Boolean wrapper class example programs

Posted by: Instanceofjava Posted date: 12:31 AM / comment : 0

  • java.lang.Boolean Class is used to represent primitive boolean value in the form of object.

  1.  public final class Boolean
  2.    extends Object
  3.       implements Serializable, Comparable<Boolean>
     

Boolean Class Constructors:

1. public Boolean(boolean value)

  1. Boolean obj=new Boolean(true);

Java code to convert boolean primitive to Boolean object
  1. package com.instanceofjavatutorial;
  2.  
  3. public class BooleanDemo {
  4.  
  5. public static void main(String[] args) {
  6.  


  9.  Boolean obj=new Boolean(false);
  10.  
  11.  System.out.println(obj);
  12.  
  13.  
  14. }
  15. }

Output:
  1. true

2. public Boolean(String s)

  1. Boolean b=new Boolean("true");

Java Program to Convert String Object to Boolean Object

  1. package com.instanceofjavatutorial;
  2.  
  3. public class BooleanDemo {
  4.  
  5. public static void main(String[] args) {
  6.  
  7.  String str="true";

  9.  Boolean obj=new Boolean(str);
  10.  
  11.  System.out.println(obj);
  12.  
  13.  
  14. }
  15. }

Output:
  1. true

Boolean class Methods:

1.public boolean booleanValue()

  • This method gives the boolean primitive value from Boolean object
  • See below java code which explains how to get boolean primitive value and assig to primitive variable from Boolean Object.

Java Program To Convert boolean primitive value to Boolean Object and To get boolean value from Boolean Object

  1. package com.instanceofjavatutorial;
  2.  
  3. public class BooleanDemo {
  4.  
  5. public static void main(String[] args) {
  6.  
  7. // converting boolean variable in to Boolean Object
  8.  Boolean bobj=new Boolean(true);
  9.  
  10.  System.out.println(bobj);
  11.  
  12. //converting Bolean object into boolean primitive value
  13.  boolean bval=bobj.booleanValue();
  14.  
  15.  System.out.println(bval);
  16.  
  17. }
  18. }

Output:
  1. true
  2. true

Google IO 2015 Main Features

Posted by: Instanceofjava Posted date: 11:53 PM / comment : 0

  • Google announced a new smart home platform called Brillo on stage at its annual Google I/O developers 
  • At Google I/O 2015 Google officially announced Project Brillo, a new platform for the Internet of Things.
watch live HereGoogle IO 2015

Whats NEW????.... It's a LOT !!!!

  • Google announced a new smart home platform called Brillo on stage at its annual Google I/O developers 
  • At Google I/O 2015 Google officially announced Project Brillo, a new platform for the Internet of Things.
  • This is one of the biggest announcements from today's developer conference. Unlimited photo and video
  • Google Maps, including turn-by-turn voice directions, to be available offline.. 
  • Android M is big brother for sure. Google's latest version of Android will remind you to pick up your dry 
  • Have you tried this fun Google trick yet? #YouNeedToTryThis #BarrelRoll #Google #Funfact  
  • Google Cardboard received a big update today and it's also compatible with iOS now.
  • Google is adding a family friendly section to Google Play! 
  • Google I/O 2015 Keynote slides look pretty sleek and consistent - good to see! 
  • Google reveals their second virtual reality headset for phones, a larger Google Cardboard. Clay Bavor of Google 

public static void main(String[] args) in Java

Posted by: Instanceofjava Posted date: 4:29 PM / comment : 4 ,

  • main method is a standard method used by JVM to execute any java program(Java SE).
  • Lets see an example class without main method.

  1. package instanceofjava;
  2. public Demo{
  3.  
  4. }


  • This class will compile fine but while executing JVM fails to find the main method in that class
  • So it will throw an exception: NoSuchMethodError:main

  1. package instanceofjava;
  2. public Demo{
  3. public static void main(String [] args){
  4.  
  5. }
  6. } 
  • This program will compile and executed. Because at run time JVM calls the main method on that class.

Why main method is public?

  •  To call by JVM from anywhere main method should be public.
  • If JVM wants to call our method outside of our package then our class method should be public.

