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 Q11. Write code to print a Pyramid pattern ?Core Java
Ans. public static void printTriagle(int n)
{
// outer loop to handle number of rows
// n in this case
for (int i=0; i1; j--)
{
// printing spaces
System.out.print(" ");
}

// inner loop to handle number of columns
// values changing acc. to outer loop
for (int j=0; j<=i; j )
{
// printing stars
System.out.print("* ");
}

// ending line after each row
System.out.println();
}
}

// Driver Function
public static void main(String args[])
{
int n = 5;
printTriagle(n);
}

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   Like         Discuss         Correct / Improve          Asked in 1 Companies

 Q12. Describe structure of a Web application.architecture
Ans. WEB APP |WEB-INF - META-INF | | | META-INF.MF | lib - WEB.xml - Classes

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   Like         Discuss         Correct / Improve     web application     Asked in 3 Companies      intermediate        frequent

 Q13. Can we have multiple inheritance in java using abstract class ?Core Java
Ans. No

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   Like         Discuss         Correct / Improve     abstract class   multiple inheritance  object oriented programming (oops)  oops concepts     Asked in 2 Companies

 Q14. What is the difference between abstraction and abstract class ?Core Java
Ans. Abstract class is one of the facility for achieving abstraction in Java.

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  Q15. Difference between == and === ?JavaScript
Ans. == compares values === is used in scripting languages for comparing two values as well as there data tpe. like in js,php.

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   Like         Discuss         Correct / Improve     ==  ===  == vs ===     Asked in 13 Companies      basic        frequent

  Q16. Write code to sort an array.Algorithm
Ans. https://www.geeksforgeeks.org/arrays-sort-in-java-with-examples/

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   Like         Discuss         Correct / Improve     array  sorting     Asked in 16 Companies      basic        frequent

 Q17. Create a stack of specific size using arrayList?Data Structure
Ans. public class StackUsingArrayList {
   public static void main(String[] args) {
      List list = new ArrayList();
      list.add("A");
      list.add("B");
      list.add("C");
      
      Stack stack = new Stack();

      list.forEach(a -> stack.add(a));
      System.out.println(stack);
   
   }
}

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 Q18. Is HttpServlet an abstract class ?Java EE
Ans. Yes

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 Q19. what is fail fast and Fail Safe in collections?Core Java
Ans. Iterators in java are used to iterate over the Collection objects.

Fail-Fast iterators immediately throw ConcurrentModificationException if there is any addition, removal or updation of any element.

Fail-Safe iterators don't throw any exception if a collection is structurally modified while iterating over it. This is because, they operate on the clone of the collection and not on the original collection.

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   Like         Discuss         Correct / Improve     ail fast   fail saf     Asked in 9 Companies      intermediate

Frequently asked Spring interview question.
 Q20. What are the components of Spring framework ?Spring
Ans. Core: [Core, Bean, Context, Expression Language]

Web: [Web, Portlet, Servlet, etc]

Data Access [JMS, JDBC, etc]

AOP, Aspect

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   Like         Discuss         Correct / Improve     spring  spring framework components     Asked in 12 Companies

Usually asked to entry level software developers.
  Q21. Write a program to swap two variables without using thirdCore Java
Ans. public static void main(String[] args) {
   int num1 = 1;   
   int num2 = 2;
   num1 = num1^num2;
   num2 = num1^num2;
   num1 = num1^num2;
   System.out.print("num1 = " + num1 +", num2 = "+num2);
}

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   Like         Discuss         Correct / Improve     code  coding     Asked in 37 Companies      basic        frequent

 Q22. What is a binary tree ?Algorithm
Ans. Binary tree is a tree in which each node has up to two children.Tree is a data structure composed of nodes.Each tree has a root node(not necessary in graph theory). The root node has zero or more child nodes.Each child node has zero or more child nodes, and so on.The tree cannot contain cycles.

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   Like         Discuss         Correct / Improve     binary tree     Asked in 1 Companies

 Q23. Write an algorithm / Java Program to show duplicates in an array of n elements?Algorithm
Ans. int duplicateArray[] = { 1, 2, 2, 3, 4, 5, 6, 8, 9}
Set unique = new HashSet();
for (int i = 0; i < duplicateArray.length; i) {
if (unique.contains(duplicateArray[i])) {
System.out.println(duplicateArray[i]);
} else {
unique.add(duplicateArray[i]);
}
}

Complexity O(n) = nHashSet contains and add has O(n) = 1

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   Like         Discuss         Correct / Improve     coding  code     Asked in 2 Companies

 Q24. Write an algorithm / java program for Heap Sort ?Algorithm
Ans. https://www.geeksforgeeks.org/heap-sort/

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   Like         Discuss         Correct / Improve     sorting  heap sort     Asked in 1 Companies        frequent

  Q25. What is an exception and exception handling in Java ?Core Java
Ans. An Exception in java is the occurrence during computation that is anomalous and is not expected.

