Centelon Software Engineer Interview Questions and Answers

JDK is a development kit for creating Java applications, while JRE is a runtime environment for executing Java programs.

• JDK stands for Java Development Kit and includes tools for developing Java applications, such as compiler and debugger.

• JRE stands for Java Runtime Environment and is used to run Java applications. It includes JVM, libraries, and other necessary components.

• JDK is needed for developing Java applications...read more

No, Java programs require JRE to run.

• Java programs are compiled into bytecode which is executed by the Java Runtime Environment (JRE)

• JRE is necessary to interpret and run Java programs

• Without JRE, Java programs cannot be executed

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

• HashMap uses an array of linked lists to store key-value pairs.

• When a key-value pair is added, the key is hashed to determine its index in the array.

• If multiple keys hash to the same index, a linked list is used to handle collisions.

• HashMap allows null keys and values, and does not guarantee the order of key-

A key with a null value in HashMap will be stored in the HashMap as a key-value pair with a null value.

• In a HashMap, keys can have null values but only one key with a null value is allowed.

• If you try to store a key with a null value multiple times, the previous value associated with that key will be overwritten.

• Example: HashMap map = new HashMap<>(); map.put(null, 5); // key=null, value=5

Fail-fast stops immediately upon detecting an error, while fail-safe continues to operate despite errors. Fail-safe is synchronized.

• Fail-fast immediately stops the operation upon encountering an error, ensuring that no further damage is done.

• Fail-safe continues to operate despite errors, often by handling the error and allowing the program to continue running.

• Fail-safe is synchronized, meaning that it is designed to ha...read more

HashMap is unordered while LinkedHashMap maintains insertion order.

• HashMap does not maintain insertion order while LinkedHashMap does.

• LinkedHashMap extends HashMap and adds a doubly-linked list to maintain order.

• HashMap is faster for iteration while LinkedHashMap is slower due to maintaining order.

Load factor in HashMap determines when the HashMap should be resized to maintain performance.

• Load factor is a measure of how full the HashMap is allowed to get before its capacity is automatically increased.

• The default load factor of HashMap is 0.75, which means the HashMap will be resized when it is 75% full.

• Increasing the load factor reduces the space overhead but increases the time cost of resolving collisions.

• Examp...read more

A Daemon thread is a background thread that runs without blocking the main program from exiting.

• Daemon threads are used for tasks that do not require user interaction or continuous monitoring.

• They are automatically terminated by the JVM when all non-daemon threads have finished.

• An example of a Daemon thread is the garbage collector in Java.

Multithreading in Java allows multiple threads to execute concurrently, improving performance and responsiveness.

• Multithreading allows multiple threads to run concurrently within a single process.

• Each thread has its own stack and program counter, but share the same heap memory.

• Java provides built-in support for multithreading through the java.lang.Thread class.

• Example: Creating a new thread by extending the Thread clas

Executor framework in Java is a framework that provides a way to manage and control the execution of tasks in a multithreaded environment.

• Allows for decoupling task submission from task execution

• Provides a way to manage thread creation, pooling, and scheduling of tasks

• Helps in handling exceptions and errors that occur during task execution

• Examples include ThreadPoolExecutor and ScheduledThreadPoolExecutor classes

Some methods in Executor framework include execute(), submit(), shutdown(), awaitTermination(), and invokeAll().

• execute() - Used to execute the given command at some time in the future.

• submit() - Submits a Runnable or Callable task for execution and returns a Future representing the task.

• shutdown() - Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.

• awaitT...read more

The join method in Java is used to concatenate the elements of an array into a single string.

• The join method is a part of the String class in Java.

• It takes an array of strings as input and concatenates them with a specified delimiter.

• Example: String[] arr = {"Hello", "World"}; String result = String.join(", ", arr); // Output: Hello, World

One-to-many and many-to-one relationships in Hibernate are used to define the relationship between entities in a database.

• One-to-many relationship: One entity can be associated with multiple instances of another entity. For example, one department can have multiple employees.

• Many-to-one relationship: Multiple instances of one entity can be associated with a single instance of another entity. For example, multiple emplo

An ID in Hibernate is a unique identifier for an entity in a database. It is used to uniquely identify each record.

• ID is a field in an entity class annotated with @Id in Hibernate.

• It can be of different types like int, long, String, etc.

• ID is used to uniquely identify each record in a database table.

• It is often generated automatically using strategies like @GeneratedValue.

• Example: @Id @GeneratedValue(strategy = Generat

Custom queries in Spring Data JPA can be created using @Query annotation or method naming conventions.

• Use @Query annotation with JPQL or native SQL to define custom queries

• Follow method naming conventions to automatically generate queries based on method names

• Use parameters in custom queries by adding method parameters and referencing them in the query

Some common annotations used in Spring Boot are @RestController, @Autowired, @RequestMapping, @Component, @Service, @Repository.

• @RestController - Used to define a controller class in Spring MVC.

• @Autowired - Used for automatic dependency injection.

• @RequestMapping - Used to map web requests to specific handler methods.

• @Component - Indicates that a class is a Spring component.

• @Service - Indicates that a class is a Spring ...read more

To create a repository interface in Spring Data JPA, you need to define an interface that extends JpaRepository.

• Create an interface that extends JpaRepository, where T is the entity type and ID is the type of the entity's primary key.

• Add custom query methods by defining method signatures in the repository interface.

• Spring Data JPA will automatically generate the implementation for the repository interface at run

Custom queries in repository interface are defined using @Query annotation in Spring Data JPA.

• Use @Query annotation above the method declaration in the repository interface.

• Write the custom query in JPQL or native SQL within the @Query annotation.

• You can also pass parameters to the query using method parameters.

The main difference is that @RestController is a specialized version of @Controller that is used for RESTful web services.

• Both @Controller and @RestController are used in Spring MVC to handle HTTP requests, but @RestController is specifically used for RESTful web services.

• @Controller is used to create web pages that return HTML, while @RestController is used to create RESTful web services that return data in JSON or XM...read more

DispatcherServlet is the front controller in Spring Boot that receives incoming requests and dispatches them to the appropriate handler.

• DispatcherServlet is a servlet that manages the flow of incoming requests in a Spring Boot application.

• It acts as the front controller, receiving all requests and then dispatching them to the appropriate handler for processing.

• DispatcherServlet is configured in the web.xml file or thro...read more

Microservices are a software development technique where applications are broken down into smaller, independently deployable services.

• Microservices allow for easier scalability and maintenance of complex systems.

• Each microservice typically focuses on a specific business function.

• Communication between microservices is usually done through APIs.

• Examples of companies using microservices include Netflix, Amazon, and Uber.

Communication between microservices can happen through synchronous or asynchronous protocols like HTTP, messaging queues, or gRPC.

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

• Alternatively, microservices can communicate asynchronously using messaging queues like RabbitMQ or Kafka, where messages are sent between serv...read more

Messaging between microservices involves communication through message brokers or direct API calls.

• Microservices communicate through message brokers like RabbitMQ or Kafka.

• Messages are sent in a format like JSON or XML.

• Microservices can also communicate through direct API calls using REST or gRPC.

• Message queues help in decoupling services and ensuring reliable communication.

• Service discovery mechanisms like Consul or E

Find max and min sum after removing one element from array. Time complexity analysis required.

• Iterate through array to find sum of all elements

• For each element, calculate sum after removing it

• Track max and min sum as you iterate

• Time complexity: O(n) where n is the number of elements in the array