  1. package instanceofjava;
  2. public MainDemo{
  3. public static void main(String [] args){
  4.  
  5. }
  6. }

public static void main(string args) interview Questions

Why main method is static?

  • To execute main method without creating object then the main method oshould be static so that JVM will call main method by using class name itself.

Why main method return type is void?

  • main method wont return anything to JVM so its return type is void.

 Why the name main?

  • This is the name which is configured in the JVM.

Why the arguments String[] args?

  •  Command line arguments.
  • String array variable name args : by the naming conventions they named like that we can write any valid variable name.

Can we write static public void main(String [] args)?

  • Yes we can define main method like static public void main(String[] args){}
  •  Order of modifiers we can change.

  1. package instanceofjava;
  2. public MainDemo{
  3. static public void main(String [] args){
  4.  
  5. }
  6. }

What are the possible public static void main(String [] args)method declaration with String[] args?

  • public static void main(String[] args){ }
  • public static void main(String []args){ }
  • public static void main(String args[] ){ }
  • public static void main(String... args){ } 
  • Instead of args can place valid java identifier.

Can we declared main method with final modifier?

  • Yes we can define main method with final modifier.

  1. package instanceofjava;
  2. public MainDemo{
  3. public static final void main(String [] args){
  4.  
  5. }
  6. } 

Main method with all possible modifiers:

 

  1. package instanceofjava;
  2. public MainDemo{
  3. static final synchronized public void main(String [] args){
  4.  
  5.  System.out.println("Main method");
  6.  
  7. }
  8. } 

Output:

  1. Main method

Overloading main method:

  • We can declare multiple main methods with same name as per overloading concept.  
  • So we can overload main method but every time JVM looks for main method with String[] args method.
  • Lets see an example program on main method overloading.
  • main(String[] args) method will be called automatically by JVM.
  • If we want to call other methods we need to call explicitly.

 

  1. package instanceofjava;
  2. public MainDemo{
  3. public static void main(String [] args){
  4.  
  5.  System.out.println("Main method String [] args");
  6.  
  7. } 
  8. public static void main(int[] args){
  9.  
  10.  System.out.println("Main method int[] args");
  11.  
  12. }
  13.  
  14. } 





Output:

  1. Main method String [] args


Main method Inheritance:

  • Inheritance concept is applicable for main method.
  • If a sub class not having main method and extending a super class which is having main method.
  • Then if we execute super class main method will be executed fine.
  • If we execute sub class which is not having main method also executed fine and super class main method will be called.

  1. package instanceofjava;
  2. public A{
  3. public static void main(String [] args){
  4.  
  5.  System.out.println("Main method of class A");
  6.  
  7. } 
  8.  
  9. } 



Output:

  1. Main method of class A

 

  1. package instanceofjava;
  2. public B extends A{
  3.  
  4. } 
Output:

  1. Main method of class A

What happen if both super and sub class having main method?

  • Actually its not overriding concept here. it is a method hiding concept in this scenario regarding with main method.

  1. package instanceofjava;
  2. public A{
  3. public static void main(String [] args){
  4.  
  5.  System.out.println("Main method of class A");
  6.  
  7. } 
  8.  
  9. } 

Output:

  1. Main method of class A


  1. package instanceofjava;
  2. public B extends A{
  3. public static void main(String [] args){
  4.  
  5.  System.out.println("Main method of class B");
  6.  
  7. } 
  8.  
  9. } 


Output:

  1. Main method of class B

Static block in Java

Posted by: Instanceofjava Posted date: 3:08 PM / comment : 0 ,

  • Static variables are class level variables and without instantiating class we can access these variables.

    1. Static int a;


  • Class loading time itself these variables gets memory
  • Static methods are the methods with static keyword are class level. without creating the object of the class we can call these static methods.

    1. public static void show(){ 
    2.  
    3. }

  • Now its time to discuss about static blocks.
  • Static block also known as static initializer
  • Static blocks are the blocks with static keyword.
  • Static blocks wont have any name in its prototype.
  • Static blocks are class level.
  • Static block will be executed only once.
  • No return statements.
  • No arguments.
  • No this or super keywords supported.
    •  
    1. static{ 
    2.  
    3.  }

     

What is the need of static block?

