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OAuth 2.0

OAuth 2.0 is an open authorization framework and mainly focuses on authorization flows fand secures access to many well-known web APIs.

Java 8 features

The key features of JDK 8 are Project Lambda (JSR 335), the Nashorn JavaScript Engine, a new Date and Time API (JSR 310), a set of Compact Profiles and the removal of the "permanent generation" from the HotSpot Java Virtual Machine (JVM). A complete list of the new features and capabilities of JDK 8 is available.

Lambda Expression

A lambda expression represents an anonymous function. It comprises of a set of parameters, a lambda operator (->) and a function body.


SOAP : Web service use XML messages that follow the Simple Object Access Protocol (SOAP) standard.
RESTful : Web service, often better integrated with HTTP than SOAP-based services are, do not require XML messages or WSDL service–API definitions.

Softwares Engineering / SDLC

Software engineers apply the principles of engineering to the design, development, maintenance, testing, and evaluation of the software and systems that make computers or anything containing software work.

Monday, September 13, 2010

Introduction to Java Language

Java is an object-oriented programming language with a built-in application programming interface (API) that can handle graphics and user interfaces and that can be used to create applications or applets. Because of its rich set of API's, similar to Macintosh and Windows, and its platform independence, Java can also be thought of as a platform in itself. Java also has standard libraries for doing mathematics.

Much of the syntax of Java is the same as C and C++. One major difference is that Java does not have pointers. However, the biggest difference is that you must write object oriented code in Java. Procedural pieces of code can only be embedded in objects. In the following we assume that the reader has some familiarity with a programming language. In particular, some familiarity with the syntax of C/C++ is useful.

In Java we distinguish between applications, which are programs that perform the same functions as those written in other programming languages, and applets, which are programs that can be embedded in a Web page and accessed over the Internet. Our initial focus will be on writing applications. When a program is compiled, a byte code is produced that can be read and executed by any platform that can run Java.

Download jdk1.6 from java.sun.com/javase/downloads/index.jsp. Double click on jdk1.6 and install java. Set path to c:\program files\java\jdk1.6\bin;

Java Directory Structure
Directory Structure
(The name may be different, for example, jdk5.0)
The compiler and tools
Look here for demos
Library documentation in HTML format (after expansion of j2sdkversion-doc.zip)
Files for compiling native methods (see Volume 2)
Java runtime environment files
Library files
The library source (after expanding src.zip)


James Gosling initiated the Java language project in June 1991 for use in one of his many set-top box projects. The language, initially called Oak after an oak tree that stood outside Gosling's office, also went by the name Green and ended up later renamed as Java, from a list of random words. Gosling aimed to implement a virtual machine and a language that had a familiar C/C++ style of notation. 

Sun released the first public implementation as Java 1.0 in 1995. It promised "Write Once, Run Anywhere" (WORA), providing no-cost run-times on popular platforms. Fairly secure and featuring configurable security, it allowed network- and file-access restrictions. Major web browsers soon incorporated the ability to run Java applets within web pages, and Java quickly became popular. With the advent of Java 2 (released initially as J2SE 1.2 in December 1998), new versions had multiple configurations built for different types of platforms. For example, J2EE targeted enterprise applications and the greatly stripped-down version J2ME for mobile applications. J2SE designated the Standard Edition. In 2006, for marketing purposes, Sun renamed new J2 versions as Java EE, Java ME, and Java SE, respectively.

In 1997, Sun Microsystems approached the ISO/IEC JTC1 standards body and later the Ecma International to formalize Java, but it soon withdrew from the process. Java remains a de facto standard, controlled through the Java Community Process. At one time, Sun made most of its Java implementations available without charge, despite their proprietary software status. Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System. Sun distinguishes between its Software Development Kit (SDK) and Runtime Environment (JRE) (a subset of the SDK); the primary distinction involves the JRE's lack of the compiler, utility programs, and header files.

