JpaRepository interface in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this JpaRepository method call?

Given a JpaRepository for an entity Product, what will be the result of calling findById(10L) if no product with ID 10 exists?

Spring Boot

Optional<Product> product = productRepository.findById(10L);
System.out.println(product.isPresent());

A false

B Throws NoSuchElementException

C null

D true

💡 Hint

Remember that findById returns an Optional, which may be empty if no entity is found.

The findById method returns an Optional. If the entity is not found, the Optional is empty, so isPresent() returns false.

📝 Syntax

intermediate

Which option correctly declares a JpaRepository for an entity User with primary key type Long?

Choose the correct interface declaration for a JpaRepository managing User entities with Long IDs.

A public interface UserRepository implements JpaRepository<User, Long> {}

B public interface UserRepository extends JpaRepository<User, Long> {}

C public class UserRepository extends JpaRepository<User, Long> {}

D public interface UserRepository extends JpaRepository<Long, User> {}

💡 Hint

JpaRepository is an interface and uses generics with entity type first, then ID type.

The correct syntax uses extends for interfaces and the generic order is Entity, ID.

❓ state_output

advanced

What is the state of the database after calling deleteById(5L) if no entity with ID 5 exists?

Assuming productRepository is a JpaRepository, what happens when productRepository.deleteById(5L) is called but no product with ID 5 exists?

A Throws EmptyResultDataAccessException

B Throws NullPointerException

C Deletes all entities in the repository

D No change; no exception is thrown

💡 Hint

Check the behavior of deleteById when the ID is not found.

The deleteById method throws EmptyResultDataAccessException if the entity with the given ID does not exist.

🧠 Conceptual

advanced

Which JpaRepository method returns a paginated list of entities?

You want to retrieve entities in pages instead of all at once. Which JpaRepository method should you use?

A saveAll(Iterable<T> entities)

B findById(Long id)

C deleteAll()

D findAll(Pageable pageable)

💡 Hint

Look for a method that accepts a Pageable parameter.

The findAll(Pageable pageable) method returns a Page of entities, allowing pagination.

🔧 Debug

expert

Why does this JpaRepository query method cause a runtime error?

Consider this method in a JpaRepository interface: List findByAgeGreaterThan(int age); When called, it throws an exception. What is the most likely cause?

Spring Boot

public interface UserRepository extends JpaRepository<User, Long> {
    List<User> findByAgeGreaterThan(int age);
}

A The method name is invalid syntax for Spring Data JPA

B The parameter type should be Integer, not int

C The User entity does not have a field named 'age'

D JpaRepository does not support query methods with parameters

💡 Hint

Check if the entity has the field used in the method name.

Spring Data JPA derives queries from method names. If the entity lacks the field 'age', it cannot create the query, causing a runtime error.