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Frequently asked Java Interview Questions and Answers - 258 question(s) found - Order By Newest

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Very Very frequently asked. Among the first few questions in almost all interviews.Among Top 5 frequently asked questions.
  Q1. Difference between == and .equals() ?Core Java
Anonymous
Ans. "equals" is the method of object class which is supposed to be overridden to check object equality, whereas "==" operator evaluate to see if the object handlers on the left and right are pointing to the same object in memory.

x.equals(y) means the references x and y are holding objects that are equal. x==y means that the references x and y have same object.

Sample code:

String x = new String("str");
String y = new String("str");

System.out.println(x == y); // prints false
System.out.println(x.equals(y)); // prints true

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Advanced level question. Frequently asked in High end product companies. Frequently asked in Cognizant ( Based on 2 feedbacks )
  Q2. Why is String immutable in Java ?Core Java
Anonymous
Ans. 1. String Pool - When a string is created and if it exists in the pool, the reference of the existing string will be returned instead of creating a new object. If string is not immutable, changing the string with one reference will lead to the wrong value for the other references.

Example -

String str1 = "String1";
String str2 = "String1"; // It doesn't create a new String and rather reuses the string literal from pool

// Now both str1 and str2 pointing to same string object in pool, changing str1 will change it for str2 too

2. To Cache its Hashcode - If string is not immutable, One can change its hashcode and hence it's not fit to be cached.

3. Security - String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment.

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Very frequently asked in different variations. Frequently asked in Deloitte ( 2 feedback ) , HCL Tech ( 3 feedback ) and Coginizant (CTS)
  Q3. Explain the scenerios to choose between String , StringBuilder and StringBuffer ?

or

What is the difference between String , StringBuilder and StringBuffer ?
Core Java
Anonymous
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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Frequently asked to fresh graduates and less experienced developers. Among the first few questions in many interviews.
  Q4. What are the difference between composition and inheritance in Java?Core Java
Admin
info@buggybread.com
Ans. Composition - has-a relationship between classes.
Inheritance - is-a relationship between classes.

Composition - Composing object holds a reference to composing classes and hence relationship is loosely bound.
Inheritance - Derived object carries the base class definition in itself and hence its tightly bound.

Composition - Used in Dependency Injection
Inheritance - Used in Runtime Polymorphism

Composition - Single class objects can be composed within multiple classes.
Inheritance - Single class can only inherit 1 Class.

Composition - Its the relationship between objects.
Inheritance - Its the relationship between classes.

  Sample Code for inheritance

  Sample Code for composition

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 Q5. Which are the different segments of memory ?Core Java
Anonymous
Ans. 1. Stack Segment - Contains primitives, Class / Interface names and references.

2. Heap Segment - Contains all created objects in runtime, objects only plus their object attributes (instance variables), Static variables are also stored in heap.

3. Code Segment - The segment where the actual compiled Java bytecodes resides when loaded

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 Q6. What are different ways to create String Object? Explain.Core Java
Admin
info@buggybread.com
Ans.
String str = new String("abc");
String str1 = "abc";


When we create a String using double quotes, JVM looks in the String pool to find if any other String is stored with same value. If found, it just returns the reference to that String object else it creates a new String object with given value and stores it in the String pool.

When we use new operator, JVM creates the String object but dont store it into the String Pool. We can use intern() method to store the String object into String pool or return the reference if there is already a String with equal value present in the pool.

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 Q7. What do you mean by "Java is a statically typed language" ?Core Java
Admin
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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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 Q8. What is the difference between declaration, instantiation and initialization ?Core Java2016-05-26 14:17:48

Ans. Declaration is intimation to the compiler about the nature of Data a reference is going to hold.

For example - List myList;

Instantiation is reservation of memory.

For example

myList = new ArrayList();

Initialization or construction is setting the default values for member elements.