On 13 November 2006, Sun released much of Java as free and open source software under the terms of the GNU General Public License (GPL). On 8 May 2007 Sun finished the process, making all of Java's core code available under free software / open-source distribution terms, aside from a small portion of code to which Sun did not hold the copyright.


Tuesday, August 31, 2010

Struts FAQs

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Q24.What are the important tags of struts-config.xml ?
  <!-- ========== Form Bean Definitions ============ -->
<form-bean name="login" type=" LoginForm" />
  <!-- ========== Global Forward Definitions ========= -->
  <!-- ========== Action Mapping Definitions ======== -->
        type="LoginAction" >
<!-- ========== Properties Definitions ============ -->
<message-resources parameter="MessageResources" />
<!-- ========== Validator framework Definitions ============ --> 
<plug-in className="org.apache.struts.validator.ValidatorPlugIn"> 
Q25. What are the core classes of the Struts Framework?

A: Core classes of Struts Framework are ActionForm, Action, ActionMapping, Action Forward, ActionServlet etc. 

Q26. What is action mappings?

         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.

Q27. Describe validate() and reset() methods ?
          validate () and reset() methods defined inActionForm class.

validate() : Used to validate properties after they have been populated; Called before FormBean is handed to Action. Returns a collection of ActionMessage as ActionErrors. Following is the method signature for the validate() method.

public ActionErrors validate(ActionMapping mapping, HttpServletRequest request)

reset(): reset() method is called by Struts Framework with each request that uses the defined ActionForm. The purpose of this method is to reset all of the ActionForm's data members prior to the new request values being set. 

public void reset() {}

Q28. Give the Details of XML files used in Validator Framework?

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. 

Q29. How you will enable front-end validation based on the xml in validation.xml?

The <html:javascript> tag to allow front-end validation based on the xml in validation.xml. For  example the code: <html:javascript formName="logonForm" dynamicJavascript="true" staticJavascript="true" /> generates the client side java script for the form "logonForm" as defined in the validation.xml file. The <html:javascript> when added in the jsp file generates the client site validation script.

Q30. What is the difference between perform() and execute() methods?

perform() method defined in Struts 1.0. but it is 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 one is available.

Q31. What are the various Struts tag libraries?

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 

Q32. What are the difference between <bean:message> and <bean:write>?

<bean:message>: This tag is used to output locale-specific text (from the properties files) from a MessageResources bundle.

<bean:write>: This tag is used to output property values from a bean. <bean:write> is a commonly used tag which enables the programmers to easily present the data.

Q33. What are difference between ActionErrors and ActionMessage?

      ActionMessage: A class that encapsulates messages. Messages can be either global or they are specific to a particular bean property.

Each individual message is described by an ActionMessage object, which contains a message key (to be looked up in an appropriate message resources database), and up to four placeholder arguments used for parametric substitution in the resulting message.

         ActionErrors: A class that encapsulates the error messages being reported by the validate() method of an ActionForm. Validation errors are either global to the entire ActionForm bean they are associated with, or they are specific to a particular bean property (and, therefore, a particular input field on the corresponding form). 

Q34. What is the use of ForwardAction?

The ForwardAction class is useful when you’re trying to integrate Struts into an existing application that uses Servlets to perform business logic functions. You can use this class to take advantage of the Struts controller and its functionality, without having to rewrite the existing Servlets. Use ForwardAction to forward a request to another resource in your application, such as a Servlet that already does business logic processing or even another JSP page. By using this predefined action, you don’t have to write your own Action class. You just have to set up the struts-config file properly to use ForwardAction.

Q35. What is IncludeAction?

The IncludeAction class is useful when you want to integrate Struts into an application that uses Servlets. Use the IncludeAction class to include another resource in the response to the request being processed. 

Q36. What are the steps need to use DynaActionForm?

