Based Web Services (JAX-WS)

Q. What is the difference between JAX-RPC and JAX-WS?

A. One of the main differences between JAX-RPC and JAX-WS is the programming model.
A JAX-WS-based service uses annotations, such as @WebService, to declare web service endpoints.
Using these annotations obviates the need for deployment descriptors.
With JAX-WS, you can have a web service deployed on a Java EE-compliant application server without a single deployment descriptor.
JAX-WS also provides many additional features, such as asynchronous callback support. Refer to the JAX-WS specification for more information.

Q. Does the Java EE 5 platform support JAX-WS?

A. Yes, the Java EE 5 platform supports JAX-WS-based services (in addition to JAX-RPC-based services) and makes these services portable through JSR 109.

Q. Can a JAX-WS-based service and a JAX-RPC-based service co-exist?

A. Yes.

Q. How do I develop a simple JAX-WS-based service?

A. See this tip.

Q. Can a JAX-WS-based service that was developed and deployed on Java WSDP 2.0 be deployed and run on any Java EE 5-based platform?

A. No.

Q. How do I make my JAX-WS service that was developed and deployed on Java WSDP 2.0 portable across Java EE 5 platforms?

A. See this tip.

Q. Will a JAX-WS-based web service work with a JAX-RPC client and a JAX-RPC-based web service work with a JAX-WS client?

A. Yes.

Q. Can I have a JAX-WS web service with deployment descriptors?

A. Yes. The same deployment descriptors, such as webservices.xml, web.xml, and ejb-jar.xml, can be used, just as they were with JAX-RPC-based services. If a deployment descriptor is specified, the values specified in the annotation attributes will be overridden by the corresponding values specified in the deployment descriptors.

Q. Can the deployment descriptor values be overridden?

A. No.

Q. Do I have to specify all attributes for annotations?

A. No. Well-defined default values have been specified for all attributes of annotations.





Java API for XML Processing (JAXP)


Q. What is Java API for XML Processing (JAXP)?

The Java API for XML Processing, or "JAXP" for short, enables applications to parse and transform XML documents using an API that is independent of a particular XML processor implementation. JAXP also provides a pluggability feature which enables applications to easily switch between particular XML processor implementations.

To achieve the goal of XML processor independence, an application should limit itself to the JAXP API and avoid implementation-dependent APIs. This may or may not be easy depending on the application. JAXP includes industry standard APIs such as DOM and SAX.

The reason for the existance of JAXP is to facilitate the use of XML on the Java platform. For example, current APIs such as DOM Level 2 do not provide a method to bootstrap a DOM Document object from an XML input document, JAXP does. (When DOM Level 3 provides this functionality, a new version of the JAXP specification will probably support the new Level 3 scheme also.) Other parts of JAXP such as the javax.xml.transform portion do not have any other equivalent APIs that are XSLT processor independent.

Q. Where can I read more about JAXP?

A. See the JAXP FAQ for more information.




Java Architecture for XML Binding (JAXB)

Q. What is Java Architecture for XML Binding (JAXB)?

A. The Java Architecture for XML Binding (JAXB) simplifies the creation and maintenance of XML-enabled Java applications. JAXB provides a binding compiler and a runtime framework to support a two-way mapping between XML documents and Java objects. The binding compiler translates W3C XML Schema into one or more Java classes without requiring the developer to write complex parsing code. The schema-derived classes and binding framework enable error and validity checking of incoming and outgoing XML documents, thereby making it possible to ensure that only valid, error-free messages are accepted, processed, and generated by a system. For more information, see the Reference Implementation and the Public Draft Specification, both available for download from the JAXB homepage.

Q. What is the difference between JAXB, SAX, and DOM? Which one should I use?

A. SAX is an event-driven XML parser that is appropriate for high-speed processing of XML because it does not produce a representation of the data in memory. DOM, on the other hand, produces an in-memory data representation, which allows an application to manipulate the contents in memory. Both SAX and DOM automatically perform structure validation. An application could perform content validation with SAX and DOM, but such an application must provide the necessary extra code, which might be complicated, error-prone, and difficult to maintain.

A JAXB application can perform structure and content validation with Java classes that it generates from a schema. A JAXB application builds an in-memory data structure, like a DOM, by marshalling an XML document to build a content tree, which contains objects that are instances of the derived classes. However, unlike a DOM tree, a content tree is specific to one source schema, does not contain extra tree-manipulation functionality, allows access to its data with the derived classes' accessor methods, and is not built dynamically. For these reasons, a JAXB application uses memory more efficiently than a DOM application does. If the content of a document is more dynamic and not well-constrained, DOM and SAX are more appropriate than JAXB for processing XML content that does not have a well-known schema prior to processing the content.

Q. How does JAXB work?

A. To build a JAXB application, start with an XML schema. The beta release requires that the schema language be W3C 2001 Recommendation for XML Schema.


