20+ Interview Questions and Answers

Java Developer interview questions covering array list, linkedlist, hashmap, synchronization, abstract vs interface, singleton class, Spring framework, database configuration, and Java 8 features.

• ArrayList and LinkedList differ in terms of implementation, performance, and usage

• HashMap uses hashing to store and retrieve key-value pairs

• Synchronized is used to ensure thread safety and prevent race conditions

• Abstract classes cannot be instantiated and can have both abstract and n...read more

This interview question covers various topics including programming, design patterns, data structures, and exception handling in Java.

• Write a program to generate the Fibonacci series

• Write a program using boolean variables

• Explain the ClassCastException and how to handle it

• Discuss the Singleton design pattern and its implementation with different JDK versions

• Explain abstract classes and interfaces in your current project

• Explain hashing in HashMap and the number of buckets

• Differ...read more

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

Interview questions for Java Developer

• JVM is a virtual machine that executes Java bytecode

• OOPS is a programming paradigm based on objects and classes

• SQL queries are used to retrieve data from a database

• REST API is a web service that uses HTTP requests to access and manipulate data

• Sorting an ArrayList can be done using Collections.sort() method

• Known sorting algorithms include Bubble Sort, Quick Sort, and Merge Sort

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

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

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() {} public static synchronized Singleton getInstance() { ...read more

Use Java 8 features to find the second largest number and reverse a string.

• Use streams and collectors to find the second largest number in an array.

• Use StringBuilder to reverse a string in Java 8.

• For Spring Boot questions, focus on dependency injection, auto-configuration, and RESTful services.

• For core Java questions, focus on OOP concepts, collections, and exception handling.

Idempotency is the property of an operation where multiple identical requests have the same effect as a single request.

• Idempotency ensures that even if the same request is made multiple times, the result will be the same.

• In RESTful APIs, using HTTP methods like GET and PUT can help achieve idempotency.

• Design patterns like Command Pattern and State Pattern can be used to ensure idempotency in projects.

To sort an array list, use the Collections.sort() method.

• Import the java.util.Collections class

• Call the sort() method and pass the array list as parameter

• Ensure the objects in the array list implement the Comparable interface or pass a Comparator object as second parameter

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 not thread-safe and requires synchronization for concurrent...read more

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

HashMap is a data structure that stores key-value pairs and uses hashing to retrieve values quickly.

• HashMap uses hashing to store and retrieve values based on their keys.

• It uses an array of buckets to store the key-value pairs.

• The hash code of the key is used to determine the index of the bucket where the pair will be stored.

• If two keys have the same hash code, they are stored in the same bucket as a linked list.

• HashMap allows null keys and values, but only one null key.

• The i...read more

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.

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 different parameters are defined in the same class.

• Runtim...read more

To make a class immutable in Java, you can follow certain guidelines to ensure that its state cannot be changed once it is created.

• Make the class final so that it cannot be extended.

• Make all fields private and final to prevent modification.

• Do not provide setter methods for the fields.

• If the fields are mutable objects, return copies of them instead of the original objects in getter methods.

Java and Python are programming languages used for developing software applications.

• Java is an object-oriented language used for developing desktop, web, and mobile applications.

• Python is a high-level language used for web development, data analysis, artificial intelligence, and scientific computing.

• Java is compiled and runs on a virtual machine, while Python is interpreted.

• Java is statically typed, while Python is dynamically typed.

• Java has a strong emphasis on type safety a...read more

Static method is a method that belongs to the class itself, not to any specific instance of the class.

• Static methods can be called without creating an instance of the class.

• They are commonly used for utility methods that do not require access to instance variables.

• Static methods are declared using the 'static' keyword.

• Example: Math.max() is a static method in the Math class that returns the maximum of two numbers.

OOPs concepts are the fundamental principles of object-oriented programming.

• Encapsulation: Hiding the implementation details of an object and exposing only the necessary information.

• Inheritance: Creating new classes from existing ones, inheriting their properties and methods.

• Polymorphism: The ability of objects to take on many forms, allowing them to behave differently in different situations.

• Abstraction: Focusing on the essential features of an object and ignoring the non-es...read more