idoow Fullstack Java Developer Interview Questions and Answers

JVM is the virtual machine that runs Java bytecode, JRE is the runtime environment for Java programs, and JDK is the development kit for creating Java applications.

• JVM (Java Virtual Machine) is responsible for running Java bytecode on different platforms.

• JRE (Java Runtime Environment) includes JVM, libraries, and other components necessary to run Java applications.

• JDK (Java Development Kit) is a software development ki...read more

String is immutable in Java to ensure security, thread safety, and optimization.

• Immutable strings prevent accidental changes to sensitive data like passwords.

• Immutable strings allow for safe sharing of string literals across multiple threads.

• Immutable strings enable string pooling for memory optimization.

• Example: String str = "Hello"; str.concat(" World"); // str remains "Hello"

SAGA pattern is a design pattern used in microservices architecture to manage distributed transactions.

• SAGA pattern breaks down a long-running transaction into a series of smaller, independent transactions.

• Each step in the SAGA pattern is a separate service that communicates with other services through events.

• If a step fails, compensating transactions are executed to rollback the changes made by previous steps.

• Example:...read more

API stands for Application Programming Interface. It defines the methods and data formats that applications can use to communicate with each other.

• API is a set of rules and protocols that allows different software applications to communicate with each other.

• To secure an API, you can use authentication methods like OAuth, API keys, or JWT tokens.

• Implementing encryption (HTTPS) and rate limiting can also enhance API secu...read more

Find duplicate characters in a given array of strings

• Iterate through each string in the array

• For each string, iterate through each character and store the count in a hashmap

• If the count of any character is greater than 1, it is a duplicate

Code for REST API using Spring Boot with annotations

• Use @RestController annotation to define the class as a REST controller

• Use @RequestMapping annotation to map HTTP requests to handler methods

• Use @GetMapping, @PostMapping, @PutMapping, @DeleteMapping annotations for specific HTTP methods

• Use @PathVariable annotation to extract values from the URI

• Use @RequestBody annotation to bind HTTP request body to a method paramete

A simple program to print 'fizz' for multiples of 3, 'buzz' for multiples of 5, and 'fizzbuzz' for multiples of both.

• Iterate through numbers from 1 to n.

• Use modulo operator to check divisibility by 3 and 5.

• Print 'fizz', 'buzz', or 'fizzbuzz' accordingly.

Yes, classes implementing an interface must implement interface methods.

• Classes implementing an interface must provide concrete implementations for all methods declared in the interface.

• Failure to implement all interface methods will result in a compilation error.

• Interfaces are used to define a contract that implementing classes must adhere to.

Synchronization is the process of coordinating multiple threads to ensure safe and consistent access to shared resources.

• Synchronization is used to prevent race conditions and ensure thread safety.

• It can be achieved using synchronized blocks or methods.

• Synchronization can cause performance issues if not used properly.

• Example: Synchronizing access to a shared counter variable in a multi-threaded application.

Encapsulation is the process of hiding implementation details and providing access to only necessary information.

• Encapsulation is achieved through access modifiers such as public, private, and protected.

• It helps in achieving data abstraction and information hiding.

• Encapsulation provides better control over the data and prevents unauthorized access.

• Example: A class with private variables and public methods to access the...read more

An interface in Java is a collection of abstract methods and constants that can be implemented by a class.

• Interfaces are used to achieve abstraction and provide a way to implement multiple inheritance in Java.

• All methods in an interface are abstract by default and cannot have a body.

• A class can implement multiple interfaces but can only extend one class.

• Interfaces can also have default and static methods with a body.

• Ex...read more

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

• It provides interfaces like List, Set, Queue, etc. for storing collections of objects.

• It also provides classes like ArrayList, HashSet, PriorityQueue, etc. that implement these interfaces.

• It allows easy manipulation of collections using methods like add, remove, contains, etc.

• It provides algorithms like ...read more

ArrayList is not synchronized while Vector is synchronized.

• ArrayList is faster than Vector as it is not synchronized.

• Vector is thread-safe while ArrayList is not.

• Vector is a legacy class while ArrayList is not.

• Vector can grow by a specified amount while ArrayList grows by 50% of its size.

• Example: ArrayList list = new ArrayList<>(); Vector vector = new Vector<>();

• Example: list.add(1); vector.add(1);

• Example: list.get(0);

Legacy classes in Java are the classes that have been replaced by newer versions but are still supported for backward compatibility.

• Legacy classes are part of the Java API and are still supported for backward compatibility.

• They have been replaced by newer versions but are still used in older applications.

• Examples of legacy classes include Vector, Hashtable, and Enumeration.

• Legacy classes are not recommended for use in ...read more

Joins in SQL are used to combine data from two or more tables based on a related column.

• Joins are used to retrieve data from multiple tables in a single query.

• There are different types of joins such as inner join, left join, right join, and full outer join.

• Inner join returns only the matching rows from both tables, left join returns all rows from the left table and matching rows from the right table, right join returns...read more

Autoboxing is the automatic conversion of primitive data types to their corresponding object wrapper classes.

• Autoboxing allows primitive data types to be used as objects.

• Auto-unboxing is the reverse process of autoboxing.

• Autoboxing and auto-unboxing are performed automatically by the compiler.

• Example: int i = 10; Integer j = i; // autoboxing

• Example: Integer j = 10; int i = j; // auto-unboxing

Where clause filters rows before grouping, having clause filters groups after grouping.

• Where clause is used to filter rows based on a condition before grouping.

• Having clause is used to filter groups based on a condition after grouping.

• Where clause is used with SELECT, UPDATE, DELETE statements.

• Having clause is used with SELECT statement and GROUP BY clause.

• Where clause cannot contain aggregate functions.

• Having clause c...read more