Using a DynaActionForm instead of a custom subclass of ActionForm is relatively straightforward. You need to make changes in two places:
In struts-config.xml: change your <form-bean> to be an org.apache.struts.action.Dyna ActionForm instead of some subclass of ActionForm

  • <form-bean name="loginForm"   type="org.apache.struts.action.DynaActionForm" >
        <form-property name="userName" type="java.lang.String"/>
        <form-property name="password" type="java.lang.String" />

  • In your Action subclass that uses your form bean:
    • import org.apache.struts.action.DynaActionForm
    • downcast the ActionForm parameter in execute() to a DynaActionForm
    • access the form fields with get(field) rather than getField()
Q.37 In struts what happens if made any changes in actionservlet?

The ActionServlet plays the role of controller wich is responsible for handling the request and selecting the correct Application Module and storing ApplicationConfig and MessageResource bundle in the request object.

If we modify the ActionServlet the Controller may or may not work what happens that depends on your modification, You have not specify whether you want to create your own custom ActionServlet by extending ActionServlet and overriding the methods in it or what exactly you want to modify.

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Struts FAQs

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Q13. How you will handle exceptions in Struts?
In Struts you can handle the exceptions in two ways:

   a) Declarative Exception Handling: You can either define global exception handling tags in your struts-config.xml or define the exception handling tags within
<action>..</action> tag.


   b) Programmatic Exception Handling: Here you can use try{}catch{} block to handle the exception.

Q14. What are the different kinds of actions in Struts?
The different kinds of actions in Struts are:

ForwardAction,   IncludeAction,   DispatchAction,  LookupDispatchAction,    SwitchAction  

Q15. What is DispatchAction?
       The DispatchAction class is used to group related actions into one class. Using this class, you can have a method for each logical action compared than a single execute method. The DispatchAction dispatches to one of the logical actions represented by the methods. It picks a method to invoke based on an incoming request parameter. The value of the incoming parameter is the name of the method that the DispatchAction will invoke.

Q16. How to use DispatchAction? 

To use the DispatchAction, follow these steps :
  1. Create a class that extends DispatchAction (instead of Action)
  2. In a new class, add a method for every function you need to perform on the service – The method has the same signature as the execute() method of an Action class.
  3. Do not override execute() method – Because DispatchAction class itself provides execute() method.
  4. Add an entry to struts-config.xml

    Q17. What is LookupDispatchAction? 

    The LookupDispatchAction is a subclass of DispatchAction. It does a reverse lookup on the resource bundle to get the key and then gets the method whose name is associated with the key into the Resource Bundle.

    Q18. What is the use of LookupDispatchAction?
    LookupDispatchAction is useful if the method name in the Action is not driven by its name in the front end, but by the Locale independent key into the resource bundle. Since the key is always the same, the LookupDispatchAction shields your application from the side effects of I18N.

    Q19. What is difference between LookupDispatchAction and DispatchAction?
    The difference between LookupDispatchAction and DispatchAction is that the actual method that gets called in LookupDispatchAction is based on a lookup of a key value instead of specifying the method name directly.

    Q20. What is SwitchAction?
    The SwitchAction class provides a means to switch from a resource in one module to another resource in a different module. SwitchAction is useful only if you have multiple modules in your Struts application. The SwitchAction class can be used as is, without extending.

    Q21. What if <action> element has <forward> declaration with same name as global forward? 

    In this case the global forward is not used. Instead the <action> element’s <forward> takes precendence.

    Q22. What is difference between ActionForm and DynaActionForm?
    An ActionForm represents an HTML form that the user interacts with over one or more pages.
    You will provide properties to hold the state of the form with getters and setters to access them. Whereas, using DynaActionForm there is no need of providing properties to hold the state. Instead these properties and their type are declared in the struts-config.xml.

    The DynaActionForm bloats up the Struts config file with the xml based definition. This gets annoying as the Struts Config file grow larger.

