Multi-Threading Interview questions


Thread concept is the most important topic for all the interviews. This post contains most important interview questions on Thread concepts. The answers for these questions will be covered in further posts.

  1. What is a thread ? What are the ways we have in Java to create a thread ?
  2. Write a program to create a thread using Thread class or Runnable interface.
  3. Which is the default thread that runs in every java program internally ?
  4. What is the difference between extends Thread or implements Runnable ? Which one is more useful.
  5. What is thread deadlock? Describe it.
  6. Which methods are used in thread communication.
  7. How can you set priorities to a thread ? And, what is the default priority number of a thread ?
  8. What is a Thread Scheduler ?
  9. What is a Daemon Thread ?
  10. What id thread life cycle ? Explain.
  11. How can you improve communication between two threads?
  12. What is ThreadGroup ? What are the benefits of ThreadGroup ?
  13. What is the difference between Process and Thread ?
  14. What is the difference between notify() and notifyAll() methods ?
  15. Explain some of the methods in Thread class.

Access Specifiers in Java


An access specifier is a keyword that specifies how to access or read the members of a class or the class itself.

There are four access specifiers in Java as mentioned below:

  1. private
  2. public
  3. protected
  4. default

1. Private : Private members of a class are not accessible in other class either in the same package or in another package. The scope of private specifier is class scope.

2. Public : Public members of a class are accessible any where in the same package or another package. The scope of public specifier is Global.

3.Protected : Protected members are available in the same package. They are not available in the class of another package. You can access the protected members in sub class of same package or another package.

4. Default : Default members are available in the class of same package but they are not available in another package. The scope of default specifier is Package level.

Other Useful Links:

Avoid nested loops using Collection Framework in Java

Replace special characters in a String using java

Singleton Design pattern in JAVA

Convert Array to Vector in Java

Marker or Tag Interface in Java

equals() and hashCode() methods of Object Class

Difference between Iterator and ListIterator

Inner classes in Java

Difference between Abstract Class and Interface:

Avoid nested loops using Collection Framework in Java


High performance is essential for any software implemented in any programming language. And, loops plays major role in this regard. This post explains how to avoid the loops using Java’s Collection framework.

Below are the two Java programs to understand how the performance could be increased using the Collection framework.

Using nested loops

package in.javatutorials;

/**
* Finds out the Duplicates is String Array using Nested Loops.
*/
public class UsingNesteadLoops {
  private static String[] strArray = { "Cat", "Dog", "Tiger",     "Lion", "Lion" };

  public static void main(String[] args) {
   isThereDuplicateUsingLoops(strArray);
  }

  /**
   * Iterates the String array and finds out the duplicates 
   */
   public static void isThereDuplicateUsingLoops(String[]     strArray) {

   boolean duplicateFound = false;
   int loopCounter = 0;

   for (int i = 0; i < strArray.length; i++) {
   String str = strArray[i];
   int countDuplicate = 0;

   for (int j = 0; j < strArray.length; j++) {
      String str2 = strArray[j];
      if (str.equalsIgnoreCase(str2)) {
         countDuplicate++;
      }
      if (countDuplicate > 1) {
         duplicateFound = true;
         System.out.println("Duplicates Found for " + str);
      }
      loopCounter++;
   }// end of inner nested for loop

   if (duplicateFound) {
    break;
   }
}// end of outer for loop

System.out.println("Looped " + loopCounter + " times to find the result");
}

}

If we run the above program, it will be looped 20 times to find out the duplicates in the string array which has the length of 5. Number of loops increases exponentially depending on size of array, hence the performance takes a hit. These are not acceptable to use in applications which require high performance.