After obtaining an XML Schema, you build and use a JAXB application by performing these steps:

Generate the Java source files by submitting the XML Schema to the binding compiler.

You can use custom binding declarations to override the default binding of XML Schema components to Java representations

Compile the Java source code.

With the classes and the binding framework, write Java applications that:

Build object trees representing XML data that is valid against the XML Schema by either unmarshalling the data from a document or instantiating the classes you created.

Access and modify the data.

Optionally validate the modifications to the data relative to the constraints expressed in the XML Schema

Marshal the data to new XML documents.

Q. Who is involved in developing JAXB?

A. JAXB is being developed through the Java Community Process (JCP) with an expert group consisting of IBM, Software AG, BEA Systems, Hewlett-Packard, TIBCO Software Inc., Oracle, Fujitsu Limited, Breeze Factor LLC, Macromedia, Inc. and Intalio, Inc. Sun is an active member of the W3C XML Schema Working Group and is also working with other industry consortia such as OASIS and xml.org.

Q. Where can I read more about JAXB?

A. For a higher-level explanation of JAXB, refer to the JAXB chapters in the Java Web Services Tutorial. Also note that a detailed user's guide is included as part of the JAXB distribution. For a more technical and detailed description of JAXB, see the the latest version of the Specification, which you can download from the JAXB homepage. Please note that the Specification is in Adobe Acrobat PDF format. Download Adobe Acrobat for free.




Java API for XML Messaging (JAXM)

Q. What is the Java API for XML Messaging (JAXM)?

A: The Java API for XML Messaging (JAXM) is an API designed specifically for the exchange of XML business documents over the Internet. Examples of XML documents that might typically be exchanged are purchase orders, order confirmations, and invoices. You can send non-XML data by adding attachments to your message.

Q: What standards is JAXM based on?

A: JAXM is based on the Simple Object Access Protocol (SOAP) 1.1 and SOAP with Attachments specifications. JAXM also allows the implementation of standard protocols on top of the SOAP implementation, such as SOAP-RP or the ebXML Transport, Routing & Packaging V1.0 - Message Service Specification.

Q. Do I have to use the J2EE platform to use JAXM?

A: No, you are free to use the Java 2 Platform, Standard Edition (J2SE) as well as the Java 2 Platform, Enterprise Edition (J2EE). A stand-alone client (a client that does not use a messaging provider) can use the J2SE platform to send request-response messages to Web services that process request-response messages. This requires no deployment or configuration from the client, so it is easy to do.

Q. What is a messaging provider?

A: A messaging provider is a service that works with the messaging infrastructure to route and transmit messages. What it does is completely transparent to the client sending or receiving a message. An application that uses a messaging provider must use a connection that goes to the provider, called a ProviderConnection object in the JAXM API. Using a messaging provider also requires some deployment and configuration. Normally, a client using a messaging provider runs in a container -- either a servlet or a J2EE container. At deployment time, the client needs to give the container information about the messaging provider. In the future, there will be a deployment tool that makes this easy.

Q. Do I have to use a messaging provider?

A: No. You need to use a messaging provider only when your application requires one-way (asynchronous) messaging. In this type of messaging, a message is sent to a recipient as one operation, and the recipient responds at some later time in a different operation. If you application uses a request-response style of messaging, in which the response to a message is sent back as part of the same operation, you do not need a messaging provider. When you do not use a messaging provider, you use a SOAPConnection object, which supports the simpler request-response messaging model.

Q. Can a JAXM message be routed to more than one destination?

Yes. Intermediate recipients can be specified in a message's header. One way this capability can be used is to automate business processes. For example, two businesses can agree to the conditions under which they exchange XML documents so that they can implement the automatic generation of messages and responses. Assume that two businesses have an arrangement specifying that purchase orders will go first to the order entry department, then to the order confirmation department, then to the shipping department, and finally to the billing department. Each department is an intermediate recipient (called an actor). After an actor has done its part, it removes everything in the header that relates to it and sends the message on to the next actor listed in the header.

Q. Can I use ebXML headers in a JAXM message?

A: Yes, you can use ebXML headers if you use an ebXML profile that is implemented on top of SOAP. A profile is a standard protocol, such as ebXML TRP or SOAP-RP, that works on top of SOAP to give you added functionality. You need to use a messaging provider that supports the profile, and you need to arrange with your recipients to use the same profile.




Java API for XML Registries (JAXR)

Q. What is the Java API for XML Registries (JAXR)?

A. The Java API for XML Registries (JAXR) API provides a uniform and standard Java API for accessing different kinds of XML Registries. XML registries are an enabling infrastructure for building, deployment, and discovery of Web services.

Q. What is the relationship between the JAXR API and other XML APIs?

A. Implementations of JAXR providers may use the Jav API for XML-Based RPC (JAX-RPC) for communication between JAXR providers and registry providers that export a SOAP based RPC-like interface (for example, UDDI).

