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 Q121. How are the concept of Association related to Composition and Inheritance ?Core Java
Ans. Composition and Inheritance are the different types of Associations for Classes.

Composition is a has-a association between classes.
Inheritance is a is-a association between classes.

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   Like         Discuss         Correct / Improve     composition  object oriented programming (oops)  oops concepts  inheritance  object oriented programming (oops)  oops concepts  association  relationship between objects

  Q122. What are the core OOPs concepts ?Core Java
Ans. Abstraction, Encapsulation, Polymorphism , Composition and Inheritance

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   Like         Discuss         Correct / Improve     core oops concepts     Asked in 65 Companies      basic        frequent

Frequently asked Spring interview question.
 Q123. 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

 Q124. What is a Webdriver ?Testing
Ans. Selenium WebDriver is a tool for automating web application testing.It helps in replicating the manual tester behavior like keyboard entry, mouse events etc and then matching the output against the expected.

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   Like         Discuss         Correct / Improve     selenium  webdriver     Asked in 37 Companies

 Q125. Upon compiling the following class, What .class files will get created

public class OuterClass{
class InnerClass{
}
}
Core Java
Ans. OuterClass.class AND OuterClass$InnerClass.class

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   Like         Discuss         Correct / Improve     class files  compilation  compile  inner classes   nested classes

 Q126. Write a Program to print count of each character in a String.Core Java
Ans. public class BuggyBread {
public static void main(String args[]) {
Map<Character, Integer> countMap = new HashMap();
String str = "hello world";
for (char character : str.toCharArray()) {
if (countMap.containsKey(character)) {
countMap.put(character, countMap.get(character) + 1);
} else {
countMap.put(character, 1);
}
}
System.out.println(countMap);
}
}

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   Like         Discuss         Correct / Improve     String  Code  Coding  Character     Asked in 1 Companies

 Q127. What is the difference between compilation and decompilation ?Core Java
Ans. Compilation is the process of converting Java source files to class files which can be executed by the Java runtime environment whereas Decompilation is the process of converting class files back to the Java Source code.

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   Like         Discuss         Correct / Improve     compilation vs decompilation  compiler vs decompiler

 Q128. What is the difference between

File f = new File("homexyz.txt");

and

File f = File.createTempFile("xyz",".txt","home"); ?		Core Java
Ans. First will not create physical file in storage whereas the second will.

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   Like         Discuss         Correct / Improve     file handling  File.createTempFile  file handling      intermediate        rare

 Q129. Is it a good practice to use Asterick(*) in import statement to load multiple packages with single import statement ?Core Java
Ans. No, we should be lean in loading only packages which are required. Loading packages which are not being used result in memory overheads.

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   Like         Discuss         Correct / Improve     import  import packages     Asked in 1 Companies

 Q130. Does importing a package imports all its sub packages too ?Core Java
Ans. No, but we can use wild card (*) to do so

For example -

import java.util.*

will import all packages with name starting with java.util

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   Like         Discuss         Correct / Improve     package   import packages     Asked in 1 Companies      basic

 Q131. Which environment variables do we need to set in order to run Java programs?Core Java
Ans. PATH, CLASSPATH and JAVA_HOME

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   Like         Discuss         Correct / Improve     environment variables  path  classpath  java_home     Asked in 3 Companies      basic        frequent

 Q132. Scale yourself on Java on the scale of 1 to 10General
Ans. [Open Ended Answer]

The objective of the question is to check how efficient one is in the language so that appropriate level questions are asked. Don't tell too high number if you are being interviewed for a junior or mid level position as the interviewer may throw advance level questions.

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   Like         Discuss         Correct / Improve          Asked in 17 Companies

 Q133. What is a factory design pattern ?Design
Ans. Factory Design Patterns is the pattern that recommends creation of separate Factory Object for creation of other object.

So its like saying - If you want to create an object of ClassA, Talk to FactoryObject ( which is an object of FactoryClass ). FactoryObject in itself encapsulates the inputs and logic required to make the decision regarding the creation and construction of object.

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   Like         Discuss         Correct / Improve     design pattern  factory design pattern     Asked in 9 Companies

  Q134. Why java doesn't support multiple Inheritance ?

or

Explain Java Diamond Problem.		Core Java
Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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   Like         Discuss         Correct / Improve     multiple inheritance  object oriented programming (oops)  oops concepts  diamond problem     Asked in 20 Companies      basic        frequent

 Q135. With a Table employee having columns Id,Name,ManagerEmployeeId, Write a SQL to find the Manager with maximum number of employees ?Database
Ans. Select Name from EMPLOYEE where ID in (Select ManagerEmployeeId from EMPLOYEE
Group By ManagerEmployeeId
order by count(Id)
LIMIT 1)

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   Like         Discuss         Correct / Improve     database  sql     Asked in 1 Companies

  Q136. What is Maven ?Maven
Ans. Maven is a build automation tool used primarily for Java projects.