For example

myList = new ArrayList(mySet);

** Example 2nd is both for instantiation as well as initialization. The only difference is that 2nd will initialized the member elements to their default values whereas 3rd will initialized it with the elements from set.


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 Q9. What are the benefits of JSON over XML ?Json
Anonymous
Ans. Lighter and faster than XML as on-the-wire data format

Object Representation - Information is presented in object notations and hence better understandable.

Easy to parse and conversion to objects for information consumption.

Support multiple data types - JSON supports string, number, array, boolean whereas XML data are all string.

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Frequently asked.
 Q10. If you are given a choice to use either ArrayList and LinkedList, Which one would you use and Why ?Core Java
Admin
info@buggybread.com
Ans. ArrayList are implemented in memory as arrays and hence allows fast retrieval through indices but are costly if new elements are to be inserted in between other elements. LinkedList allows for constant-time insertions or removals using iterators, but only sequential access of elements
1. Retrieval - If Elements are to be retrieved sequentially only, Linked List is preferred.
2. Insertion - If new Elements are to be inserted in between other elements , Array List is preferred.
3. Search - Binary Search and other optimized way of searching is not possible on Linked List.
4. Sorting - Initial sorting could be pain but lateral addition of elements in a sorted list is good with linked list.
5. Adding Elements - If sufficiently large elements needs to be added very frequently ,Linked List is preferable as elements don't need consecutive memory location.

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Frequently asked question in companies using Hibernate.
  Q11. What is Lazy Initialization in Hibernate ?Hibernate
Admin
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Ans. It's a feature to lazily initialize dependencies , relationship and associations from the Database. Any related references marked as @OneToMany or @ManyToMany are loaded lazily i.e when they are accessed and not when the parent is loaded.

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  Q12. What is a Lambda Expression ? What's its use ?Core Java
Admin
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Ans. Its an anonymous method without any declaration.

Lambda Expression are useful to write shorthand Code and hence saves the effort of writing lengthy Code.

It promotes Developer productivity, Better Readable and Reliable code.

  Sample Code for lambda

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 Q13. How can we run a java program without making any object?Core Java
Anonymous
Ans. By putting code within static method. With Java 6 and earlier versions, even static block can be used.

  Sample Code for static block

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Very frequently asked to Fresh graduates.
  Q14. What is the difference between Encapsulation and Abstraction?Core Java
Anonymous
Ans. 1.Abstraction solves the problem at design level while encapsulation solves the problem at implementation level

2.Abstraction is used for hiding the unwanted data and giving relevant data. while Encapsulation means hiding the code and data into a single unit to protect the data from outside world.

3. Abstraction lets you focus on what the object does instead of how it does it while Encapsulation means hiding the internal details or mechanics of how an object does something.

4.For example: Outer Look of a Television, like it has a display screen and channel buttons to change channel it explains Abstraction but Inner Implementation detail of a Television how CRT and Display Screen are connect with each other using different circuits , it explains Encapsulation.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. Frequently asked in HCL Technologies.
  Q15. What is the difference between final, finally and finalize() ?Core Java
Admin
info@buggybread.com
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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Recently asked in Accenture and Fidelity.
 Q16. Explain Flow of Spring MVC ?Spring
Admin
info@buggybread.com
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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Advanced level question frequently asked in US based companies. Recently asked in EMC and Intuit.
  Q17. Can you provide some implementation of a Dictionary having large number of words ? Solution
Admin
info@buggybread.com
Ans. Simplest implementation we can have is a List wherein we can place ordered words and hence can perform Binary Search.

Other implementation with better search performance is to use HashMap with key as first character of the word and value as a LinkedList.

Further level up, we can have linked Hashmaps like ,

hashmap {
a ( key ) -> hashmap (key-aa , value (hashmap(key-aaa,value)
b ( key ) -> hashmap (key-ba , value (hashmap(key-baa,value)
....................................................................................
z( key ) -> hashmap (key-za , value (hashmap(key-zaa,value)
}

upto n levels ( where n is the average size of the word in dictionary.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q18. Difference between TreeMap and HashMap ?Core Java
Anonymous
Ans. They are different the way their elements are stored in memory. TreeMap stores the Keys in order whereas HashMap stores the key value pairs randomly.

