CRUD methods (save, findById, findAll, delete) in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does the save method do in a Spring Data repository?

Consider a Spring Data JPA repository interface. What is the main behavior of the save method?

A It inserts a new entity or updates an existing one in the database.

B It only inserts a new entity and throws an error if the entity exists.

C It deletes the entity from the database.

D It retrieves an entity by its ID.

💡 Hint

Think about whether save can handle both new and existing data.

The save method in Spring Data repositories either inserts a new entity if it doesn't exist or updates it if it does. It does not delete or retrieve entities.

❓ state_output

intermediate

What is the output of findById when the entity is not found?

Given a Spring Data repository, what does findById return if no entity matches the given ID?

Spring Boot

Optional<Entity> result = repository.findById(999L);

A An empty <code>Optional</code> object.

B A <code>null</code> value.

C Throws <code>NoSuchElementException</code> immediately.

D Returns a default entity instance.

💡 Hint

Remember how Optional works in Java.

The findById method returns an Optional. If the entity is not found, the Optional is empty, not null or an exception.

📝 Syntax

advanced

Which code snippet correctly deletes an entity by ID using Spring Data repository?

Choose the correct way to delete an entity by its ID using a Spring Data repository.

A repository.deleteEntityById(entityId);

B repository.deleteById(entityId);

C repository.delete(entityId);

D repository.remove(entityId);

💡 Hint

Check the official Spring Data repository method names.

The correct method to delete by ID is deleteById. Other methods like remove or deleteEntityById do not exist in the standard repository interface.

🔧 Debug

advanced

Why does this findAll call cause a runtime error?

Given this code snippet, why does calling findAll cause a runtime error?

List list = repository.findAll();

Spring Boot

List<Entity> list = repository.findAll();

A The <code>findAll</code> method returns a Stream, not a List.

B The <code>findAll</code> method requires an argument specifying a filter.

C The entity class is missing a no-argument constructor.

D The repository interface does not extend <code>JpaRepository</code> or <code>CrudRepository</code>.

💡 Hint

JPA entities require a no-argument constructor for instantiation.

JPA/Hibernate requires a no-argument constructor on entity classes to instantiate objects from database results. Without it, findAll throws a runtime exception (e.g., InstantiationException) when mapping entities.

🧠 Conceptual

expert

What happens if you call deleteById with a non-existing ID?

In Spring Data JPA, what is the behavior when deleteById is called with an ID that does not exist in the database?

A It silently does nothing without throwing an error.

B It creates a new entity with that ID and then deletes it.

C It returns a boolean indicating success or failure.

D It throws <code>EmptyResultDataAccessException</code> immediately.

💡 Hint

Think about how Spring Data handles deleting non-existing records.

Calling deleteById with a non-existing ID throws EmptyResultDataAccessException to indicate no entity was found to delete.