Without using nested loops

package in.javatutorials;

import java.util.HashSet;
import java.util.Set;

/**
* Finds out the Duplicates is String Array using Collection.
*/
public class AvoidNesteadLoopsUsingCollections {

private static String[] strArray = { "Cat", "Dog", "Tiger", "Lion", "Lion" };

public static void main(String[] args) {
 isThereDuplicateUsingSet(strArray);
}

/**
* Iterates the String array and finds out the duplicates
*/
public static void isThereDuplicateUsingSet(String[] strArray) {
  boolean duplicateFound = false;
  int loopCounter = 0;
  Set setValues = new HashSet();

  for (int i = 0; i < strArray.length; i++) {
    String str = strArray[i];

    if(setValues.contains(str)){
        duplicateFound = true;
        System.out.println("Duplicates Found for " + str);
    }
    setValues.add(str);
    loopCounter++;

    if (duplicateFound) {
       break;
    }
   }// end of for loop

   System.out.println("Looped " + loopCounter + " times to find the result");
 }

}
  • Above approach takes only 5 loops to identify the duplicates in the same array.
  • It is more readable , easier to maintain and performs better.
  • If you have an array with 1000 items, then nested loops will loop through 999000 times and utilizing a collection will loop through only 1000 times.

Other Useful links:

Difference between sorted and ordered collection in hibernate


Below are the differences between Sorted collection and Ordered collection in Hibernate.

sorted collection 

order collection 

A sorted collection is sorting a collection by utilizing the sorting features provided by the Java collections framework. The sorting occurs in the memory of JVM which running Hibernate, after the data being read from database using java comparator. Order collection is sorting a collection by specifying the order-by clause for sorting this collection when retrieval.
If your collection is not large, it will be more efficient way to sort it. If your collection is very large, it will be more efficient way to sort it .

 

Other Useful Links:

Caching in Hibernate

Hibernate Interview Questions

Advantages of Hibernate over jdbc

Create a Java web service using top down approch


In the bottom up approach, we will write the java class and generates the WSDL file and other dependent components. The same will be deployed into the web containers.

In Top down approach, Architects will write the WSDL file based on the requirements. Developer need to make the corresponding service implementation using the WSDL provided. This post will explain how to create a service using the WSDL file.

Step 1: Create a dynamic or java project as mentioned here

Here, I have created a sample web dynamic project with the name SampleWS as given below.

Dyanmic web project

Step 2: generate the service using top down approach

Right click on the SamplWS project name -> New -> Other

SelectOther

Select the Web Service from the wizard as below and click on Finish button.

select webservice

Select the Web service type as ‘Top down Java bean Web service’ and provide the WSDL url in the Service definition drop down and click on Finish button.

Sample WSDL URL is: http://localhost:8080/SampleWebService/wsdl/Calculator.wsdl

providethewsdl

Your Web service is ready with the Java bean methods as below and the Final folder structure looks like below:

service Folder structure

Write the business logic into the Service class as given below:

Generated class:

/**
* CalculatorSoapBindingImpl.java
*
* This file was auto-generated from WSDL
* by the Apache Axis 1.4 Apr 22, 2006 (06:55:48 PDT) WSDL2Java emitter.
*/

package in.malliktalksjava;

public class CalculatorSoapBindingImpl implements in.malliktalksjava.Calculator{
public int addition(int var1, int var2) throws java.rmi.RemoteException {
return -3;
}

public int multiplication(int var1, int var2) throws java.rmi.RemoteException {
return -3;
}

public int division(int var1, int var2) throws java.rmi.RemoteException {
return -3;
}

}

Implemented class:

/**
* CalculatorSoapBindingImpl.java
*
* This file was auto-generated from WSDL
* by the Apache Axis 1.4 Apr 22, 2006 (06:55:48 PDT) WSDL2Java emitter.
*/

package in.malliktalksjava;

public class CalculatorSoapBindingImpl implements in.malliktalksjava.Calculator{
public int addition(int var1, int var2) throws java.rmi.RemoteException {
return var1+var2;
}

public int multiplication(int var1, int var2) throws java.rmi.RemoteException {
return var1*var2;
}

public int division(int var1, int var2) throws java.rmi.RemoteException {
return var1/var2;
}

}

Deploy the application into server and use the below url as a WSDL for this to have the client.

http://localhost:8080/SampleWebService/wsdl/Calculator.wsdl

 

Other Useful Links:

 Click here to know more about webservices

Click here to know more about RESTfull web services.

Click here for Web services Question and Answers.