  Sample Code for treemap

  Sample Code for hashmap

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Very frequently asked across all types of companies.
  Q19. Difference between HashMap and Hashtable?Core Java
Anonymous
Ans. Hashtable is synchronized whereas HashMap is not.HashMap allows null values whereas Hashtable doesnt allow null values.

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 Q20. What will this code print ?

String a = new String ("TEST");
String b = new String ("TEST");
if(a == b) {
System.out.println ("TRUE");
} else {
System.out.println ("FALSE");
}
Core Java
Anonymous
Ans. FALSE. == operator compares object references, a and b are references to two different objects, hence the FALSE. .equals method is used to compare string object content.

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Advanced level question usually asked in High end product companies. Have been asked in Google and Amazon (Based on 1 Feedback)
  Q21. Describe, in general, how java's garbage collector works ?Core Java
Anonymous
Ans. The Java runtime environment deletes objects when it determines that they are no longer being used. This process is known as garbage collection. The Java runtime environment supports a garbage collector that periodically frees the memory used by objects that are no longer needed. The Java garbage collector is a mark-sweep garbage collector that scans Java dynamic memory areas for objects, marking those that are referenced. After all possible paths to objects are investigated, those objects that are not marked (i.e. are not referenced) are known to be garbage and are collected.

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Frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q22. What are the methods of Object Class ?Core Java
Anonymous
Ans. clone() - Creates and returns a copy of this object.

equals() - Indicates whether some other object is "equal to" this one.

finalize() - Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.

getClass() - Returns the runtime class of an object.

hashCode() - Returns a hash code value for the object.

toString() - Returns a string representation of the object.

notify(), notifyAll(), and wait() - Play a part in synchronizing the activities of independently running threads in a program.

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  Q23. Difference between SAX and DOM Parser ?Xml
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Ans. A DOM (Document Object Model) parser creates a tree structure in memory from an input document whereas A SAX (Simple API for XML) parser does not create any internal structure.

A SAX parser serves the client application always only with pieces of the document at any given time whereas A DOM parser always serves the client application with the entire document no matter how much is actually needed by the client.

A SAX parser, however, is much more space efficient in case of a big input document whereas DOM parser is rich in functionality.

Use a DOM Parser if you need to refer to different document areas before giving back the information. Use SAX is you just need unrelated nuclear information from different areas.

Xerces, Crimson are SAX Parsers whereas XercesDOM, SunDOM, OracleDOM are DOM parsers.

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Frequently asked to experienced developers. Recently asked in many US interviews.
 Q24. What is database deadlock ? How can we avoid them?Database
Admin
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Ans. When multiple external resources are trying to access the DB locks and runs into cyclic wait, it may makes the DB unresponsive.

Deadlock can be avoided using variety of measures, Few listed below -

Can make a queue wherein we can verify and order the request to DB.

Less use of cursors as they lock the tables for long time.

Keeping the transaction smaller.



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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q25. What is the difference between ArrayList and LinkedList ?Core Java
admin
info@buggybread.com
Ans. Underlying data structure for ArrayList is Array whereas LinkedList is the linked list and hence have following differences -

1. ArrayList needs continuous memory locations and hence need to be moved to a bigger space if new elements are to be added to a filled array which is not required for LinkedList.

2. Removal and Insertion at specific place in ArrayList requires moving all elements and hence leads to O(n) insertions and removal whereas its constant O(1) for LinkedList.

3. Random access using index in ArrayList is faster than LinkedList which requires traversing the complete list through references.

4. Though Linear Search takes Similar Time for both, Binary Search using LinkedList requires creating new Model called Binary Search Tree which is slower but offers constant time insertion and deletion.