  • Static blocks will be executed at the time of class loading.
  • So if you want any logic that needs to be executed at the time of class loading that logic need to place inside the static block so that it will be executed at the time of class loading. 
  • To initialize the static variables while class loading itself we need static block.
  • If we have multiple classes then if you want to see the order of those classes loading then static block needed.

When to use static blocks?

  • If you want to access static variables before executing the constructor.
  • If you want to execute your code only once even any number of objects created.
  • If you want to initialize static constants.

When and where static blocks will be executed?

  • Static blocks will be executed at the time of class loading by the JVM  by creating separate stack frames in java stacks area (Please refer JVM Architecture  to know about stacks area).
  • Static blocks will be executed in the order they defined from top to bottom.



  1. package com.instanceofjava;
  2.  
  3. class StaticBlockDemo 
  4. {
  5.  
  6. static{
  7.       System.out.println("First static block executed");
  8. }
  9.  
  10. static{
  11.      System.out.println("Second static block executed");
  12. }
  13.  
  14. static{
  15.      System.out.println("Third static block executed");
  16. }
  17.  
  18. }
     
Output:
  1. First static block executed
  2. Second static block executed
  3. Third static block executed

Order of execution of static block and main method:

  1. package com.instanceofjava;
  2.  
  3. class StaticBlockDemo 
  4. {
  5.  
  6. public static void main (String[] args) throws java.lang.Exception
  7. {
  8.         System.out.println("Main method executed");
  9. }
  10.  
  11. static{
  12.       System.out.println("First static block executed");
  13. }
  14.  
  15. static{
  16.      System.out.println("Second static block executed");
  17. }
  18.  
  19. static{
  20.      System.out.println("Third static block executed");
  21. }
  22.  
  23. }
     
Output:
  1. First static block executed
  2. Second static block executed
  3. Third static block executed
  4. Main method executed


Order of execution of Static block and constructor:

 

  1. package com.instanceofjava;
  2.  
  3. class StaticBlockDemo 
  4. {
  5.  
  6. StaticBlockDemo(){
  7.  
  8.   System.out.println("Constructor executed");
  9.  
  10. }
  11. public static void main (String[] args) throws java.lang.Exception
  12. {
  13.  
  14.     System.out.println("Main method executed");
  15.     StaticBlockDemo obj = new StaticBlockDemo();
  16.  
  17. }
  18.  
  19. static{
  20.       System.out.println("First static block executed");
  21. }
  22.  
  23. static{
  24.      System.out.println("Second static block executed");
  25. }
  26.  
  27. static{
  28.      System.out.println("Third static block executed");
  29. }
  30.  
  31. }
     


Output:
  1. First static block executed
  2. Second static block executed
  3. Third static block executed
  4. Main method executed
  5. Constructor executed

What is the output of following program:


  1. package com.instanceofjava;
  2.  
  3. class StaticBlockDemo 
  4. {
  5.  
  6.   static int a=getNum();

  8. public static int getNum(){
  9.  
  10. System.out.println("In method m1()");
  11. return 10;
  12.  
  13. }

  15. public static void main (String[] args) throws java.lang.Exception
  16. {
  17.  
  18.     System.out.println("Main method executed");
  19.   
  20.  
  21. }
  22.  
  23. static{
  24.       System.out.println("First static block executed");
  25. }
  26.  
  27. static{
  28.      System.out.println("Second static block executed");
  29. }
  30.  
  31. static{
  32.      System.out.println("Third static block executed");
  33. }
  34.  
  35. }
     
Output:
  1. In method m1()
  2. First static block executed
  3. Second static block executed
  4. Third static block executed
  5. Main method executed


Java Jsp Interview Questions

Posted by: Instanceofjava Posted date: 10:14 PM / comment : 0 , ,

1.Explain JSP life Cycle methods?

  • jsp engine controls the life cycle of jsp
  • Jsp life cycle is described by three life cycle methods and six life cycle phases


Jsp life cycle methods :

  • jspInit()
  • _jspService()
  • jspDestroy()


  • Jspinit() method is invoked when JSP page is initialized.
  • The _jspService() method corresponds to the body of the jsp page.
  • This method is defined automatically by the JSp container and should never be defined y the JSP page author.

