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Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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Very frequently asked in phone and walk in interviews.

Q122. What are Marker Interfaces ? Name few Java marker interfaces ?

Ans. An interface without any method declaration is called as marker interface. there are 3 in-built interfaces in JVM i.e. serializable, clonable, remote

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Frequently asked to fresh graduates.

Q123. What is a Deadlock ?

Operating System

Ans. When two threads are waiting each other and cant precede the program is said to be deadlock.

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Very frequently asked. Usually difference between String,StringBuffer and StringBuilder is asked in different variations.

Q124. Difference between StringBuffer and StringBuilder ?

Ans. StringBuffer is synchronized whereas StringBuilder is not.

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Very frequently asked in different variations. Frequently asked in Deloitte ( 2 feedback ) , HCL Tech ( 3 feedback ), TCS and Coginizant (CTS)

Q125. Explain the scenerios to choose between String , StringBuilder and StringBuffer ?

or

What is the difference between String , StringBuilder and StringBuffer ?

Ans. If the Object value will not change, use String Class because a String object is immutable.

If the Object value can change and will only be modified from a single thread, use StringBuilder because StringBuilder is unsynchronized(means faster).

If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).

Sample Code for String

Sample Code for StringBuffer

Sample Code for StringBuilder

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Q126. What is reflection ?

Ans. It is the process of examining / modifying the behaviour of an object at runtime.

Sample Code for reflection

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Q127. what is the difference between collections class vs collections interface ?

Ans. Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. Frequently asked in HCL Technologies, TCS and Accenture.

Q128. What is the difference between final, finally and finalize() ?

Ans. final - constant variable, objects cannot be de-referenced, restricting method overriding, restricting class sub classing.

finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of what happens within the try block. Use the finally block to close files or to release other system resources like database connections, statements etc.

finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state.

Sample Code for final

Sample Code for finally

Sample Code for finalize

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Q129. When do you get ClassCastException?

or

What is ClassCastException ?

Ans. As we only downcast class in the hierarchy, The ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance.

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Frequently asked in CTS ( Based on 2 feedback )

Q130. What is the difference between comparable and comparator in java.util pkg?

Ans. Comparable interface is used for single sequence sorting i.e.sorting the objects based on single data member where as comparator interface is used to sort the object based on multiple data members.

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Recently asked in TCS , Tech Mahindra, HCL , Accenture and Fidelity.

Q131. Explain Flow of Spring MVC ?

Spring

Ans. The DispatcherServlet configured in web.xml file receives the request.

The DispatcherServlet finds the appropriate Controller with the help of HandlerMapping and then invokes associated Controller.

Then the Controller executes the logic business logic and then returns ModeAndView object to the DispatcherServlet.

The DispatcherServlet determines the view from the ModelAndView object.

Then the DispatcherServlet passes the model object to the View.

The View is rendered and the Dispatcher Servlet sends the output to the Servlet container.

Finally Servlet Container sends the result back to the user.

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Very Frequently asked Spring question.

Q132. What is bean auto wiring?

Spring

Ans. The Spring container is able to autowire relationships between collaborating beans. This means that it is possible to automatically let Spring resolve collaborators (other beans) for your bean by inspecting the contents of the BeanFactory without using and elements.

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Very frequently asked.Usually among first few questions.

Q133. What is MVC ?

Design

Ans. MVC is a Design Pattern that facilititates loose coupling by segregating responsibilities in a Web application

1. Controller receives the requests and handles overall control of the request
2. Model holds majority of the Business logic, and
3. View comprise of the view objects and GUI component

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Q134. What is a stream and what are the types of Streams and classes of the Streams?

Ans. A Stream is an abstraction that either produces or consumes information. There are two types of Streams :

Byte Streams: Provide a convenient means for handling input and output of bytes.

Character Streams: Provide a convenient means for handling input & output of characters.

Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream.

Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer.

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Q135. Advantage of Collection classes over Arrays ?

Ans. Collections are re-sizable in nature. We can increase or decrease the size as per recruitment.
Collections can hold both homogeneous and heterogeneous data's.
Every collection follows some standard data structures.
Collection provides many useful built in methods for traversing,sorting and search.

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Frequently asked in Infosys and HCL Technologies ( Based on 2 feedback )

Q136. What are different ways of object creation in Java ?