Exception handling is the mechanism which is used to handle such situations.


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   Like         Discuss         Correct / Improve     exception handling     Asked in 18 Companies      basic        frequent

Frequently asked in high end product companies. Frequently asked in Deloitte.
  Q26. How is Hashmap internally implemented in Java ?Core Java
Ans. https://medium.com/javarevisited/internal-working-of-hashmap-in-java-97aeac3c7beb#:~:text=HashMap%20internally%20uses%20HashTable%20implementation,the%20entries%20into%20the%20map.

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   Like         Discuss         Correct / Improve     hashmap  collections  hashmap internal implementation     Asked in 20 Companies      expert        frequent

Basic and Very Frequently asked.
  Q27. What is Polymorphism in Java ?Core Java
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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   Like         Discuss         Correct / Improve     polymorphism  object oriented programming (oops)  oops concepts  oops concepts     Asked in 108 Companies      Basic        frequent
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Very frequently asked.
 Q28. What is the use of synchronized keyword ?Core Java
Ans. Synchronize is used to achieve mutual exclusion i.e at one time, the segment of the code, method that has been declared synchronized should be executed by single thread only and hence the lock needs to be retrieved before executing the segment and then released.

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   Like         Discuss         Correct / Improve     threads  multithreading  synchronized     Asked in 5 Companies      basic        frequent

 Q29. What are the disadvantages of multithreading ?Core Java
Ans. 1. Switching Overheads - Even though multi threading aims at improving performance by reducing the wait time and hence improving overall throughput, there is a cost of switching resources between threads and sometime this cost can surpass the benefits if there isnt much wait for IO or external communication.

2. Debugging is hard with multi threaded code.

3. Deadlock - Execution of multi threaded code many a times lead to deadlock due to shared resources.

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   Like         Discuss         Correct / Improve     multithreading  threads     Asked in 4 Companies

Frequently asked at HCL Technologies ( Based on 3 feedback )
  Q30. Difference between Checked and Unchecked exceptions ?Core Java
Ans. Checked exceptions are the exceptions for which compiler throws an errors if they are not checked whereas unchecked exceptions are caught during run time only and hence can't be checked.

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 Q31. Can we serialize static variables ?Core Java
Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object.

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   Like         Discuss         Correct / Improve     serialization   java   oops   static   static variables     Asked in 1 Companies      intermediate        rare
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Advanced level question. Frequently asked in High end product companies. Frequently asked in Google , Cognizant and Deloitte ( Based on 2 feedback )
  Q32. Why is String immutable in Java ?Core Java
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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  Q33. Difference between Vector and ArrayList ?Core Java
Ans. Vectors are synchronized whereas Array lists are not.

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  Q34. What is the use of Transient Keyword ?Core Java
Ans. It in Java is used to indicate that a field should not be serialized.

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Very Frequently asked. Usually asked along with String Class related questions.
  Q35. What is an immutable class ?Core Java
Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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 Q36. How to implement an immutable class ?Core Java
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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   Like         Discuss         Correct / Improve     java   oops   immutable  immutability   immutable  immutability class   technical lead     Asked in 5 Companies      intermediate        frequent
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Very frequently asked in different variations. Frequently asked in Deloitte ( 2 feedback ) , HCL Tech ( 3 feedback ), TCS and Coginizant (CTS)
  Q37. Explain the scenerios to choose between String , StringBuilder and StringBuffer ?

or

What is the difference between String , StringBuilder and StringBuffer ? 		Core Java
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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Advanced level question. Recently asked in few Indian service companies ( Based on 3 inputs )
 Q38. What are various types of Class loaders used by JVM ?Core Java
Ans. Bootstrap - Loads JDK internal classes, java.* packages.

Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE

System - Loads classes from system classpath.

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   Like         Discuss         Correct / Improve     java   jvm   memory management   class loaders   bootstrap   extensions   system  classloaders   advanced   technical lead   technical architect     Asked in 7 Companies

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.
  Q39. What is rule regarding overriding equals and hashCode method ?Core Java
Ans. A Class must override the hashCode method if its overriding the equals method.

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   Like         Discuss         Correct / Improve     java   collections   hashcode  hash code   equals   collections     Asked in 44 Companies      intermediate        frequent
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  Q40. Can we have null keys in TreeMap ?Core Java
Ans. No, results in exception.

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