Implementations of JAXR providers may use the Java API for XML Messaging (JAXM) API for communication between JAXR providers and registry providers that export an XML Messaging-based interface (for example, ebXML TRP).

The Java API for XML Processing (JAXP) and Java Architecture for XML Binding (JAXB) may be used by implementers of JAXR providers and JAXR clients for processing XML content that is submitted to or retrieved from the Registry.

Q. Why do we need a new JAXR API when we have the Java Naming and Directory Interface (JNDI)?

A. The JNDI API was designed with a very different set of requirements from the JAXR API. Both are abstraction APIs over existing specifications. However, the abstraction in directory services differs considerably from that of XML Registries used for publishing and discovery of Web services. The JAXR API needs richer metadata capabilities for classification and association, as well as richer query capabilities.

Q. Would it not be better to have enhanced the JNDI API with the added functionality of the JAXR API?

A. That option was considered. Meeting the additional requirements of XML Registries requires an elaborate information model. The JNDI API has an existing information model that is constrained by design to address the requirements for directory services. Extending the JNDI API would overly constrain the JAXR API and would create backward compatibility issues for the JNDI API.

Q. What is the purpose of Association in the JAXR information model? It is not used anywhere in the API.

A. An Association relates two RegistryObjects to each other. An Association may be defined between two objects in the registry and submitted using the GenericLifeCycleManager's saveObjects method.

Q. What is the purpose of Classification in the JAXR information model? It is not used anywhere in the API.

A. A Classification classifies a RegistryObject. A Classification may be defined for a RegistryObject and submitted using the GenericLifeCycleManager's saveObjects method.

Q. Why is JAXR an abstraction API and not targeted to a specific registry such as UDDI or ebXML?

A. An abstraction-based JAXR API gives developers the ability to write registry client programs that are portable across different target registries. This is consistent with the Java philosophy of Write Once, Run Anywhere. It also enables value-added capabilities beyond what the underlying registries are capable of. For example, a non-JAXR UDDI client does not have the ability to do taxonomy browsing and taxonomy-aware smart queries, which are available to a JAXR client for UDDI.

Q. Why does the JAXR API not use UDDI terms and concepts?

A. The JAXR API is not specific to UDDI or any other registry specification. It is an abstraction API that covers multiple specifications. It is designed to enable developer choice in the use of a Web service registry and/or repository. The JAXR API uses UDDI terms and concepts when they fit the JAXR information model (for example, Service, ServiceBinding, and method names in BusinessQueryManager and BusinessLifeCycleManager).

Q. Why did the JAXR information model use the ebXML Registry Information Model as its basis rather than the UDDI data structures?

A. The JAXR API is designed to support multiple registries. The ebXML Registry Information Model is more generic and extensible than the UDDI data structures. Because of this characteristic, it was possible to extend the ebXML Registry Information Model to satisfy the needs of UDDI and other registries.

Q. Why was the JAXR information model not designed from the ground up?

A. Information models take time to develop. It was easier to start with an existing information model and improve upon it.




Java API for XML-Based RPC (JAX-RPC)

Q. What is the Java API for XML-Based RPC (JAX-RPC)?

A. The Java API for XML-Based RPC (JAX-RPC) enables Java technology developers to build Web applications and Web services incorporating XML-based RPC functionality according to the SOAP (Simple Object Access Protocol) 1.1 specification.

Q. How does JAX-RPC use SOAP?

A. Please refer to JSR-101.

Q. What is RPC?

A. RPC stands for remote procedure call, a mechanism that allows a client to execute procedures on other systems. The RPC mechanism is often used in a distributed client/server model. The server defines a service as a collection of procedures that may be called by remote clients.

Q. How is XML related to RPC?

A. The remote procedure call is represented by an XML-based protocol, such as SOAP. In addition to defining envelope structure and encoding rules, the SOAP specification defines a convention for representing remote procedure calls and responses.

Q. What does JAX-RPC have to do with Web services?

A. An XML-based RPC server application can define, describe, and export a Web service as an RPC-based service. WSDL (Web Service Description Language) specifies an XML format for describing a service as a set of endpoints operating on messages. With the JAX-RPC API, developers can implement clients and services described by WSDL.

Q. What are the modes of interaction between clients and JAX-RPC services?

A. There are three different modes:

Synchronous Request-Response: The client invokes a remote procedure and blocks until it receives a return or an exception.


One-Way RPC: The client invokes a remote procedure but it does not block or wait until it receives a return. The runtime system for the JAX-RPC client may throw an exception.


Non-Blocking RPC Invocation: The client invokes a remote procedure and continues processing in the same thread without waiting for a return. Later, the client processes the remote method return by blocking for the receive or polling for the return.