Sample Code for object initialization using getInstance

Ans. Using new operator - new xyzClass()

Using factory methods - xyzFactory.getInstance( )

Using newInstance( ) method - (Class.forName(xyzClass))emp.newInstance( )

By cloning an already available object - (xyzClass)obj1.clone( )

Sample Code for object initialization using clone

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Q137. Write a method to check if input String is Palindrome?

Ans. private static boolean isPalindrome(String str) {

if (str == null)
return false;

StringBuilder strBuilder = new StringBuilder(str);

strBuilder.reverse();

return strBuilder.toString().equals(str);

}

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Asked multiple times in Capgemini.

Q138. Why Char array is preferred over String for storing password?

Ans. String is immutable in java and stored in String pool. Once it's created it stays in the pool until unless garbage collected, so even though we are done with password it's available in memory for longer duration and there is no way to avoid it. It's a security risk because anyone having access to memory dump can find the password as clear text.

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Q139. Why String is popular HashMap key in Java?

Ans. Since String is immutable, its hashcode is cached at the time of creation and it doesnt need to be calculated again. This makes it a great candidate for key in a Map and its processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.

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Q140. What is the use of HashCode in objects ?

Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc.

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Frequently asked to fresh graduates and less experienced.

Q141. Difference between Composition and Inheritance ?

Ans. Inheritance means a object inheriting reusable properties of the base class. Compositions means that an abject holds other objects.

In Inheritance there is only one object in memory ( derived object ) whereas in Composition , parent object holds references of all composed objects.

From Design perspective - Inheritance is "is a" relationship among objects whereas Composition is "has a" relationship among objects.

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Very frequently asked in HCL Tech ( Based of 4 inputs )

Q142. Write a program to reverse a string iteratively and recursively ?

Ans. Using String method -

new StringBuffer(str).reverse().toString();

Iterative -

public static String getReverseString(String str){
StringBuffer strBuffer = new StringBuffer(str.length);
for(int counter=str.length -1 ; counter>=0;counter--){
strBuffer.append(str.charAt(counter));
}
return strBuffer;
}

Recursive -

public static String getReverseString(String str){
if(str.length <= 1){
return str;
}
return (getReverseString(str.subString(1)) + str.charAt(0);
}

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Very frequently asked in companies using SOA.

Q143. What are RESTful Web Services ?

Rest

Ans. REST or Representational State Transfer is a flexible architecture style for creating web services that recommends the following guidelines -

1. http for client server communication,
2. XML / JSON as formatiing language ,
3. Simple URI as address for the services and,
4. stateless communication.

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Very Frequently asked. Favorite question in walkins and telephonic interviews. Usually among first few questions. Asked in different variants. Must know for intermediate and expert professionals.Among Top 10 frequently asked questions.

Q144. What is rule regarding overriding equals and hashCode method ?

Ans. A Class must override the hashCode method if its overriding the equals method.

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Q145. What do you mean by "Java is a statically typed language" ?

Ans. It means that the type of variables are checked at compile time in Java.The main advantage here is that all kinds of checking can be done by the compiler and hence will reduce bugs.

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Frequently asked question in companies using hibernate.

Q146. Difference between first level and second level cache in hibernate ?

Ans. 1. First level cache is enabled by default whereas Second level cache needs to be enabled explicitly.

2. First level Cache came with Hibernate 1.0 whereas Second level cache came with Hibernate 3.0.

3. First level Cache is Session specific whereas Second level cache is shared by sessions that is why First level cache is considered local and second level cache is considered global.

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Q147. What are different types of second level cache ?

Ans. 1. EHCache ( Easy Hibernate )
2. OSCache ( Open Symphony )
3. Swarm Cache ( JBoss )
4. Tree Cache ( JBoss )

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Very frequently asked Hibernate interview question. Frequently asked in TCS ( based on 2 feedback )

Q148. What are different types of associations in Hibernate ?

Ans. There are 4 types of associations in Hibernate

One to One
One to Many
Many to One
Many to Many

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Q149. What are the contents of Hibernate configuration file ( hibernate.cfg.xml ) ?

Ans. HBM Files ( Mapping )
DB Connection ( DB Connection String , User Name , Password , Pool Size )
SQL Dialect ( SQL variant to be generated )
Show SQL ( Show / No show SQL on Console )
Auto Commit ( True / False )

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Very frequently asked if being interviewed for hibernate. Frequently asked in Tata Consultancy (TCS) and Overstock.com

Q150. Difference between load and get ?