    The DynaActionForm is not strongly typed as the ActionForm. This means there is no compile time checking for the form fields. Detecting them at runtime is painful and makes you go through redeployment.
    ActionForm can be cleanly organized in packages as against the flat organization in the Struts Config file.
    ActionForm were designed to act as a Firewall between HTTP and the Action classes, i.e. isolate and encapsulate the HTTP request parameters from direct use in Actions. With DynaActionForm, the property access is no different than using request.get Parameter( .. ).
    • DynaActionForm construction at runtime requires a lot of Java Reflection (Introspection) machinery that can be avoided.
    Q23. What is the life cycle of ActionForm?
    The lifecycle of ActionForm invoked by the RequestProcessor is as follows:
    • Retrieve or Create Form Bean associated with Action
    • "Store" FormBean in appropriate scope (request or session)
    • Reset the properties of the FormBean
    • Populate the properties of the FormBean
    • Validate the properties of the FormBean
    • Pass FormBean to Action

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      Struts FAQs

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       Q12. What is RequestProcessor?
      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.


      All request URIs with the pattern *.do are mapped to this servlet in the deployment descriptor as follows.


                A request URI that matches this pattern will have the following form.


      The preceding mapping is called extension mapping, however, you can also specify path mapping where a pattern ends with /* as shown below.


      A request URI that matches this pattern will have the following form.

      http://localhost:8080/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 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.

              path= "/somerequest"
          <forward name="Success" path="/action/xys" redirect="true"/>
          <forward name="Failure" path="/somejsp.jsp" redirect="true"/>

        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.

              path= "/somerequest"
          <forward name="Success" path="/action/xys" redirect="true"/>
          <forward name="Failure" path="/somejsp.jsp" redirect="true"/>

       The action forward mappings also can be specified in a global section, independent of any specific action mapping.

          <forward name="Success" path="/action/somejsp.jsp" />
          <forward name="Failure" path="/someotherjsp.jsp" />

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      Struts FAQs

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      Q 1. What is MVC?
      Model-View-Controller (MVC) is a design pattern put together to help control change. MVC decouples interface from business logic and data.

      Model: The model contains the core of the application's functionality. The model enca psulates the state of the application. Sometimes the only functionality it contains is state. It knows nothing about the view or controller.

      View: The view provides the presentation of the model. It is the look of the application. The view can access the model getters, but it has no knowledge of the setters. In addition, it knows nothing about the controller. The view should be notified when changes to the model occur.

      Controller: The controller reacts to the user input. It creates and sets the model. 

      Q 2. What is a framework?
      Framework is made up of the set of classes which allow us to use a library in a best possible way for a specific requirement.

      Q 3. What is Struts framework?

      Struts framework is an open-source framework for developing the web applications in Java EE, based on MVC-2 architecture. It uses and extends the Java Servlet API. 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. Struts provides its own Controller component and integrates with other technologies to provide the Model and the View. 

      For the Model, Struts can interact with standard data access technologies, like JDBC and EJB, as well as most any third-party packages, like Hibernate, iBATIS, or Object Relational Bridge. For the View, Struts works well with JavaServer Pages, including JSTL and JSF, as well as Velocity Templates, XSLT, and other presentation systems.

      Q 4. What is Jakarta Struts Framework

      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.

      Q 5. What is ActionServlet?

      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.

      Q 6. What is role of ActionServlet?
      ActionServlet performs the role of Controller:
      • Process user requests
      • Determine what the user is trying to achieve according to the request
      • Pull data from the model (if necessary) to be given to the appropriate view,
      • Select the proper view to respond to the user
      • Delegates most of this grunt work to Action classes
      • Is responsible for initialization and clean-up of resources
      Q 7.  What is Action Class?

      Any java class which extends from org.apache.struts.action.Action is called Action class. 

      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..

      Q 8. Write code of any Action Class?

      import javax.servlet.http.*;
      import org.apache.struts.action.*;
      public class TestAction extends Action
      public ActionForward execute(ActionMapping mapping, ActionForm form,
       HttpServletRequest request,HttpServletResponse response) throws Exception 
            return mapping.findForward("success");

      Q 9. What is ActionForm?

                  Any java class which extends from org.apache.struts.action.ActionForm is called ActionForm. An ActionForm is also called JavaBean. 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.
      Q10. What is Struts Validator Framework?