Jsp Life cycle Phases: 

  • Translation phase
  • Compilation phase
  • Instantiation phase
  • initialization phase
  • Servicing phase
  • Destruction phase

Read More at : JSP Life Cycle

2. What are the Jsp Implicit Objects?


  • Implicit objects in jsp are the objects that are created by the container automatically and the container makes all these 9 implicit objects to the developer.
  • We do not need to create them explicitly, since these objects are created and automatically by container and are accessed using standard variables(objects) are called implicit objects.
  • The following are of implicit variables available in the JSP.  


  1. out
  2. request
  3. response
  4. config
  5. application
  6. session
  7. pagecontext
  8. page
  9. exception
Read More at: 9 Jsp Impicit Objects in java

3. Explain Jsp Scripting Elements?

  • Jsp scripting elements are used to embed jva code in to the jsp.
  • Jsp scripting elements are three types
    1. Declaration
    2. Expression
    3. Scriplet
Read More at: Jsp Scripting Elements



4. What are the Directives in Jsp?

  • A jsp directive is a translation time instruction to the jsp engine.
  • we have three kinds of directives.
  1. Page directive
  2. Include directive
  3. Taglib directive

 Read More at: Jsp Directive Elements


 5. What are the differences between include directive and include action?


Java Jsp interview Questions

 

6. How to disable session in JSP?

  • By using page directive
  • <%@ page session="false" %> 

7. How can we handle exceptions in a JSP page?

  • We can handle exception in jsp by using error page element of page directive 

  • The errorPage attribute tells the JSP engine which page to display if there is an error while the current page runs.
    The value of the errorPage attribute is a relative URL.
    The following directive displays MyErrorPage.jsp when all uncaught exceptions are thrown

    Syntax:

    errorPage="url"

    Example:

    <%@ page errorPage="error.jsp"%>

8.How can we override jspInit() and jspDestroy() methods in a jsp?

  • Both methods are executed once in a life cycle of a jsp.
  • We can override these two method as shown below.
  • <%!
        public void jspInit() {
            . . .
        }
    %>
  • <%!    
        public void jspDestroy() {
            . . .   
        }
    %>

9.Can a JSP extend a java class?

  • Yes we can extend a java class in jsp
  • <%@ include page extends="classname" %>
  • We can do this because jsp will convert to a servlet so a servlet can extend a class its fine

10.How can we pass information from one jsp to included jsp page?

  • <jsp:include page="employeedetail.jsp" flush="true">
  • <jsp:param name="name" value="abc"/>
  • <jsp:param name="id" value="220"/>

Java Servlet Interview Questions

Posted by: Instanceofjava Posted date: 10:51 PM / comment : 2 , ,

 

1.Explain Servlet life cycle in java.

  • Servlet engine controls the life cycle of servlet
  • Servlet life cycle is described by three life cycle methods under 5 life cycle phases
  • life cycle methods are
  1. init(ServletConfig obj)
  2. service(servletRequest, servletResponse)
  3. destroy() 

  • When ever somethings happens in the life of servlet. servlet engine calls these methods on the servlet instance and hence the name.

Read more here: Servlet Life Cycle


2.What is ServletConfig ?

  • Request parameters are used by the servlet to receive data from client to process the request.
  • Some specific data must be supplied to the servlet at the time of initialization of the servlet and this data is specific to the client.But data is not supplied by the client via request parameters
  • Use initialization parameters to supply the data to the servlet at the time of initialization of the servlet. initialization parameters are set in deployment descriptor   (Web.xml).
  • Servlet can access these parameters during the run time
  • Request parameters change form Request to request 
  • Initialization parameters change from servlet to servlet.

Read more at : ServletConfig

3. What is ServletContext?

  • We can deploy multiple web applications in a servlet container.
  • Each web application contains its own resources in a separate  environment. This environment called ad Web Application context or ServletContext.
  • The resources belongs to the one web application context are not available to the other web application context.
  • A Servlet context contains zero or more number of servlets. For every servlet object the container creates separate ServletConfig object.
Read More at : ServlectContext


4. Can we define a Constructor in servlet?

  • Yes we can define a constructor in servlet but we can not call the constructor explicitly because servlet container will create the object of servlet so it will be called by the servlet container.