5. For a set of integers you want to sort using quicksort, it's probably faster to use an array; for a set of large structures you want to sort using selection sort, a linked list will be faster.

  Sample Code for ArrayList

  Sample Code for LinkedList

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Frequently asked to fresh graduates and less experienced.
 Q26. Why do we write public static void main ? Can we use some other syntax too for main ?Core Java
Admin
info@buggybread.com
Ans.
1. public is the access modifier that makes the method accessible from anywhere, static is the keyword that makes it accessible even without creating any object and using class name only , void means it doesnt return anything , String args[] is the array of argument that this method receives.

2. If I use Main , it will compile correctly as Java will treat it as just another method but it wont be the method "main" which Java looks for when it looks to execute the class and hence will throw

Error: Main method not found in class , please define the main method as:
public static void main(String[] args)

3. Main is not a keyword but a special string that Java looks for while initiating the main thread.

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  Q27. What is a String Pool ?Core Java
Anonymous
Ans. String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.

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Very frequently asked. Usually asked in different variants like Diff between StringBuffer , String Builder ; Difference between StringBuilder and String class; Choice between these classes etc.
  Q28. What is the difference between StringBuffer and String class ?Core Java
Anonymous
Ans. A string buffer implements a mutable sequence of characters. A string buffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls.

The String class represents character strings. All string literals in Java programs, such as "abc" are constant and implemented as instances of this class; their values cannot be changed after they are created.

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 Q29. What is JDBC? Describe the steps needed to execute a SQL query using JDBC.Database
Anonymous
Ans. The JDBC is a pure Java API used to execute SQL statements. It provides a set of classes and interfaces that can be used by developers to write database applications.

The steps needed to execute a SQL query using JDBC:

1. Open a connection to the database.
2. Execute a SQL statement.
3. Process th results.
4. Close the connection to the database.

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Very Frequently asked to fresh graduates and less experienced. Favorite question in Walk in drives.
  Q30. Difference between Overloading and Overriding ?Core Java
Anonymous
Ans. Overloading - Similar Signature but different definition , like function overloading.

Overriding - Overriding the Definition of base class in the derived class.

  Sample Code for overloading

  Sample Code for overriding

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  Q31. Difference between Vector and ArrayList ?Core Java
Anonymous
Ans. Vectors are synchronized whereas Array lists are not.

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Very frequently asked in phone and walk in interviews.
  Q32. What are Marker Interfaces ? Name few Java marker interfaces ?Core Java
Anonymous
Ans. These are the interfaces which have no declared methods.Serializable and cloneable are marker interfaces.

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 Q33. Similarity and Difference between static block and static method ?Core Java
Admin
info@buggybread.com
Ans. Both belong to the class as a whole and not to the individual objects. Static methods are explicitly called for execution whereas Static block gets executed when the Class gets loaded by the JVM.

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Frequently asked to fresh graduates and less experienced.
 Q34. Difference between Compositions and Inheritance ?Core Java
Admin
info@buggybread.com
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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 Q35. Difference between Static and Singleton Class ?Core Java2016-11-30 21:00:43

Ans. 1. Static class is a class which cannot be instantiated and all its members are static whereas Singleton is the class that only permit creation of single object and then the object is reused.

2. As there is no object in Static class, it cannot participate in runtime Polymorphism.

3. As Static class doesnt allow creating objects and hence it cannot be serialized.

4. Static class body is initialized eagerly at application load time whereas Singleton object can be initiated eagerly using static blocks or lazily on first need.

5. Its not recommended to use pure static class as it fails to use many OOPs concepts.

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Frequently asked question in companies using hibernate.
  Q36. Difference between first level and second level cache in hibernate ?Hibernate
Admin
info@buggybread.com
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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Very Frequently asked.
 Q37. Explain throw, throws , try and catch in Java ?Core Java
Admin
info@buggybread.com
Ans. throw is used to re throw an exception.

throws is used to declare that the method throws the respective exceptions.

try block is used to identify if the respective block has thrown any exception.

catch is used to catch the exception that has been thrown by the respective try block.