Click here to know how to write web service client suing java.

Write a Client for web service


Below steps explains how to write a web service client in java using STS IDE.

Step 1: Create a Java project using the steps mentioned here.

Step 2: Generate the stubs for the Java web service using below steps

Mouse Right click on Client project and select New -> Other

select other option

Select the Web service client from the wizard

Select webservice cliet

Provide the service WSDL url in the Service Definition text box and click on finish button.

Enterwsdl into service defination

Web service client stubs will be generated into the package and final folder structure looks below.

Client stubs generated

Write the Client class using the stubs and test the client project.

Write a client

 

Use the below sample code to write the client:

 

package in.malliktalksjava.client;

import java.rmi.RemoteException;

import in.malliktalksjava.Calculator;
import in.malliktalksjava.CalculatorServiceLocator;
import javax.xml.namespace.QName;
import javax.xml.rpc.ServiceException;

/**
* @author Javatutorials
* @since version 1.0
*
*/
public class SampleWSClient {

/**
* @param args
*/
public static void main(String[] args) {

SampleWSClient sc = new SampleWSClient();
sc.callCalculatorWebservice();
}

/**
* used to call web service
*/
public void callCalculatorWebservice(){

String wsdl = “http://localhost:8080/SampleWebService/wsdl/Calculator.wsdl&#8221;;
QName queue = new QName(“http://malliktalksjava.in&#8221;, “CalculatorService”);

try {
//create the servicelocator object
CalculatorServiceLocator calServiceLoc = new CalculatorServiceLocator(wsdl, queue);
//create the service object
Calculator calculator = calServiceLoc.getCalculator();
//call the service methods
System.out.println(“addition result : “+calculator.addition(10, 11));
System.out.println(“division result : “+calculator.division(10, 5));
System.out.println(“multiplication result : “+calculator.multiplication(10, 10));
} catch (ServiceException e) {
e.printStackTrace();
} catch (RemoteException e) {
e.printStackTrace();
}
}
}

 

With this your client application is ready to use.

 

Other Useful links:

Click here to know how to create the web service project.

Click here to know the difference between SOAP and RESTfull web services.

Create a java webservice using STS


Below steps explains the how to create a web-service in java in bottom-up approach using the STS(Spring tool suite) IDE. In the bottom-up approach, first we will create a template class, using the template class we will generate the WSDL and deploy the service in servers. Follow the below steps to create a webservice.

Step 1: Create a new java project using the steps mentioned here

Step 2 : Create a template Java class as service

Create a Java class in src folder of the in.javatutorials package as below

Writeaclass

Provide the package name and class name into respective fields and click on finish button.

Namethetheclass

Implement the Java methods as give below:

write the class

Use the below source code to write the class:

package in.malliktalksjava;

/**
* Calculator class exposed as a webservice
* @author malliktalksjava
* @since Version 1.0
*
*/
public class Calculator {

/**
* adds the two input parameters and return the result
* @param var1
* @param var2
* @return
*/
public Integer addition(int var1, int var2) {

Integer result = var1 + var2;
System.out.println(“addition result in service : ” + result);
return result;
}

/**
* multiply the two input parameters and return the result
* @param var1
* @param var2
* @return
*/
public Integer multiplication(int var1, int var2) {

Integer result = var1 * var2;
System.out.println(“multiplication result in service : ” + result);
return result;
}

/**
* divide the two input parameters and return the result
* @param var1
* @param var2
* @return
*/
public Integer division(int var1, int var2) {

Integer result = var1 / var2;
System.out.println(“division result in service : ” + result);
return result;
}

}

Step 3: make the java class as web service

Select the Java class as below

maketheclassas service

Choose Web Service from the  and click on Next button

createservice

Select the ‘Bottom up Java bean Web Service’  from the Web service type and service implementation class as mentioned in the below picture and click on Next button.

bottomup webservice

Select the service methods from the menu and click on Finish button. The Service implementation style used is document/literal as shown in below picture.

choose service methods

The folder structure of the web application looks like below:

webservice folder structure

Deploy the service into tomcat web server and access the WSDL file using the below url:

http://localhost:8080/SampleWebService/wsdl/Calculator.wsdl

wsdlfile

Finally your web service is ready to use and WSDL url is the end point url for your web service.

