10+ Java Back End Developer Interview Questions and Answers

Design a layered architecture for saving user data to db with appropriate annotations.

• Create a data access layer (DAO) to interact with the database using annotations like @Repository

• Implement a service layer to handle business logic with annotations like @Service

• Use @Entity annotation on model classes to map them to database tables

• Add @Id annotation on primary key fields in model classes

• Utilize @Transactional annotation on service methods for transaction management

The @Primary annotation is used to give a higher priority to a bean when multiple beans of the same type are present. The @Qualifier annotation is used to specify which bean to inject when multiple beans of the same type are present.

• Use @Primary annotation to specify the primary bean to be used when multiple beans of the same type are present.

• Use @Qualifier annotation along with the bean name to specify which bean to inject when multiple beans of the same type are present.

• Exa...read more

SQL query to find nth highest salary using LIMIT and OFFSET.

• Use ORDER BY salary DESC to sort salaries in descending order.

• Use LIMIT 1 OFFSET n-1 to get the nth highest salary.

• For example, SELECT salary FROM employees ORDER BY salary DESC LIMIT 1 OFFSET 2 will give the 3rd highest salary.

LinkedHashSet is the data structure to use for storing data in order of insertion and uniqueness.

• LinkedHashSet maintains insertion order and ensures uniqueness of elements.

• It uses a combination of LinkedList and HashSet to achieve this.

• Example: LinkedHashSet set = new LinkedHashSet<>();

Implemented multithreading using Java's Thread class and Executor framework.

• Used Thread class to create and manage threads.

• Utilized Executor framework for managing thread pools and executing tasks.

• Implemented synchronization mechanisms like locks and semaphores to prevent race conditions.

• Used Java's concurrent data structures like ConcurrentHashMap and BlockingQueue for thread-safe operations.

Lock isolation in Spring Framework ensures that each transaction operates independently without interfering with other transactions.

• Lock isolation prevents concurrent transactions from affecting each other's data.

• Different levels of lock isolation can be set in Spring, such as READ_COMMITTED and REPEATABLE_READ.

• For example, setting a higher level of lock isolation like REPEATABLE_READ ensures that a transaction will not see changes made by other transactions until it is compl...read more

Using RestControllerAdvice to handle exceptions at controller level in Java

• Create a class annotated with @RestControllerAdvice to handle exceptions globally

• Use @ExceptionHandler annotation in the class to define methods to handle specific exceptions

• Return ResponseEntity with appropriate status code and error message in the exception handling methods

Saga pattern is used to manage distributed transactions in microservices by breaking them into smaller, independent transactions.

• Saga pattern involves breaking down a long transaction into a series of smaller, independent transactions.

• Each step in the saga is a separate transaction that can be rolled back if needed.

• Compensating transactions are used to undo the effects of a previously completed step in case of failure.

• Sagas can be implemented using choreography or orchestrati...read more

Use counting sort to achieve O(N) complexity.

• Count the frequency of each string in the array.

• Create a new array based on the frequency counts.

• Fill the new array with the strings based on their frequency counts.