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Usually asked to experienced developers.
 Q38. What is a cyclic dependency ?Maven
Admin
info@buggybread.com
Ans. A has dependency of B, B has dependency of C and C has dependency of A,architecture,technical lead

With Maven 2 , came transitive dependency wherein in above scenario, C will acts as a dependency of A as if this dependency has been defined directly in A but the negative side is that if it leads to cyclic dependency , it creates problems.

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Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 2 feedback ). Among first few questions in many interviews.
  Q39. Differences between abstract class and interface ?Core Java
Admin
info@buggybread.com
Ans. Abstract classes can have both abstract methods ( method declarations ) as well as concrete methods ( inherited to the derived classes ) whereas Interfaces can only have abstract methods ( method declarations ).

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Almost sure to be asked in every company using any Dependency Injection framework ( Spring, Guice etc )
  Q40. What is Dependency Injection or IOC ( Inversion of Control ) ?Design

Ans. It is a Design Pattern that facilitates loose coupling by sending the dependency information ( object references of dependent object ) while building the state of the object. Objects are designed in a manner where they receive instances of the objects from other pieces of code, instead of constructing them internally and hence provide better flexibility.

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Frequently asked. Among first few questions in the J2EE segment of interview.
  Q41. What is the use of deployment descriptor ?Java EE2016-06-04 22:46:40

Ans. Deployment Descriptor which is usually web.xml is used to specify the classes, resources and configuration of the application and how the web server uses them to serve web requests.This file is usually added to WEB-INF folder and contains following

* Servlet entries and url mapping
* Plugins
* Some info regarding authentication / filters
* Landing Page
* Event Handlers

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Basic and Very Frequently asked.
  Q42. What is Polymorphism in Java ?Core Java2016-11-06 09:10:54

Ans. Polymorphism means the condition of occurring in several different forms.

Polymorphism in Java is achieved in two manners

1. Static polymorphism is the polymorphic resolution identified at compile time and is achieved through function overloading whereas

2. Dynamic polymorphism is the polymorphic resolution identified at runtime and is achieved through method overriding.

  Sample Code for overloading

  Sample Code for overriding

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  Q43. What are Inner , Outer , Left and Right Joins in SQL ?Database2017-01-09 13:07:55

Ans. Inner join is the intersection of two tables on the condition defined by the where clause i.e will get records from both tables matched by a column.

Outer join is the union of two tables i.e will get all records from both tables and will put null in the columns where related records are not present.

Left Outer join is the left union of two tables i.e all records from the table on the left and values from the right table for related records else null for the columns from right table.

Right Outer join is the right union of two tables i.e all records from the table on the right and values from the left table for related records else null for the columns from left table.

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 Q44. Which elements of a class are ignored during serialization ?Core Java2017-01-12 11:08:21

Ans. 1. Objects are serialized and not classes and hence Static variables are ignored.

2. Transient is an explicit declaration to ignore the variable during serialization and hence transient instance variables are ignored too.

3. Base class instance variables if the base class hasn't been declared serializable.

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  Q45. What are the different types of inner classes ?Core Java
Anonymous
Ans. Simple Inner Class,
Local Inner Class,
Anonymous Inner Class,
Static Nested Inner Class.

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Very Frequently asked across all type of companies and across all levels.
  Q46. Difference between Public, Private, Default and Protected ?Core Java
Anonymous
Ans. Private - Not accessible outside object scope.

Public - Accessible from anywhere.

Default - Accessible from anywhere within same package.

Protected - Accessible from object and the sub class objects.

