Cognizant Java Developer Interview Questions, Process, and Tips

Easy to moderate level questions based on arrays , matrix and strings

Q1. Explain runtime polymorphism and compile time polymorphism
Ans. 
Runtime polymorphism occurs when a method is overridden in a subclass, while compile time polymorphism occurs when method overloading is used.
• Runtime polymorphism is achieved through method overriding, where a subclass provides a specific implementation of a method that is already provided by its parent class.

• Compile time polymorphism is achieved through method overloading, where multiple methods with the same name but...read more
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Q2. Given a string , print the letter and the number of times it occurs
Ans. 
Print each letter in a string along with the number of times it occurs
• Iterate through the string and count the occurrences of each letter

• Use a HashMap to store the letter as key and count as value

• Print the letter and count for each unique letter
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Collection, stream api

Q1. How do you get data from database in spring boot?
Ans. 
Data can be retrieved from a database in Spring Boot using JPA repositories or JDBC templates.
• Use JPA repositories to create custom queries and retrieve data from the database.

• Use JDBC templates to execute SQL queries and retrieve data from the database.

• Configure the data source in the application.properties file.

• Use annotations like @Repository, @Entity, @Table, etc. to map Java objects to database tables.
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Q1. Core java , jdbc template, basic rest questions...restcontroller vs controller, springbootappliaction annotation,multithreading.
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Q1. Asks about current salary..gets shocked with current CTC. I tell her my expectations, she says you better go with current offers in hand. Why do I had to go through all interview process if you could not p...read more
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Q1. Basic oops concepts, basic sql queries, some questions on spring framework.
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Q1. Basic hr questions were asked like strengths, weakness, what you have learned in your college life, how have you changed as a person during your college life etc
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Q1. Internal working of hashmap?
Ans. 
HashMap is a data structure that stores key-value pairs and uses hashing to retrieve values.
• HashMap uses an array of buckets to store key-value pairs

• Each bucket contains a linked list of entries

• Hashing is used to determine the index of the bucket where the entry should be stored

• If multiple entries have the same hash code, they are stored in the same bucket as a linked list

• HashMap allows null keys and values

• HashMap is n...read more
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Q2. How to create immutable classes?
Ans. 
Immutable classes cannot be modified after creation. They ensure thread safety and prevent bugs.
• Make the class final so it cannot be extended

• Make all fields private and final

• Do not provide any setters, only getters

• If a field is mutable, return a copy of it in the getter

• Consider using a builder pattern for complex objects
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Q3. How to create singleton class ?
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Singleton class is a class that can only have one instance at a time.
• Create a private constructor to prevent instantiation from outside the class.

• Create a private static instance of the class.

• Create a public static method to get the instance of the class.

• Ensure thread safety by using synchronized keyword or static initializer.

• Example: public class Singleton { private static Singleton instance = null; private Singleton(...read more
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Q4. We have a employee class having empId, name and salary fields. I have around 1000 employee objects, I want to sort them according employee name is descending order?
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Q5. Basic OOPS related questions?
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Q6. Difference between List and Set? which one to prefer in which requirements and why ?
Ans. 
List allows duplicates and maintains insertion order, Set doesn't allow duplicates and doesn't maintain order.
• Use List when duplicates are allowed and order is important

• Use Set when duplicates are not allowed and order is not important

• List examples: ArrayList, LinkedList

• Set examples: HashSet, TreeSet
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Q7. Spring Boot basic questions?
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Q8. How to implement Spring security with JWT token?
Ans. 
Implementing Spring security with JWT token involves configuring security filters and authentication providers.
• Add dependencies for Spring Security and JWT

• Configure security filters to validate JWT tokens

• Implement authentication provider to authenticate users

• Generate and sign JWT tokens for successful authentication

• Example: https://www.baeldung.com/spring-security-jwt
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