20+ Senior Java Developer Interview Questions and Answers for Freshers

Immutable objects are those whose state cannot be changed after creation.

• Use final keyword to make class immutable

• Make all fields private and final

• Do not provide setter methods

• If a field is mutable, return a copy of it instead of the original object

• Examples: String, Integer, LocalDate

Singleton is a design pattern that restricts the instantiation of a class to one object.

• To create a singleton class, make the constructor private, create a static method to get the instance of the class, and create a private static variable to hold the instance.

• Benefits of singleton include ensuring only one instance of the class exists, providing a global point of access to the instance, and reducing the number of objects created in the system.

• Example: java.lang.Runtime is a...read more

SQL questions on Truncate, Delete, Drop, Composite Key and Query to find customer details

• Truncate removes all data from a table, Delete removes specific data, Drop removes the entire table

• Composite key is a combination of two or more columns that uniquely identifies a row in a table

• SELECT * FROM customer c JOIN order o ON c.customer_id = o.customer_id WHERE AVG(o.amount) BETWEEN 1000 AND 2000

The question asks to multiply all elements in an array except for a given number.

• Iterate through the array and calculate the product of all elements except the given number.

• Use a variable to keep track of the product.

• If the given number is 1, multiply all other elements.

• Return the resulting array.

get() method returns null if object is not found in database, while load() method throws ObjectNotFoundException.

• get() method is eager loading while load() method is lazy loading.

• get() method returns a fully initialized object while load() method returns a proxy object.

• get() method is slower than load() method as it hits the database immediately.

• load() method is faster than get() method as it returns a proxy object and hits the database only when required.

Yes, we can create our own immutable class in Java.

• Make the class final

• Make all fields private and final

• Do not provide any setter methods

• Provide only getter methods

• If any mutable object is used, return a copy of it instead of the original object

ClassLoader loads classes at runtime. JIT compiles bytecode to native machine code.

• ClassLoader loads classes dynamically at runtime

• It searches for classes in classpath and loads them into JVM memory

• JIT (Just-In-Time) compiler compiles bytecode to native machine code for faster execution

• It optimizes frequently executed code by compiling it at runtime

• JIT is enabled by default in most JVMs

JDK is a development kit that includes JRE, while JRE is a runtime environment for executing Java programs.

• JDK stands for Java Development Kit and includes tools for developing, debugging, and monitoring Java applications.

• JRE stands for Java Runtime Environment and includes the minimum set of tools required to run Java applications.

• JDK includes JRE, so if you have JDK installed, you also have JRE.

• JDK includes a compiler, while JRE does not.

• JDK is required for developing Java ...read more

Volatile is a keyword in Java used to indicate that a variable's value may be modified by multiple threads.

• Volatile variables are not cached in thread's local memory

• Changes made to volatile variables are immediately visible to other threads

• Volatile keyword is used to ensure visibility and ordering of variables in multithreaded environment

• Example: volatile int count = 0;

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

• HashMap uses an array of buckets to store key-value pairs

• The hash code of the key is used to determine the index of the bucket where the key-value pair is stored

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

• When retrieving a value, the hash code of the key is used to find the bucket and then the linked list is searched for the key

• HashMa...read more

The @Component annotation is used to mark a class as a bean, while @Configuration is used to define a configuration class.

• The @Component annotation is used to auto-detect and auto-configure beans using classpath scanning.

• @Configuration classes are used to define beans and their dependencies explicitly.

• A @Configuration class can contain @Bean methods that return bean instances.

• A @Component class can be used as a bean in a Spring application context.

A program to create custom exceptions in Java.

• Create a class that extends Exception or RuntimeException

• Add constructors to the custom exception class

• Throw the custom exception in the code when needed

Microservices communicate through lightweight protocols like HTTP/REST or messaging systems like RabbitMQ.

• Microservices can communicate through synchronous protocols like HTTP/REST, where one service sends a request to another service and waits for a response.

• Microservices can also communicate through asynchronous messaging systems like RabbitMQ, where one service publishes a message to a queue and other services consume the message when they are ready.

• Microservices can use l...read more

Garbage collector is an automatic memory management system in Java that reclaims memory by deallocating objects no longer in use.

• Garbage collector uses different algorithms like Mark-Sweep, Mark-Compact, and Copying to reclaim memory.

• Mark-Sweep algorithm marks objects as reachable or unreachable, then sweeps through and deletes the unreachable objects.

• Mark-Compact algorithm moves reachable objects to one end of the memory space and compacts them to free up space.

• Copying algor...read more

To count duplicate names in SQL, use the GROUP BY clause with the COUNT function.

• Use GROUP BY clause to group the names together

• Use COUNT function to count the number of occurrences

• Use HAVING clause to filter out names with count less than 2

Spring Security is a powerful and customizable authentication and access control framework for Java applications.

• Spring Security provides comprehensive security services for Java EE-based enterprise software applications.

• It offers authentication, authorization, and protection against common security vulnerabilities.

• Configurations are typically done using XML or Java-based configuration.

• It supports various authentication mechanisms such as form-based, HTTP Basic, and OAuth.

• Dev...read more

Collection framework is a set of classes and interfaces that provide a standard way to store and manipulate groups of objects.

• It provides interfaces like List, Set, and Map for storing collections of objects.

• It includes classes like ArrayList, LinkedList, HashSet, and HashMap for implementing these interfaces.

• Collections framework simplifies the process of storing, retrieving, and manipulating data in Java.

Java is a popular programming language used for developing various applications. Spring Boot is a framework that simplifies the development of Java applications.

• Java is an object-oriented programming language known for its platform independence and versatility.

• Spring Boot is a framework that simplifies the development of Java applications by providing pre-configured settings and tools.

• Spring Boot allows for rapid development and deployment of Java applications, making it popu...read more

Java 1.8 introduced lambda expressions, default methods, and functional interfaces.

• Lambda expressions allow functional programming in Java.

• Default methods allow adding new methods to interfaces without breaking existing implementations.

• Functional interfaces are interfaces with a single abstract method, used for lambda expressions.

• Stream API for processing collections and arrays.

• Date and Time API for easier handling of date and time.

• Nashorn JavaScript engine for embedding Java...read more

OAuth2.0 is an authorization framework that allows a user to grant limited access to their resources without sharing their credentials.

• OAuth2.0 involves the client, resource server, authorization server, and the user.

• The client requests authorization from the user and receives an access token from the authorization server.

• The client then uses the access token to access the user's resources on the resource server.

• OAuth2.0 supports different grant types such as authorization co...read more

Using Java Stream operations to manipulate student objects

• Use Stream API to perform operations on a collection of student objects

• Apply filter, map, reduce, and other stream operations to transform and manipulate data

• Use lambda expressions to define the operations to be performed on each student object

• Example: filtering students based on a certain criteria, mapping student names to uppercase, calculating the average grade

Spring Boot annotations are used to simplify the development process by providing shortcuts for common tasks.

• Annotations like @SpringBootApplication, @RestController, @Autowired, @GetMapping, @PostMapping are commonly used in Spring Boot applications.

• These annotations help in configuring the application, defining RESTful endpoints, injecting dependencies, and mapping HTTP requests to controller methods.

• Annotations like @Component, @Service, @Repository are used for component ...read more

API gateway is a server that acts as an API front-end, receiving API requests, enforcing throttling and security policies, passing requests to the back-end service, and then passing the response back to the requester.

• Acts as a single entry point for all API requests

• Enforces security policies and authentication

• Handles request routing and load balancing

• Provides caching and rate limiting

• Logs and monitors API usage

• Examples: Amazon API Gateway, Apigee, Kong