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  Q47. Does Java support Multiple Inheritance ?Core Java
Anonymous
Ans. Java doesn't support multiple inheritance. Interfaces does't facilitate inheritance and hence implementation of multiple interfaces doesn't make multiple inheritance.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q48. What is a Final Variable ?Core Java
Anonymous
Ans. Final variable is a variable constant that cannot be changed after initialization.

  Sample Code for final variable

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q49. What is a final method ?Core Java
Anonymous
Ans. Its a method which cannot be overridden. Compiler throws an error if we try to override a method which has been declared final in the parent class.

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Frequently asked to fresh graduates and less experienced developers.
 Q50. Explain multithreading in Java ?Core Java
Anonymous
Ans. 1. Multithreading provides better interaction with the user by distribution of task

2. Threads in Java appear to run concurrently, so it provides simulation for simultaneous activities.The processor runs each thread for a short time and switches among the threads to simulate sim-ultaneous execution (context-switching) and it make appears that each thread has its own processor.By using this feature, users can make it appear as if multiple tasks are occurring simultaneously when, in fact, each is running for only a brief time before the context is switched to the next thread.

3. We can do other things while waiting for slow I/O operations.In the java.iopackage, the class InputStreamhas a method, read(), that blocks until a byte is read from the stream or until an IOExceptionis thrown. The thread that executes this method cannot do anything elsewhile awaiting the arrival of another byte on the stream.

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Frequently asked in all types of companies especially Indian Services companies. Frequently asked in CTS (Based on 2 feedback)
 Q51. What is the use of hashcode in Java ?Core Java
Anonymous
Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. The value received from hashcode() is used as bucket number for storing elements. This bucket number is the address of the element inside the set/map. when you do contains() then it will take the hashcode of the element, then look for the bucket where hashcode points to and if more than 1 element is found in the same bucket (multiple objects can have the same hashcode) then it uses the equals() method to evaluate if object are equal, and then decide if contain() is true or false, or decide if element could be added in the set or not.

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 Q52. What are wrapper classes ?Core Java
Anonymous
Ans. They are wrappers to primitive data types. They allow us to access primitives as objects.

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  Q53. What is casting?Core Java
Anonymous
Ans. There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference

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 Q54. What is JSON ?Json
Anonymous
Ans. JSON is "JavaScript Object Notation", primarily used for client-server or server-server communication. Its a much lighter and readable alternative to XML. JSON is language independent and is easily parse-able in all programming languages.

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 Q55. What is an API ( Application Programming Interface ) ?
Anonymous
Ans. An API is a kind of technical contract which defines functionality that two parties must provide: a service provider (often called an implementation) and an application. an API simply defines services that a service provider (i.e., the implementation) makes available to applications.

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 Q56. What is HTTP ?Java EE
Anonymous
Ans. HTTP or Hypertext Transfer Protocol is internet protocol for tranmission of hypertext ( text with meta data ) over internet.

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 Q57. Which access specifier can be used with Class ?
Anonymous
Ans. For top level class we can only use "public" and "default". We can use private with inner class.

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 Q58. Difference between Abstract and Concrete Class ?Core Java
Anonymous
Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

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 Q59. Different ways of implementing Threads in Java ?Core Java
Anonymous
Ans. Threads in Java can be implement either by Extending Thread class or implementing runnable interface.

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Frequently asked question for intermediate developers.
  Q60. What is Volatile keyword used for ?Core Java
Anonymous
Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldnt be cached.

  Sample Code for volatile

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  Q61. What is Serialization ? Why do we need it ?Core Java
Anonymous
Ans. Storing the state of an object in a file or other medium is called serialization.

Classes can communicate only if they are built together ( as they need Byte code for communication ). What if we need to enable communication between different applications ( i.e they have been built independently or even they reside at different locations ), We need a mechanism that will transfer the Bean state to a Medium than can be transferred to the receiving application.

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 Q62. What is the use of Transient Keyword ?Core Java
Anonymous
Ans. It in Java is used to indicate that a field should not be serialized.