Q. Can a remote method call or response carry service context information?

A. Yes. For example, it may carry a unique transaction identifier or digital signature.

Q. Why doesn't xrpcc generate a WSDL file?

A. The xrpcc tool does in fact generate the WSDL file, but due to a bug it gets deleted along with the source files if the -keep option is not specified. You can use the -keep option which will cause xrpcc to not delete the WSDL or .java source files. If you also use the -s sourcepath option, all of the source files will be placed in the sourcepath directory and then you can easily delete them. The WSDL file will still be placed in the current directory or the directory specified by the -d option.




Java 2 Platform, Enterprise Edition

Q. Does the Java 2 Platform, Enterprise Edition (J2EE) use XML?

A. The Java 2 Platform, Enterprise Edition (J2EE) promotes the use of XML for data messaging between loosely-coupled business systems. The J2EE reference implementation includes the Java API for XML Parsing (JAXP).

JavaServer Pages (JSP) can generate and consume XML between multi-tier servers or between server and client. Java Message Service (JMS) provides an asynchronous transport mechanism for XML data messaging. Enterprise JavaBeans (EJB) offers a robust, synchronous transport mechanism by allowing a business service object to be invoked by XML tags. EJB also uses XML to describe its deployment properties, such as transactions and security.

Q. Can I generate dynamic XML documents using JSP pages?

A. JSP pages enables the authoring of XML pages. XML pages can be generated using JSP pages, which include elements to produce the dynamic portions of the document. The JSP specification includes a powerful tag extension mechanism that can be used to perform XML-based operations, such as applying an XSLT transformation to an XML document.

JAVA/Swing Quest

1.What is a class?
A class is a blueprint, or prototype, that defines the variables and the methods common to all objects of a certain kind.

2.What is a object?
An object is a software bundle of variables and related methods.An instance of a class depicting the state and behavior at that particular time in real world.

3.What is a method?
Encapsulation of a functionality which can be called to perform specific tasks.

4.What is encapsulation? Explain with an example.
Encapsulation is the term given to the process of hiding the implementation details of the object. Once an object is encapsulated, its implementation details are not immediately accessible any more. Instead they are packaged and are only indirectly accessible via the interface of the object

5.What is inheritance? Explain with an example.
Inheritance in object oriented programming means that a class of objects can inherit properties and methods from another class of objects.

6.What is polymorphism? Explain with an example.
In object-oriented programming, polymorphism refers to a programming language’s ability to process objects differently depending on their data type or class. More specifically, it is the ability to redefine methods for derived classes. For example, given a base class shape, polymorphism enables the programmer to define different area methods for any number of derived classes, such as circles, rectangles and triangles. No matter what shape an object is, applying the area method to it will return the correct results. Polymorphism is considered to be a requirement of any true object-oriented programming language

7.Is multiple inheritance allowed in Java?
No, multiple inheritance is not allowed in Java.

8.What is interpreter and compiler?
Java interpreter converts the high level language code into a intermediate form in Java called as bytecode, and then executes it, where as a compiler converts the high level language code to machine language making it very hardware specific

9.What is JVM?
The Java interpreter along with the runtime environment required to run the Java application in called as Java virtual machine(JVM)

10.What are the different types of modifiers?
There are access modifiers and there are other identifiers. Access modifiers are public, protected and private. Other are final and static.

11.What are the access modifiers in Java?
There are 3 access modifiers. Public, protected and private, and the default one if no identifier is specified is called friendly, but programmer cannot specify the friendly identifier explicitly.

12.What is a wrapper class?
They are classes that wrap a primitive data type so it can be used as a object

13.What is a static variable and static method? What’s the difference between two? The modifier static can be used with a variable and method. When declared as static variable, there is only one variable no matter how instances are created, this variable is initialized when the class is loaded. Static method do not need a class to be instantiated to be called, also a non static method cannot be called from static method.

14.What is garbage collection?
Garbage Collection is a thread that runs to reclaim the memory by destroying the objects that cannot be referenced anymore.

15.What is abstract class?
Abstract class is a class that needs to be extended and its methods implemented, aclass has to be declared abstract if it has one or more abstract methods.

16.What is meant by final class, methods and variables?
This modifier can be applied to class method and variable. When declared as final class the class cannot be extended. When declared as final variable, its value cannot be changed if is primitive value, if it is a reference to the object it will always refer to the same object, internal attributes of the object can be changed.

17.What is interface?
Interface is a contact that can be implemented by a class, it has method that need implementation.

18.What is method overloading?
Overloading is declaring multiple method with the same name, but with different argument list.

19.What is method overriding?
Overriding has same method name, identical arguments used in subclass.

20.What is singleton class?
Singleton class means that any given time only one instance of the class is present, in one JVM.

21.What is the difference between an array and a vector?
Number of elements in an array are fixed at the construction time, whereas the number of elements in vector can grow dynamically.

22.What is a constructor?
In Java, the class designer can guarantee initialization of every object by providing a special method called a constructor. If a class has a constructor, Java automatically calls that constructor when an object is created, before users can even get their hands on it. So initialization is guaranteed.