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   Like         Discuss         Correct / Improve     maven  build     Asked in 6 Companies      basic        frequent

 Q137. What is Spring Boot ?Spring Boot
Ans. Spring Boot is Springs convention-over-configuration solution for creating stand-alone, production-grade Spring-based Applications.

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   Like         Discuss         Correct / Improve     Spring Boot     Asked in 7 Companies

 Q138. Write a Program to reverse a Linked List ?Data Structure
 This question was recently asked at 'Myntra,Compro Technologies'.This question is still unanswered. Can you please provide an answer.


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Frequently asked in high end product companies. Frequently asked in Deloitte.
  Q139. 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

  Q140. Write a Program to check if 2 strings are Anagrams ?Core Java
Ans. public void checkIfAnagram(String str1,String str2){
boolean anagram = true;
for(char c:str1.toCharArray()){
if(!str2.contains(String.valueOf(c))){
System.out.println("Strings are Anagrams");
anagram = false;
}

if(anagram == true){
System.out.println("Strings are not Anagrams");
}
}
}

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   Like         Discuss         Correct / Improve      check if 2 strings are Anagrams     Asked in 30 Companies      basic        frequent

Basic and Very Frequently asked.
  Q141. 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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Frequently asked to fresh graduates.
 Q142. What is ACID ?Database
Ans. ACID stands for Atomicity, Consistency, Isolation, Durability is a set of properties of database transactions.

Atomicity means all or nothing. i.e parts of a transaction shouldn't commit if any one of them fails. Either the whole transaction should succeed or it should be complete rollback.

Consistency means that any transaction should lead database from one stabe state to another.

Isolation means that the execution of transaction results in a system state that would be obtained if transactions were executed serially.

Durability means that when a transaction is committed it forms the permanent state of database.

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   Like         Discuss         Correct / Improve     database  acid     Asked in 7 Companies      Intermediate
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 Q143. What is the difference between Collections.emptyList() and creating new instance of List using new ?Core Java
Ans. But Collections.emptyList() returns an Immutable list whereas new arraylist() creates a mutable list.

Advantage of getting an empty list using Collections.emptyList is that it returns a singleton list which can be shared among many references and hence made immutable. This is good fit for situations where we would like to initialize a list to basic minimum empty to avoid null pointer exception.

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   Like         Discuss         Correct / Improve     collections  Collections.emptyList  immutable  immutability  immutability

 Q144. What are the two main components of Hadoop System ?BigData
Ans. Distributed file system and Map Reduce system.

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 Q145. Where do we generally use comparable interface ?Core Java
Ans. Implementing Comparable interface means that the elements of the class are comparable i.e the class provides the implementation of compareTo method that would help comparing the elements.

This is usually required if we are planning to sort elements of a collection, If compareTo method is not defined , the sorting class / method could never understand a way to compare its elements in order to sort them.

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 Q146. What are design situations to use Singleton and Prototype Design Pattern ?Design
Ans. Both are creational design patterns.

Singleton is used when we would like to reuse an object if object is not supposed to hold request or thread specific information. Inversely Prototype is used in situations where we would like to reuse the object information but the request / thread may require it own data to be persisted.

In short, Singleton is used in situations where we can live with single object being shared across multiple requests or threads whereas Prototype is used when we need duplicate copies of objects.

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   Like         Discuss         Correct / Improve     design pattern  singleton  prototype  creational design pattrn     Asked in 7 Companies      expert
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  Q147. What is a self Join and give an example of a self Join ?

or

What is self Join and What is it's purpose ?		Database
Ans. When a Table Join itself , it's a Self Join. For example - we like to know the pair of department names where first dept has lesser employees than the later.

Select D1.name , D2.name from Dept D1, Dept D2 where D1.employee_count < D2.employee_count

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   Like         Discuss         Correct / Improve     sql  joins  self join  self-join  Pair of employee names with first having lesser salary than later  Pair of department names where first dept has lesser employees than the later     Asked in 26 Companies      basic        frequent

Frequently asked. Among first few questions in the J2EE segment of interview.
  Q148. What is deployment descriptor ?

or

What is the use of deployment descriptor ?		Java EE
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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   Like         Discuss         Correct / Improve     web.xml   servlets  deployment descriptor     Asked in 15 Companies      basic        frequent
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This question can be asked in different variants like performace improvement of an application, improving performance for DB communication etc.
 Q149. What are the ways in which execution time of the code can be improved ?Solution
Ans. This depends on entirely upon the code type. For example

1.If its just logic, we can try it to short circuit or put the if / switch case with maximum probability in the beginning.

2. Can use faster data structures, for eg - random retrieval instead of sequence / iterator

3. Working with primitive types or even bytes instead of Objects, even though it may result in marginal improvement.

4. If its service call, then service call with bulk load can help

5. If DB Operation, then with use of Indices , Views or using ORM , cache etc.


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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q150. Difference between TreeMap and HashMap ?Core Java
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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