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 Q63. What is a Final Method ?Core Java
Anonymous
Ans. A Method that cannot be overriden in the sub class.

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Very Frequently asked.Favorite question in Walk in drive for many Indian service companies.
 Q64. What is a Final Class ?
Anonymous
Ans. A Class that cannot be sub classed.

  Sample Code for final class

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Very frequently asked. Usually asked with questions related to String.
  Q65. What is an Immutable Object ?Core Java
Anonymous
Ans. Object that can't be changed after instantiation.

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Very Frequently asked. Usually asked along with String Class related questions.
 Q66. What is an immutable class ?Core Java
Anonymous
Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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 Q67. How to implement an immutable class ?Core Java
Anonymous
Ans. We can make a class immutable by

1. Making all methods and variables as private.

2. Setting variables within constructor.

Public Class ImmutableClass{

private int member;
ImmutableClass(int var){
member=var;
}
}

and then we can initialize the object of the class as

ImmutableClass immutableObject = new ImmutableClass(5);

Now all members being private , you cant change the state of the object.

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 Q68. Can we override static methods ? Why ?Core Java
Anonymous
Ans. No.

Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior.

A static method is not associated with any instance of a class so the concept of overriding for runtime polymorphism using static methods is not applicable.

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 Q69. Name few Java Exceptions ?Core Java
Anonymous
Ans. IndexOutofBoundException ,
NoClassDefFoundException ,
OutOfMemoryException ,
IllegalArgumentException,
NullPointerException,
IllegalStateException

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Frequently asked to fresh graduates and less experienced.
 Q70. Which of the following is tightly bound ? Inheritance or Composition ?Core Java
Anonymous
Ans. Inheritance.

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 Q71. How can we make sure that a code segment gets executed even in case of uncatched exceptions ?
Anonymous
Ans. By putting it within finally.

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 Q72. What is "super" used for ?
Anonymous
Ans. Used to access members of the base class.

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 Q73. What is "this" keyword used for ?
Anonymous
Ans. Used to represent an instance of the class in which it appears.

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Frequently asked to fresh graduates.
  Q74. Difference between Process and Thread ?Operating System
Anonymous
Ans. Process is a program in execution whereas thread is a separate path of execution in a program.

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Frequently asked to fresh graduates.
  Q75. What is a Deadlock ?Operating System
Anonymous
Ans. When two threads are waiting each other and cant precede the program is said to be deadlock.

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 Q76. Difference between Serialization and Deserialization ?Core Java
Anonymous
Ans. Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

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 Q77. Explain Autoboxing ?Core Java
Anonymous
Ans. Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes

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 Q78. What is an Enum type ?Core Java
Anonymous
Ans. An enum type is a special data type that enables for a variable to be a set of predefined constants

  Sample Code for enum

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 Q79. Different types of memory used by JVM ?Core Java
Anonymous
Ans. Class , Heap , Stack , Register , Native Method Stack.

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 Q80. What is a Static import ?Core Java
Anonymous
Ans. By static import , we can access the static members of a class directly without prefixing it with the class name.

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Very frequently asked. Usually difference between String,StringBuffer and StringBuilder is asked in different variations.
  Q81. Difference between StringBuffer and StringBuilder ?Core Java
Anonymous
Ans. StringBuffer is synchronized whereas StringBuilder is not.

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 Q82. Difference between Map and HashMap ?Core Java
Anonymous
Ans. Map is an interface where HashMap is the concrete class.

  Sample Code for map

  Sample Code for hashmap

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Frequently asked to fresh graduates.
 Q83. Is JVM, a compiler or interpretor ?
Anonymous
Ans. Its an interpretor.

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 Q84. Is it necessary that each try block to be followed by catch block ? Core Java
Anonymous
Ans. It should be followed by either catch or finally block.

  Sample Code for Retry in case of exception

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 Q85. Difference between nested and inner classes ?
Anonymous
Ans. Inner classes are non static nested classes.