23.What is casting?
Conversion of one type of data to another when appropriate. Casting makes explicitly converting of data.

24.What is the difference between final, finally and finalize?
The modifier final is used on class variable and methods to specify certain behaviour explained above. And finally is used as one of the loop in the try catch blocks, It is used to hold code that needs to be executed whether or not the exception occurs in the try catch block. Java provides a method called finalize( ) that can be defined in the class. When the garbage collector is ready to release the storage ed for your object, it will first call finalize( ), and only on the next garbage-collection pass will it reclaim the objects memory. So finalize( ), gives you the ability to perform some important cleanup at the time of garbage collection.

25.What is are packages?
A package is a collection of related classes and interfaces providing access protection and namespace management.

26.What is a super class and how can you call a super class?
When a class is extended that is derived from another class there is a relationship is created, the parent class is referred to as the super class by the derived class that is the child. The derived class can make a call to the super class using the keyword super. If used in the constructor of the derived class it has to be the first statement.

27.What is meant by a Thread?
Thread is defined as an instantiated parallel process of a given program.

28.What is multi-threading?
Multi-threading as the name suggest is the scenario where more than one threads are running.

29.What are two ways of creating a thread? Which is the best way and why?
Two ways of creating threads are, one can extend from the Java.lang.Thread and can implement the rum method or the run method of a different class can be called which implements the interface Runnable, and the then implement the run() method. The latter one is mostly used as first due to Java rule of only one class inheritance, with implementing the Runnable interface that problem is sorted out.

30.What is deadlock?
Deadlock is a situation when two threads are waiting on each other to release a resource. Each thread waiting for a resource which is held by the other waiting thread. In Java, this resource is usually the object lock obtained by the synchronized keyword.

31.What are the three types of priority?
MAX_PRIORITY which is 10, MIN_PRIORITY which is 1, NORM_PRIORITY which is 5.

32.What is the use of synchronizations?
Every object has a lock, when a synchronized keyword is used on a piece of code the, lock must be obtained by the thread first to execute that code, other threads will not be allowed to execute that piece of code till this lock is released.

Swings

1) Can a class be it’s own event handler? Explain how to implement this.
Answer: Sure. an example could be a class that extends Jbutton and implements ActionListener. In the actionPerformed method, put the code to perform when the button is pressed.

2) Why does JComponent have add() and remove() methods but Component does not?
Answer: because JComponent is a subclass of Container, and can contain other components and jcomponents.

3) How would you create a button with rounded edges?
Answer: there’s 2 ways. The first thing is to know that a JButton’s edges are drawn by a Border. so you can override the Button’s paintComponent(Graphics) method and draw a circle or rounded rectangle (whatever), and turn off the border. Or you can create a custom border that draws a circle or rounded rectangle around any component and set the button’s border to it.

4) If I wanted to use a SolarisUI for just a JTabbedPane, and the Metal UI for everything else, how would I do that?
Answer: in the UIDefaults table, override the entry for tabbed pane and put in the SolarisUI delegate. (I don’t know it offhand, but I think it’s "com.sun.ui.motiflookandfeel.MotifTabbedPaneUI" - anything simiar is a good answer.)

5) What is the difference between the ‘Font’ and ‘FontMetrics’ class?
Answer: The Font Class is used to render ‘glyphs’ - the characters you see on the screen. FontMetrics encapsulates information about a specific font on a specific Graphics object. (width of the characters, ascent, descent)

6) What class is at the top of the AWT event hierarchy?
Answer: java.awt.AWTEvent. if they say java.awt.Event, they haven’t dealt with swing or AWT in a while.

7) Explain how to render an HTML page using only Swing.
Answer: Use a JEditorPane or JTextPane and set it with an HTMLEditorKit, then load the text into the pane.

8) How would you detect a keypress in a JComboBox?
Answer: This is a trick. most people would say ‘add a KeyListener to the JComboBox’ - but the right answer is ‘add a KeyListener to the JComboBox’s editor component.’

9) Why should the implementation of any Swing callback (like a listener) execute quickly?
A: Because callbacks are invoked by the event dispatch thread which will be blocked processing other events for as long as your method takes to execute.

10) In what context should the value of Swing components be updated directly?
A: Swing components should be updated directly only in the context of callback methods invoked from the event dispatch thread. Any other context is not thread safe?

11) Why would you use SwingUtilities.invokeAndWait or SwingUtilities.invokeLater?
A: I want to update a Swing component but I’m not in a callback. If I want the update to happen immediately (perhaps for a progress bar component) then I’d use invokeAndWait. If I don’t care when the update occurs, I’d use invokeLater.

12) If your UI seems to freeze periodically, what might be a likely reason?
A: A callback implementation like ActionListener.actionPerformed or MouseListener.mouseClicked is taking a long time to execute thereby blocking the event dispatch thread from processing other UI events.

