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.

Practice

1. What is the primary purpose of the JpaRepository interface in Spring Boot?

easy

A. To provide built-in methods for database operations on entities

B. To define the structure of REST API endpoints

C. To manage application security and authentication

D. To handle frontend UI rendering

Solution

1. Step 1: Understand JpaRepository role

   JpaRepository is designed to simplify database access by providing ready-made methods like save, findAll, and delete for entity classes.

2. Step 2: Compare with other options

   Options A, C, and D relate to REST API endpoints, security, and UI rendering, which are not responsibilities of JpaRepository.

3. Final Answer:

   To provide built-in methods for database operations on entities -> Option A

4. Quick Check:

   JpaRepository = database helper [OK]

Hint: JpaRepository = database methods for entities [OK]

Common Mistakes:

• Confusing JpaRepository with REST controllers
• Thinking it manages security
• Assuming it handles UI rendering

2. Which of the following is the correct way to declare a repository interface for an entity User with primary key type Long using JpaRepository?

easy

A. public interface UserRepository extends Repository<User> {}

B. public class UserRepository implements JpaRepository {}

C. public interface UserRepository extends JpaRepository {}

D. public interface UserRepository extends JpaRepository {}

Solution

1. Step 1: Check JpaRepository declaration syntax

   JpaRepository is an interface that should be extended, not implemented. The generic parameters are , so is correct.

2. Step 2: Validate each option

   public interface UserRepository extends JpaRepository {} correctly extends JpaRepository with . public class UserRepository implements JpaRepository {} incorrectly uses implements and class. public interface UserRepository extends JpaRepository {} swaps generic types. public interface UserRepository extends Repository<User> {} uses Repository, not JpaRepository.

3. Final Answer:

   public interface UserRepository extends JpaRepository {} -> Option D

4. Quick Check:

   Extend JpaRepository [OK]

Hint: Extend JpaRepository interface [OK]

Common Mistakes:

• Using implements instead of extends
• Swapping generic type order
• Using Repository instead of JpaRepository

3. Given the following repository method declaration:

List<User> findByLastName(String lastName);

What will this method do when called with findByLastName("Smith")?

medium

A. Return all User entities with lastName exactly 'Smith'

B. Return all User entities with lastName containing 'Smith'

C. Return a single User entity with lastName 'Smith'

D. Throw a runtime error because the method is invalid

Solution

1. Step 1: Understand method naming convention

   JpaRepository supports query derivation by method names. 'findByLastName' means find all entities where lastName equals the given parameter.

2. Step 2: Analyze return type and behavior

   The return type is List<User>, so it returns all matching users with lastName exactly 'Smith'. It does not do partial matching or throw errors.

3. Final Answer:

   Return all User entities with lastName exactly 'Smith' -> Option A

4. Quick Check:

   findByProperty = exact match [OK]

Hint: findByX = exact match, returns list if List type [OK]

Common Mistakes:

• Assuming it does partial matching
• Expecting a single result instead of list
• Thinking method is invalid without @Query

4. Consider this repository interface:

public interface ProductRepository extends JpaRepository {
    List<Product> findByPriceGreaterThan(Double price);
}

Which of the following is a likely cause of a runtime error when calling findByPriceGreaterThan(null)?

medium

A. JpaRepository does not support comparison keywords like GreaterThan

B. Method name is invalid and causes compile error

C. Passing null causes a NullPointerException in the query generation

D. The return type List<Product> is incorrect

Solution

1. Step 1: Check method name and support

   JpaRepository supports keywords like GreaterThan for query derivation, so method name is valid and compiles fine.

2. Step 2: Analyze passing null parameter

   Passing null to a comparison query causes a NullPointerException at runtime because the query cannot compare with null.

3. Final Answer:

   Passing null causes a NullPointerException in the query generation -> Option C

4. Quick Check:

   Null param in comparison query = runtime error [OK]

Hint: Never pass null to comparison query methods [OK]

Common Mistakes:

• Thinking method name is invalid
• Assuming JpaRepository lacks GreaterThan support
• Believing return type causes error

5. You want to add a custom method to your OrderRepository that finds all orders placed between two dates. Which of the following method signatures correctly uses JpaRepository naming conventions to achieve this?

hard

A. List<Order> findOrdersBetweenDates(LocalDate start, LocalDate end);

B. List<Order> findByOrderDateBetween(LocalDate start, LocalDate end);

C. List<Order> getOrdersByDateRange(LocalDate start, LocalDate end);

D. List<Order> findByOrderDateRange(LocalDate start, LocalDate end);

Solution

1. Step 1: Recall JpaRepository method naming rules

   JpaRepository supports keywords like Between to filter values between two parameters. The property name must match entity field, here 'OrderDate'.

2. Step 2: Evaluate each method signature

   List<Order> findByOrderDateBetween(LocalDate start, LocalDate end); uses 'findByOrderDateBetween' which is correct. Options B, C, and D use unsupported or incorrect keywords and will not work.

3. Final Answer:

   List<Order> findByOrderDateBetween(LocalDate start, LocalDate end); -> Option B

4. Quick Check:

   Use Between keyword for range queries [OK]

Hint: Use 'Between' keyword for range queries in method name [OK]

Common Mistakes:

• Using unsupported keywords like 'Range' or 'BetweenDates'
• Not matching property name exactly
• Trying to create custom method without @Query but wrong name