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 Q86. What is reflection ?
Anonymous
Ans. It is the process of examining / modifying the behaviour of an object at runtime.

  Sample Code for reflection

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 Q87. Difference between static vs. dynamic class loading?Core Java
Admin
info@buggybread.com
Ans. static loading - Classes are statically loaded with Java new operator.

dynamic class loading - Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time.

Class.forName (Test className);

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 Q88. When do you get ClassCastException?
Anonymous
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 to fresh graduates.
 Q89. Explain Thread States ?Operating System
Admin
info@buggybread.com
Ans. Runnable - waiting for its turn to be picked for execution by the thread schedular based on thread priorities.

Running - The processor is actively executing the thread code. It runs until it becomes blocked, or voluntarily gives up its turn.

Waiting: A thread is in a blocked state while it waits for some external processing such as file I/O to finish.

Sleeping - Java threads are forcibly put to sleep (suspended) with Thread.sleep. they can resume using Thread.resume method.

Blocked on I/O - Will move to runnable after I/O condition like reading bytes of data etc changes.

Blocked on synchronization - Will move to Runnable when a lock is acquired.

Dead - The thread is finished working.

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 Q90. What is XML ?Xml
Admin
info@buggybread.com
Ans. XML or eXtensible Markup Language is a markup languages for describing data and its metadata.

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Very frequently asked.Usually among first few questions.
  Q91. What is MVC ? Design
Anonymous
Ans. The Model/View/Controller pattern, a strategy for dividing responsibility in a GUI component.

The model is the data for the component.

The view is the visual presentation of the component on the screen.

The controller is responsible for reacting to events by changing the model.

According to the MVC pattern, these responsibilities should be handled by different objects.

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  Q92. Write a method to check if input String is Palindrome?Core Java
Admin
info@buggybread.com
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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Very frequently asked in HCL Tech ( Based of 4 inputs )
 Q93. Write a program to reverse a string iteratively and recursively ?Core Java
Admin
info@buggybread.com
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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 Q94. How to find whether a given integer is odd or even without use of modules operator in java?Core Java
Admin
info@buggybread.com
Ans. public static void main(String ar[])
{
int n=5;
if((n/2)*2==n)
{
System.out.println("Even Number ");
}
else
{
System.out.println("Odd Number ");
}
}

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 Q95. What will be the output of following Code ?

class BuggyBread {
   public static void main(String[] args) {
      String s2 = "I am unique!";
      String s5 = "I am unique!";

      System.out.println(s2 == s5);
   }
}
Core Java
Admin
info@buggybread.com
Ans. true, due to String Pool, both will point to a same String object.

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Very frequently asked in companies using SOA.
  Q96. What are RESTful Web Services ?Rest
Admin
info@buggybread.com
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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  Q97. Can we overload main method in Java ?Core Java
Admin
info@buggybread.com
Ans. Yes, but the overloaded main methods without single String[] argument doesn't get any special status by the JVM. They are just another methods that needs to be called explicitly.

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Very frequently asked if being interviewed for hibernate. Frequently asked in Tata Consultancy (TCS) and Overstock.com
  Q98. Difference between load and get ?Hibernate
Admin
info@buggybread.com
Ans. If id doesnt exist in the DB load throws an exception whereas get returns null in that case.get makes the call to DB immediately whereas load makes the call to proxy.

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 Q99. How is Abstraction implemented in Java ?Core Java
Admin
info@buggybread.com
Ans. Abstraction is provided in Java by following ways -

Coding to the ( Interfaces / Abstract Classes ) or contracts

By Encapsulating details within classes and exposing the minimal Door ( few public methods )

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Frequently asked.
 Q100. When does the finally block gets executed ?Core Java2016-10-31 09:29:10

Ans. A finally block of code always executes, whether or not an exception has occurred.The only time finally won't be called is if you call System.exit() or if the JVM crashes first.

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