13) Which Swing methods are thread-safe?
A: The only thread-safe methods are repaint(), revalidate(), and invalidate()

14) Why won’t the JVM terminate when I close all the application windows?
A: The AWT event dispatcher thread is not a daemon thread. You must explicitly call System.exit to terminate the JVM.

Q1: What are the advantages of OOPL?
Ans: Object oriented programming languages directly represent the real life objects. The features of OOPL as inhreitance, polymorphism, encapsulation makes it powerful.

Q2: What do mean by polymorphisum, inheritance, encapsulation?
Ans: Polymorhisum: is a feature of OOPl that at run time depending upon the type of object the appropriate method is called.
Inheritance: is a feature of OOPL that represents the "is a" relationship between different objects(classes). Say in real life a manager is a employee. So in OOPL manger class is inherited from the employee class.
Encapsulation: is a feature of OOPL that is used to hide the information.

Q3: What do you mean by static methods?
Ans: By using the static method there is no need creating an object of that class to use that method. We can directly call that method on that class. For example, say class A has static function f(), then we can call f() function as A.f(). There is no need of creating an object of class A.

Q4: What do you mean by virtual methods?
Ans: virtual methods are used to use the polymorhism feature in C++. Say class A is inherited from class B. If we declare say fuction f() as virtual in class B and override the same function in class A then at runtime appropriate method of the class will be called depending upon the type of the object.

Q5: Given two tables Student(SID, Name, Course) and Level(SID, level) write the SQL statement to get the name and SID of the student who are taking course = 3 and at freshman level.
Ans: SELECT Student.name, Student.SID
FROM Student, Level
WHERE Student.SID = Level.SID
AND Level.Level = "freshman"
AND Student.Course = 3;

Q6: What are the disadvantages of using threads?
Ans: DeadLock.

Q7: Write the Java code to declare any constant (say gravitational constant) and to get its value
Ans: Class ABC
{
static final float GRAVITATIONAL_CONSTANT = 9.8;
public void getConstant()
{
system.out.println("Gravitational_Constant: " + GRAVITATIONAL_CONSTANT);
}
}

Q8: What do you mean by multiple inheritance in C++ ?
Ans: Multiple inheritance is a feature in C++ by which one class can be of different types. Say class teachingAssistant is inherited from two classes say teacher and Student.

Q9: Can you write Java code for declaration of multiple inheritance in Java ?
Ans: Class C extends A implements B
{
}

About Agile Methodology

The purpose of the Agile Method site is to provide a channel for the continuous ongoing development of the methodology and to achieve this by seeking feedback from the Agile Method community. All feedback received is evaluated and suggestions for improvements may be incorporated into future releases of the individual components that make up the Agile Method.
Components of the Agile Method:

Agile Method: scope
The Scope template captures the Goal of the project, and defines Boundaries that the Project Team will work to.
This includes the Project Team details, a listing of specific and measurable Objectives, and any specific Constraints.

Agile Method: define
The User Story template is used to capture individual customer journeys in a way that works.
Using Agile Method, User Stories are the main tool used to explore and iterate business requirements.

Agile Method: develop
The Product Backlog template is the master list of all desired product (or service) features.
Using Agile Method, the Product Backlog will keep evolving as customer and business needs are discovered.

Agile Method: evaluate
The Burndown Chart template is used to monitor delivery of features described in the project's User Stories.
On a project using Agile Method, the team tracks its progress by updating a Burndown Chart at the end of each Cycle

The Agile Method Templates provide a standard lean and lightweight pack of deliverables, ensuring adequate documentation is produced, and supporting rapid progress through Agile Method cycles.

Q&A for Java

Q: Explain Database normalization?

A: Normalization is used to describe how to break a file down into tables to create a database. It is the technique for designing rational database to minimize duplication of information and in so doing to safeguard the database against certains types of logical or structural problems.

Q: What is spring?

A: Spring is the open source application framework for the java platform. Spring Framework does not enforce any specific programming model, It has become popular in the java community as an alternative, replacement, or even addition to the EJB (Enterprise java Beans) model. It can be used by any java application, there are many extensions and improvements for building web-based applications on top of the java Enterprise platform.

Q: What is JSF?

A: JSF is Java server faces framework, JavaServer Faces technology simplifies building user interfaces for JavaServer applications.

Q: What is inheritance?

A: inheritance is a way to form new classes (instances of which are called object) using classes that have already been defined. The new classes, known as derived classes, take over (or inherit) attributes and behavior of the pre-existing classes, which are referred to as base classes (or ancestor classes). It is intended to help reuses existing code with little or no modification.

Q: What is abstract?

A: A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.

Q: What is interface?

A: 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


Q: What is the difference between interface and abstract?

A: Differences are as follows: Interfaces provide a form of multiple inheritance. A class can extend only one other class. Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc. Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class. Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast.