 

Other Useful links:

 Click here to know more about webservices

Click here to know more about RESTfull web services.

Click here for Web services Question and Answers.

Click here to know how to write web service client suing java.

Create a Dynamic web project using STS


Below steps explain the creating a dynamic web project using STS.

Step 1 : Select File -> New -> Other.

CreateWebproject

Step 2:  Select the Dynamic web project from the menu and click on Next button.

dynamixwebproject

 

Step 3: Give a name to Dynamic web project and click on Finish button.

namewbproject

 

Step 4: A new project will be created as below with web project structure

webprojectstructure

Java Source files can be kept into src folder other files can will be kept into the sub folders of the WebContent folder.

 

What are the differences between SOAP WS and RESTful WS?


SOAP Web Services RESTfull Web Services
The SOAP WS supports both remote procedure call (i.e. RPC) and message oriented middle-ware (MOM) integration styles. The Restful Web Service supports only RPC integration style.
The SOAP WS is transport protocol neutral. Supports multiple protocols like HTTP(S),  Messaging, TCP, UDP SMTP, etc. The REST is transport protocol specific. Supports only HTTP or HTTPS protocols.
The SOAP WS permits only XML data format. You define operations, which tunnels through the POST. The focus is on accessing the named operations and exposing the application logic as a service. The REST permits multiple data formats like XML, JSON data, text, HTML, etc. Any browser can be used because the REST approach uses the standard GET, PUT, POST, and DELETE Web operations. The focus is on accessing the named resources and exposing the data as a service. REST has AJAX support. It can use the XMLHttpRequest object. Good for stateless CRUD (Create, Read, Update, and Delete) operations.GET – represent()POST – acceptRepresention()

PUT – storeRepresention()

DELETE – removeRepresention()

SOAP based reads cannot be cached. REST based reads can be cached. Performs and scales better.
SOAP WS supports both SSL security and WS-security, which adds some enterprise security features like maintaining security right up to the point where it is needed, maintaining identities through intermediaries and not just point to point SSL only, securing different parts of the message with different security algorithms, etc. The REST supports only point-to-point SSL security. The SSL encrypts the whole message, whether all of it is sensitive or not.
The SOAP has comprehensive support for both ACID based transaction management for short-lived transactions and compensation based transaction management for long-running transactions. It also supports two-phase commit across distributed resources. The REST supports transactions, but it is neither ACID compliant nor can provide two phase commit across distributed transactional resources as it is limited by its HTTP protocol.
The SOAP has success or retries logic built in and provides end-to-end reliability even through SOAP intermediaries. REST does not have a standard messaging system, and expects clients invoking the service to deal with communication failures by retrying.

 

Other Useful links:

 Click here to know more about webservices

Click here to know more about RESTfull web services.

Click here for Web services Question and Answers.

Click here to know how to create a SOAP web service.

Click here to know how to write web service client suing java.

What are the differences between DispatchAction and LookupDispatchAction in Struts Framework?


Dispatch Action
LookupDispatchAction
 
It’s a parent class of  LookupDispatchAction
Subclass of Dispatch Action
DispatchAction provides a mechanism for grouping a set of related functions into asingle action, thus eliminating the need to create separate actions for each function.
An abstract Action that dispatches to the subclass mapped executes method. This is useful in cases where an HTML form has multiple submit buttons with the same name. The button name is specified by the parameter property of the corresponding ActionMapping.
If not using Internalization functionality then dispatch action is more useful.
 
Lookup Dispatch Action is useful when we are using Internalization functionality
 
DispatchAction selects the method to execute depending on the request parameter valuewhich is configured in the xml file.
LookupDispatchAction looks into the resource bundle file and find out the corresponding key name. We can map this key name to a method name by overriding the getKeyMethodMap() method. 
DispatchAction is not useful for I18N
 
LookupDispatchAction is used for I18N
 
Find the DispatchAction Example here
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