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 Frequently asked in Infosys India | ||||
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| 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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 Discuss Correct / Improve  java oops string string class string pool heap memory  Asked in 31 Companies  intermediate  frequent | ||||
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| Very Frequently asked to fresh graduates and less experienced. Favorite question in Walk in drives. Frequently asked in Indian Services companies. | ||||
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| 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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| Discuss Correct / Improve java oops overloading overriding oops concepts basic interview question overloading vs overriding Asked in 86 Companies basic frequent | ||||
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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) | ||||
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| 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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| Discuss Correct / Improve java garbage collection java memory management advanced Asked in 21 Companies intermediate frequent | ||||
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| Frequently asked in Cognizant (CTS) | ||||
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| 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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| Discuss Correct / Improve java oops serialization Asked in 18 Companies basic frequent | ||||
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| Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. | ||||
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| 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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| Discuss Correct / Improve java oops final final method Asked in 30 Companies basic frequent | ||||
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| Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. | ||||
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| 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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| Discuss Correct / Improve java collections map treemap hashmap treemap vs hashmap Asked in 31 Companies basic frequent | ||||
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| Ans. Generics , Enums , Autoboxing , Annotations and Static Import. | ||||
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| Discuss Correct / Improve java java5 generics enum autoboxing annotations static import rare | ||||
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| Advanced level question usually asked to senior developers , leads and architects. | ||||
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| Ans. Volatile is an instruction that the variables can be accessed by multiple threads and hence shouldn't be cached. As volatile variables are never cached and hence their retrieval cannot be optimized. | ||||
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| Discuss Correct / Improve java java keywords volatile synchronization compiler optimization variable caching architecture Asked in 3 Companies expert | ||||
| Frequently asked to fresh graduates and less experienced. | ||||
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| 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, void means it doesn't return anything , String args[] is the array of argument that the method receives. 2. If we use main without the string args , it will compile correctly as Java will treat it as just another method. 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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| Discuss Correct / Improve java main method Asked in 4 Companies basic frequent | ||||
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| Frequently asked to fresh graduates. | ||||
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| 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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| Discuss Correct / Improve database acid Asked in 7 Companies Intermediate | ||||
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| Very frequently asked in companies using SOA. | ||||
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| 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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| Discuss Correct / Improve java web services rest java j2ee architecture Asked in 14 Companies intermediate frequent | ||||
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| Ans. JDK(Java Development kit) = Development Kit comprising of JVM , library and development tools for developers JRE (Java Run time Environment) - Comprise of JVM and set of libraries JVM(Java Virtual Machine) = Interpreter which reads the .class file line by line. When we install JDK, JRE also get installed so we can write,compile and excute our code. Used by developer. Without JDK we can only execute the program using JRE. | ||||
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| Discuss Correct / Improve jvm jre jdk jvm vs jre vs jdk Asked in 4 Companies | ||||
| Frequently asked to fresh graduates and less experienced. | ||||
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| 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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| Discuss Correct / Improve java oops oops concepts inheritance object oriented programming (oops) oops concepts composition object oriented programming (oops) oops concepts difference between basic interview question Asked in 12 Companies basic frequent | ||||
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| Frequently asked in high end product companies. Frequently asked in Deloitte. | ||||
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| 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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| Discuss Correct / Improve hashmap collections hashmap internal implementation Asked in 20 Companies expert frequent | ||||
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| 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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| Discuss Correct / Improve hibernate configuration barclays Asked in 11 Companies | ||||