Q: Explain your experience with Java-Hibernate-

AJAX-Struts? A: Java: Used oops concepts in the application like inheritance, encapsulation, polymorphism etc

Hibernate: Hibernate is a powerful, high performance object/relational persistence and query service. Hibernate lets you develop persistent classes following object-oriented idiom - including association, inheritance, polymorphism, composition, and collections. Hibernate allows you to express queries in its own portable SQL extension (HQL), as well as in native SQL, or with an object-oriented Criteria and Example API

AJAX: Used for Server side validation.

Struts: Used in MVC architecture for the application. Used Dyano Action forms in the Struts-config.xml

Q: Explain your experience & understanding with SOA and Agile methodology?

A:SOA is Service oriented architecture which is used to the concepts of webservices. We created WSDL and soap of webservices and communicate the messages with different servers.

Q: Do you know what adobe flex is?
A: Adobe Flex is a collection of technologies released by Adobe Systems for the development and deployment of cross platform rich internet applications based on the proprietary adobe flash platform.


Q: How do you use adobe flex relating to your project?

A:It is used for Front end layer like creating the jsp pages and java classes

Q: Why would adobe flex be used?

A:Adobe Flex is a collection of technologies released by Adobe Systems for the development and deployment of cross platform rich internet applications based on the proprietary adobe flash platform.

Q: Tell me what you do know about adobe flex?

A: Adobe Flex is a collection of technologies released by Adobe Systems for the development and deployment of cross platform rich internet applications based on the proprietary adobe flash platform.

JAVA FAQs-1

Q: What is the difference between an Interface and an Abstract class?

A: An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

Q: What is the purpose of garbage collection in Java, and when is it used?

A: The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Q: Describe synchronization in respect to multithreading.

A: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

Q: Explain different way of using thread?

A: The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.


Q: What are pass by reference and passby value?

A: Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.

Q: What is HashMap and Map?

A: Map is Interface and Hashmap is class that implements that.


Q: Difference between HashMap and HashTable?

A: The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.


Q: Difference between Vector and ArrayList?

A: Vector is synchronized whereas arraylist is not.


Q: Difference between Swing and Awt?

A: AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.


Q: What is the difference between a constructor and a method?

A: A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.


Q: What is an Iterator?

A: Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.


Q: State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.

A: public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.


Q: What is an abstract class?

A: Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.


Q: What is static in java?

A: Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.


Q: What is final?

A: A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

Q: What if the main method is declared as private?

A: The program compiles properly but at runtime it will give "Main method not public." message.


Q: What if the static modifier is removed from the signature of the main method?

A: Program compiles. But at runtime throws an error "NoSuchMethodError".


Q: What if I write static public void instead of public static void?

A: Program compiles and runs properly.


Q: What if I do not provide the String array as the argument to the method?

A: Program compiles but throws a runtime error "NoSuchMethodError".


Q: What is the first argument of the String array in main method?

A: The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.


Q: If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?

A: It is empty. But not null.


Q: How can one prove that the array is not null but empty using one line of code?

A: Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.


Q: What environment variables do I need to set on my machine in order to be able to run Java programs?

A: CLASSPATH and PATH are the two variables.


Q: Can an application have multiple classes having main method?

A: Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.


Q: Can I have multiple main methods in the same class?

A: No the program fails to compile. The compiler says that the main method is already defined in the class.


Q: Do I need to import java.lang package any time? Why ?

A: No. It is by default loaded internally by the JVM.


Q: Can I import same package/class twice? Will the JVM load the package twice at runtime?

A: One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.


Q: What are Checked and UnChecked Exception?

A: A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.
Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method·
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the
exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.


Q: What is Overriding?

A: When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.


Q: What are different types of inner classes?

A: Nested -level classes, Member classes, Local classes, Anonymous classes

Nested -level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other -level class.
Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. -level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested -level variety.

Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested -level class. The primary difference between member classes and nested -level classes is that member classes have access to the specific instance of the enclosing class.

Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a
more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.

Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?

A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;


Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?

A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.


Q: What is the difference between declaring a variable and defining a variable?

A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.


Q: What is the default value of an object reference declared as an instance variable?

A: null unless we define it explicitly.


Q: Can a level class be private or protected?

A: No. A level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a level class can not be private. Same is the case with protected.


Q: What type of parameter passing does Java support?

A: In Java the arguments are always passed by value .
[ Update from Eki and Jyothish Venu]

Q: Primitive data types are passed by reference or pass by value?

A: Primitive data types are passed by value.


Q: Objects are passed by value or by reference?

A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .
[ Update from Eki and Jyothish Venu]

Q: What is serialization?

A: Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.


Q: How do I serialize an object to a file?

A: The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.


Q: Which methods of Serializable interface should I implement?

A: The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.


Q: How can I customize the seralization process? i.e. how can one have a control over the serialization process?

A: Yes 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.


Q: What is the common usage of serialization?

A: Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.


Q: What is Externalizable interface?