5. Can we call destroy() method inside init() method of a servlet. If yes what will happen?

  • Yes we can call destroy() method inside a init() method.
  • Actually container will call destroy() method if we call that method explicitly nothing will happen.
  • If we override destroy() method and called from init(). method will be called and code will be executed.

6.What are the difference between GenericServlet and HttpServlet?

GenericServlet HttpServlet
Abstract Class Abstract Class
Protocol Independent Http Protocol Dependent
Subclass of Servlet Subclass of GenericServlet
public void service(ServletRequest req,ServletResponse res ). Supports public void service() and protected void service(), doGet(),doPost(),doPut(),doDelete(),doHead(),doTrace(),doOptions()etc.


7.What are the differences between doGet() and doPost()?



doGet() doPost()
protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, java.io.IOException Protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
handles client's GET request handles client's POST request
Request parameters appended to the URL and sent along with header information Request parameters are submitted via form.
Request Parameters are not encrypted Request Parameters are encrypted
Maximum size of data that can be sent using doget() method is 240 bytes There is no maximum size for data.




8. Can we call a servlet from another servlet?

  • Yes. We can call a servlet from another servlet this is known as inter servlet communication.
  • By using RequestDispatcher object we can do this.
  • RequestDispatcher rd=request.getRequestDispatcher("other servlet url name");
  • rd.forward(req, res);
Read more at : RequestDipatcher in java

9. What are the differences between forward() and sendRedirect() methods?

 


forward() sendRedirect()
request.getRequestDispathcer("example.jsp").
forward(request, response);		 response.sendRedirect
("http://www.instanceofjava.com");
Used to forward the Request to resources available in within the server Used to redirect the Request to other resources or domains

10.How can you get the information about one servlet context in another servlet?

  • By using setAttribut() method we can set the data in one servlet and get in another
  • Context.setAttribute (“name”,” value”)
  • Context.getAttribute (“name”)

11.Explain Servlet architecture?

  • Frequently asking java interview question in servlets you may get questions like
  • explain the architecture of java servlet?
  • explain the architecture of java servlet with diagram?
  • define the servlet architecture?
  • servlet architecture overview?
  • Servlets read the explicit data sent by the clients (browsers). This includes an HTML form on a Web page or it could also come from an applet or a custom HTTP client program.
  • Read the implicit HTTP request data sent by the clients (browsers). This includes cookies, media types and compression schemes the browser understands, and so forth.
The Servlet architecture in Java is a Java-based framework for building web applications. It is a specification defined by the Java Community Process (JCP) and is implemented by various web containers such as Apache Tomcat, Jetty, and GlassFish.

The Servlet architecture consists of several components, including:

Servlets: These are the core components of the Servlet architecture. They are Java classes that handle HTTP requests and responses. Servlets are executed by a web container, which is responsible for managing their lifecycle, threading, and security.

Web Containers: Also known as servlet engines, web containers are responsible for managing the lifecycle of servlets, providing services such as request handling, thread management, and security.

JSP (JavaServer Pages): These are Java-based pages that are used to create dynamic web content. JSPs are compiled into servlets by the web container and executed by the servlet engine.

JSP Tag Libraries: These are collections of custom tags that can be used in JSP pages to simplify the creation of dynamic web content.

JavaBeans: These are Java classes that encapsulate business logic and data. They can be used in JSP pages to create dynamic web content.

Filters: These are Java classes that can intercept and modify the requests and responses sent to and from servlets. They can be used to implement security, logging, and other common functionality.

Listeners: These are Java classes that are notified of certain events that occur within the web container, such as a servlet being initialized or a session being created.

In summary, the Servlet architecture provides a powerful and flexible framework for building web applications in Java, by providing a set of components and services to handle HTTP requests and responses, providing a powerful and flexible way to handle web content, and providing a set of components to handle security, logging, and other common functionality.
Read More : servlet architecture with diagram
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