      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. 

      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.

      Q11. How you will display validation fail errors on jsp page?

      Following tag displays all the errors:


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      Collections FAQs

      Q51. What is TreeMap?

       TreeMap can be used to store a group of objects as key-value pairs where all the entries are arranged according to some sorting order of keys.
      • The underlying data structure is RED-BLACK Tree
      • Duplicates keys are not allowed but values can be duplicated.
      • Insertion order is not preserved because insertion is based on some sorting order
      • If we are depending on Natural sorting order then keys should be homogeneous(violation leads to ClassCastException)  but values need not homogeneous
      • In case of customized sorting order we can insert  heterogeneous keys and values
      • For empty TreeMap as first entry with null values are allowed but after inserting that entry if we are trying to insert any other entry we will get NullPointerException
      • For non empty TreeMap if we are trying to insert null keys we will get NullPointerException
      • There are no restrictions for null values.
        Q52. What is Hashtable
           Hashtable is a legacy Map and can be used to store objects as key value pairs.
      • The underlying data sturucture is Hashtabe
      • Duplicates keys are not allowed but duplicate values are allowed
      • null insertion is not possible for both keys and values
      • all methods are synchronized
      • insertion order is not preserved because it is  based on hashcode  of keys
      • heterogeneous Objects are allowed for both keys and values
      • introduced in 1.0 version it is legacy class
      Q53. What is PriorityQueue?

      It represents a data structure to hold group of individual objects prior to processing based on some priority .it can be natural sorting order and it can be customized sorting order described by Comparator.
      It is the implementation class of Queue interface.
      • Insertion order is not preserved because here insertion is done based on some sorting order
      • Duplicates are not allowed
      • null insertion is not possible even as first element also
      •  If we are depending on natural sorting order Objects should be homogeneous  violation leads to ClassCastException
      •  If we are depending on customized  sorting order Objects can be heterogeneous also.
      Q54. What is Arrays class?
      • It is utility class for arrays.
      • It defines several utility methods for arrays like sorting an array or searching an element in array
      • present in java.util package
      Q55. We are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList?


      Q56. Why ArrayList is faster than Vector?  

      All methods present in the Vector are synchronized  and hence  any method can be executed by only one thread at a time. It slows down the execution.
      But in ArrayList,  no method is synchronized and hence multiple thread are allowed execute simultaneously which speed up the execution.

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      Collections FAQs

      Q41. What is difference between HashSet and TreeSet?

      1The underlying data structure is Hashtable 1The underlying data structure is balanced tree
      2Heterogeneous objects are allowed 2       Heterogeneous objects are not allowed  bydefalut
      3Insertion order is not preserved and it is based on hashcode of the objects 3   Insertion order is not preserved and all the objects are inserted according to some sorting order.
      4null insertion is possible 4   As the first element only null insertion is   possible and in all other cases we will get NullPointerException

      Q42. What is Entry interface?

      It is inner interface of Map.
      In the Map each key value pair is considered as Entry object.

      interface Map
                      //more code here
         interface Entry
                                      Object  getKey()
                                      Object  getValue()
                                      Object  setValue(Object new)

      Q43. Explain about HashMap?

      It is a Map Object which can be used used to represent a group of objects as key-value pairs.
      • The underlying data structure is Hashtable
      • Duplicaes keys are not allowed duplicate values are allowed
      • Insertion order is not preserved because insertion is based on hashcode of keys.
      • Heterogeneous objects are allowed for  both keys and values
      • null key is allowed  only once
      • null values  are allowed multiple times
      • Introduced in 1.2 version
      Q44. Explain about LinkedHashMap?

      It is child class of HashMap. It is exactly same as HashMap except the following difference.
      In the case of HashMap the insertion order is not preserved but in the case of LinkedHashMap insertion order is preserved. Introduced in 1.4 version

      Q45. Differences between HashMap and LinkedHashMap ?