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| Ans. 1. Query Optimization ( Query Rewriting , Prepared Statements ) 2. Restructuring Indexes. 3. DB Caching Tuning ( if using ORM ) 4. Identifying the problems ( if any ) with the ORM Strategy ( If using ORM ) | ||||
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| Discuss Correct / Improve java db database hibernate orm at&t overstock.com performance improvement architecture technical lead architect intermediate | ||||
| Frequently asked. Among first few questions in the J2EE segment of interview. | ||||
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| 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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| Discuss Correct / Improve web.xml servlets deployment descriptor Asked in 15 Companies basic frequent | ||||
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| Ans. EC2 - Time and Reservation , Instance Type ( Memory , Computation power ) S3 - Accessibility Frequency , Data Transfer , Storage Space CloudFront - Geography, No of Locations EBS - IOPS, Access Frequency | ||||
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| Discuss Correct / Improve aws ec2 aws computing amazon cloud computing amazon ec2 aws s3 aws storage amazon cloud storage amazon s3 aws cloudfront amazon cloudfront aws ebs | ||||
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| 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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| Discuss Correct / Improve java oop oop concepts inheritence multiple inheritence basic interview question Asked in 10 Companies basic frequent | ||||
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| Ans. It in Java is used to indicate that a field should not be serialized. | ||||
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| Discuss Correct / Improve java oops serialization transient java keywords Asked in 39 Companies intermediate frequent | ||||
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| Frequently asked to fresh graduates and less experienced. | ||||
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| Ans. Inheritance. | ||||
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| Discuss Correct / Improve inheritance object oriented programming (oops) oops concepts java oops composition object oriented programming (oops) oops concepts Asked in 1 Companies basic frequent | ||||
| Very frequently asked in phone and walk in interviews. | ||||
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| 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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| Discuss Correct / Improve java oops interfaces marker interface serializable clonable Asked in 21 Companies intermediate frequent | ||||
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| Ans. Builder is a creational Design Pattern whereas Composite is a structural design pattern. Composite creates Parent - Child relations between your objects while Builder is used to create group of objects of predefined types. | ||||
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| Discuss Correct / Improve java design pattern builder composite creational design pattern structural design pattern | ||||
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| Ans. It is the process of examining / modifying the behaviour of an object at runtime. | ||||
| Sample Code for reflection | ||||
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| Discuss Correct / Improve java reflection reflection api ebay mindtree Asked in 29 Companies basic frequent | ||||
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| Ans. Garbage Collector wont remove a strong reference. A soft reference will only get removed if memory is low. A weak reference will get removed on the next garbage collection cycle. A phantom reference will be finalized but the memory will not be reclaimed. Can be useful when you want to be notified that an object is about to be collected. | ||||
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| Discuss Correct / Improve java memory management jvm garbage collections references strong reference soft reference weak reference phantom reference architecture Asked in 2 Companies | ||||
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| Ans. Yes, We can call garbage collector directly but it doesn't guarantees that the gc will start executing immediately. This gc( ) method appears in both Runtime and System classes of java.lang package. | ||||
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| Discuss Correct / Improve java garbage collection java memory management jvm gc yes no Asked in 4 Companies expert | ||||
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| Ans. Java uses Just-In-Time compiler to enable high performance. Just-In-Time compiler is a program that turns Java bytecode into instructions that can be sent directly to the processor. | ||||
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| Discuss Correct / Improve java compiler jvm bytecode justintime compiler architecture | ||||
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| Ans. The purpose of comparator interface is to compare objects of the same class to identify the sorting order. Sorted Collection Classes ( TreeSet, TreeMap ) have been designed such to look for this method to identify the sorting order, that is why class need to implement Comparator interface to qualify its objects to be part of Sorted Collections. | ||||
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| Discuss Correct / Improve java collections treemap treeset comparator Asked in 2 Companies Intermediate | ||||
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| 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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| Discuss Correct / Improve java main method overloading yes-no Asked in 6 Companies intermediate frequent | ||||
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| Ans. No. compareTo method is declared final for the Enumerations and hence cannot be overriden. This has been intentionally done so that one cannot temper with the sorting order on the Enumeration which is the order in which Enum constants are declared. | ||||
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| Discuss Correct / Improve java compareto final methods enum enumeration expert | ||||
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When String literals are compared using ==, they always returns true if the string values are same because .. 