A: Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.


Q: When you serialize an object, what happens to the object references included in the object?

A: The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.


Q: What one should take care of while serializing the object?

A: One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.


Q: What happens to the static fields of a class during serialization?

A: 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 ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.

Q: Does Java provide any construct to find out the size of an object?

A: No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.


Q: Give a simplest way to find out the time a method takes for execution without using any profiling tool?

A: Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.
To put it in code...

long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();

System.out.println ("Time taken for execution is " + (end - start));

Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing.



Q: What are wrapper classes?

A: Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.


Q: Why do we need wrapper classes?

A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.


Q: What are checked exceptions?

A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.


Q: What are runtime exceptions?

A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.


Q: What is the difference between error and an exception?

A: An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).


Q: How to create custom exceptions?

A: Your class should extend class Exception, or some more specific type thereof.


Q: If I want an object of my class to be thrown as an exception object, what should I do?

A: The class should extend from Exception class. Or you can extend your class from some more precise exception type also.


Q: If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?

A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.


Q: How does an exception permeate through the code?

A: An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.


Q: What are the different ways to handle exceptions?

A: There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.


Q: What is the basic difference between the 2 approaches to exception handling.
1> try catch block and
2> specifying the candidate exceptions in the throws clause?
When should you use which approach?

A: In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.


Q: Is it necessary that each try block must be followed by a catch block?

A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.


Q: If I write return at the end of the try block, will the finally block still execute?

A: Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.


Q: If I write System.exit (0); at the end of the try block, will the finally block still execute?

A: No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

Q: How are Observer and Observable used?

A: 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.


Q: What is synchronization and why is it important?

A: 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 leads to significant errors.


Q: How does Java handle integer overflows and underflows?

A: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.


Q: Does garbage collection guarantee that a program will not run out of memory?

A: 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
.


Q: What is the difference between preemptive scheduling and time slicing?

A: 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 the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.


Q: When a thread is created and started, what is its initial state?

A: A thread is in the ready state after it has been created and started.


Q: What is the purpose of finalization?

A: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.


Q: What is the Locale class?

A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.


Q: What is the difference between a while statement and a do statement?

A: 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.


Q: What is the difference between static and non-static variables?

A: 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.


Q: How are this() and super() used with constructors?

A: This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.


Q: What are synchronized methods and synchronized statements?

A: 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.


Q: What is daemon thread and which method is used to create the daemon thread?

A: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.


Q: Can applets communicate with each other?

A: At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable.

An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members.

It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.


Q: What are the steps in the JDBC connection?

A: While making a JDBC connection we go through the following steps :

Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );

Step 2 : Now create a database connection using :

Connection con = DriverManager.getConnection(url,username,password);

Step 3: Now Create a query using :

Statement stmt = Connection.Statement(\"select * from TABLE NAME\");

Step 4 : Exceute the query :

stmt.exceuteUpdate();


Q: How does a try statement determine which catch clause should be used to handle an exception?

A: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.

Q: Can an unreachable object become reachable again?

A: An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.
[Received from P Rajesh]

Q: What method must be implemented by all threads?

A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.



Q: What are synchronized methods and synchronized statements?

A: 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.



Q: What is Externalizable?

A: Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
[ Received from Venkateswara Manam]

Q: What modifiers are allowed for methods in an Interface?

A: Only public and abstract modifiers are allowed for methods in interfaces.



Q: What are some alternatives to inheritance?

A: Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).



Q: What does it mean that a method or field is "static"?

A: Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.





Q: What is the difference between preemptive scheduling and time slicing?

A: 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 the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.


Q: What is the catch or declare rule for method declarations?

A: If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

Q: Is Empty .java file a valid source file?

A: Yes, an empty .java file is a perfectly valid source file.
[Received from Sandesh Sadhale]

Q: Can a .java file contain more than one java classes?

A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.


Q: Is String a primitive data type in Java?

A: No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.


Q: Is main a keyword in Java?

A: No, main is not a keyword in Java.


Q: Is next a keyword in Java?

A: No, next is not a keyword.


Q: Is delete a keyword in Java?

A: No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.


Q: Is exit a keyword in Java?

A: No. To exit a program explicitly you use exit method in System object.


Q: What happens if you dont initialize an instance variable of any of the primitive types in Java?

A: Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.


Q: What will be the initial value of an object reference which is defined as an instance variable?

A: The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.


Q: What are the different scopes for Java variables?

A: The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.


Q: What is the default value of the local variables?

A: The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..


Q: How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();


A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.


Q: Can a public class MyClass be defined in a source file named YourClass.java?

A: No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.


Q: Can main method be declared final?

A: Yes, the main method can be declared final, in addition to being public static.
[ Received fromSandesh Sadhale]

Q: What will be the output of the following statement?
System.out.println ("1" + 3);

A: It will print 13.


Q: What will be the default values of all the elements of an array defined as an instance variable?

A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.