      1.The underlying data structure is Hashtable 1.The underlying data structure is a combination of Hashtable and linkedlist
      2.Insertion order is not preserved and it is based on hashcode of keys 2    Insertion order is preserved
      3.Introduced in 1.2 version 3   Introduced in 1.4 version.

      Q46. Differences between HashMap and Hashtable?

      1.The underlying data structure is Hashtable 1.The underlying data structure of Hashtable
      2.No method is synchronized and hence HashMap object is not thread safe 2 .All methods are synchronized and hence it is   thread safe
      3.Performance is high 3.   Performance is low
      4.null insertion is possible for both keys and values 4.   null insertion is not possible for both key and value violation leads to NullPointerException
      5.Introduced in 1.2 version and it is non legacy 5.   Introduced in 1.0 version and it is legacy

      Q47. What is IdentityHashMap?

      It is exactly same as HashMap except the following difference.
           In the HashMap JVM uses equals() method to identify duplicate keys  but in the  case of IdentityHashMap JVM uses == operator for this.

      Q48. What is difference between HashMap and IdentityHashMap?
           Refer Q47 for the answer.

      Q49. What is WeakHashMap?

        It is exactly same as HashMap except the following difference.
       In case of HashMap an Object is not eligible for garbage collection if it is associated with HashMap even though it dosent have any external references.  ie HashMap dominates garbage collector.
      But in case of WeakHashMap , if an Object is not having any external references then it is always eligible for garabage collectoion even though it is associated with weakHashMap.  ie  garbage collector dominates WeakHashMap

      Q50. What is difference between HashMap and WeakHashMap?

        Refer Q49 for the answer.

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      Collections FAQs

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      Q31. Explain about HashSet class?
      • The underlying data structure is Hashtable
      • null values are accepted
      • duplicates are not allowed
      • insertion order is based on hashcode of the object hence insertion order is not preserved
      • best  suitable if frequent operation is  search operations
      • HashSet  class implements Serializable and Cloneable
      • it is implementation class for Set interface
      • heterogeneous objects are allowed
      • it is introduced in 1.2 version
      Q32. If we are trying to insert duplicate values in Set what will happen?

       If we are trying to insert duplicate objects to the HashSet  , we wont get any compile time or run time errors just the add(Object o) returns false and it doesn’t add that object.

      Q33. What is LinkedHashSet?

      It is the child class of HashSet. The main difference between HashSet and LinkedHashSet is:
      In the case of HashSet insertion order is not preserved , but in the case of LinkedHashSet insertion will be preserved.

      Q34. Differences  between HashSet and LinkedHashSet?

      1The Underlying datastructure is Hashtable 1The underlying datastructure is combination of LinkedList and Hashtable
      2Insertion Order is not preserved 2     Insertion order is preserved.
      3Introduced in 1.2 version 3     Introduced in 1.4 version

      Q35. What are major enhancements in 1.4 version of collection frame work?


      Q36. Explain about TreeSet?
       It is Collection object which can be used to represent a group of objects according to some sorting order.
      • The underlying datastructure is Balanced tree
      • Duplicates are not allowed
      • All objects are stored according to some sorting order hence insertion order is not preserved
      • Heterogeneous objects are not allowed violation leads to ClassCastException
      • For an Empty TreeSet as firs element null value can be inserted but after inserting that first value if we are trying to insert any other objects then we will get NullPointerException
      • For an non empty TreeSet if we are trying to  inser null value at run time u will get NullPointerException

      Q37. What are differences between List and Set interfaces?

      1Insertion Order is preserved 1Insertion Order is not preserved
      2Duplicate Objects are allowed 2     Duplicate Objects are not allowed
      3The implemented classes are ArrayList,LinkedList , Vector and Stack classes 3   The implemented classes are HashSet,            LinkedHashSet and Tree

      Q38. What is Comparable interface?
      • This interface can be used for defining natural sorting order of the objects.
      • It is present in java.lang package
      • It contains a method public int compareTo(Object obj1)
      Q39. What is Comparator interface?
      • This interface can be used for implementing customized sorting order.
      • It is present in java.util package
      • It contains two methods
        • public int compare(Object ,Object)
        • public boolean equals(Object)

      Q40. What are differences between Comparable and Comparator?
      1 This can be used for natural sorting order 1This can be used for implementing customized sorting
      2 This interface present in java.lang package 2     This is present in java.util package
      3 Contains only one method: public int compareTo(Object obj1) 3     It contains two methods.
      public int compare(Object ,Object)
      public Boolean equals(Object)
      4   It is marker interface 4  It is not a marker interface.

      Collections FAQs

      Q21. What is difference between ArrayList and Vector?

      1. No method is synchronized in the ArrayList class 1. All methods in Vector are synchronized.
      2. ArrayList object is not thread safe. 2.  Vector is thread safe.
      3. Relatively performance is high 3. Relatively performance is low
      4. Introduced in 1.2 version and it is non legacy 4. Introduced in 1.0 version and it is legacy

      Q22. How we can get synchronized version of ArrayList?

      Collections class contains synchronizedList() method for this
                      Public static List synchronizedList(List l)
                      EX                 ArrayList l= new  ArrayList();
                                        List l2=Collections.synchronizedList(l);

        Similarly we can get synchronized versions of Set and Map objects by the following methods.
      Public static List synchronizedSet(Set s)
      Public static List synchronizedMap(Map m)

      Q23. What is difference between size and capacity of a Collection Object?

      size means number  of objects present  where as capacity means no of objects it can accommodate.

      Q24. What is difference between ArrayList and Linked List?

      1. The underlying data structure is resizable or growable array. 1. The underlying data structure is Double Linked List.
      2.  This is Best choice if frequent operation is retrieval and worst choice if frequent operation is insertion or deletion in the middle. 2.  This is Best choice  if frequent operation is insertion or deletion in the middle and worst choice if frequent operation is retrieval .
      3. This class implements Serializable , Cloneable and RandomAccess interfaces. 3. This class implements Serializable , Cloneable but not  RandomAccess interface.

      Q25. What are legacy classes and interfaces present  in Collections framework ?
      • Enumeration ---Interface
      • Dictonary ------Abstract class
      • Hashtable -----Concrete class
      • Properties -----Concrete class
      • Vector -----Concrete class
      • Stack  -----Concrete class
       Q26. what is difference Enumeration and Iterator?

      1. It is legacy interface and introduced in 1.0 version 1 It is non-legacy and introduced in 1.2 version
      2.Applicable only for legacy classes and it is not universal cursor 2.Applicable for any Collection implemented class object.
      3.While iterating the elements we are not allowed to remove the objects just we can perform only read operation 3.While iterating we can perform removal also in addition to read operation.
      4.By using elements() method we can get Enumeration object 4.By using iterator() method we can get Iterator

      Q27. What are limitations of Enumeration?
      • While iterating the elements we are not allowed to perform removal operation
      • It is applicable only for legacy classes and it is not a universal cursor.
      • It can retrieve the elements only in forward direction
        Q28. What is difference between enum and Enumeration?

      An enum can be used to define a group of named constants .It has  introduced in 1.5 version

      Ex                  Class Beer

      Enumeration is cursor to retrieve Objects one by one from Collection objects.

      Q29. What is difference between Iterator and ListIterator?
      • ListIterator is the child interface of the Iterator
      • Iterator is the single direction cursor where as ListIterator is bidirectional cursor.
      • While iterating the elements by Iterator we can perform only read and remove operations. But by using ListIterator we can perform read,removal, replace and addition of new objects also.
      • Iterator is applicable for every Collecton implemented class object but ListIterator  is applicable only for List implemented class objects.
      • Iterator can be get by using iterator() of Collection interface where as ListIterator can be get by using listIterator() method of List interface
      • both are introduced in 1.2 version
      Q30. What is relation between ListIterator and Iterator?
            ListIterator is child interface of Iterator 

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