DTO pattern for data transfer in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the main purpose of a DTO in Spring Boot?

In a Spring Boot application, why do developers use a Data Transfer Object (DTO)?

A To handle HTTP requests directly from the client

B To transfer data between layers without exposing the entity directly

C To store data permanently in the database

D To replace the service layer logic

💡 Hint

Think about how data is shared safely between parts of the app.

A DTO is used to safely transfer data between layers, like from the controller to the service, without exposing the full entity or database structure.

📝 Syntax

intermediate

Which DTO declaration is syntactically correct in Spring Boot?

Choose the correct way to declare a simple DTO class in Spring Boot.

A public class UserDTO { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } }

B public class UserDTO { String name; getName() { return name; } setName(String name) { this.name = name; } }

C class UserDTO { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } }

D public UserDTO { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } }

💡 Hint

Remember Java class syntax requires access modifiers and method return types.

Option A correctly declares a public class with private field and public getter/setter methods with proper syntax.

❓ state_output

advanced

What will be the output of this DTO mapping code?

Given this code snippet in a Spring Boot service, what will be the value of userDTO.getEmail() after execution?

Spring Boot

User user = new User("Alice", "[e-mail]");
UserDTO userDTO = new UserDTO();
userDTO.setName(user.getName());
// Note: Email is not set in userDTO

String email = userDTO.getEmail();

A Throws NullPointerException

B "[e-mail]"

C Empty string ""

D null

💡 Hint

Check which fields are assigned values in the DTO.

The email field in userDTO was never set, so calling getEmail() returns null by default.

🔧 Debug

advanced

Why does this DTO cause a JSON serialization error?

Consider this DTO class used in a Spring Boot REST controller. Why does it cause a JSON serialization error?

Spring Boot

public class ProductDTO {
  private String name;
  private CategoryDTO category;
  // getters and setters
}

public class CategoryDTO {
  private String name;
  private ProductDTO product;
  // getters and setters
}

A Because of circular reference between ProductDTO and CategoryDTO causing infinite recursion

B Because fields are private without public getters and setters

C Because DTO classes must be annotated with @Entity

D Because JSON serialization requires fields to be static

💡 Hint

Think about what happens when JSON tries to convert linked objects referencing each other.

The circular reference between ProductDTO and CategoryDTO causes infinite recursion during JSON serialization, leading to an error.

🧠 Conceptual

expert

Which statement best describes the advantage of using DTOs in microservices?

In a microservices architecture, why is using DTOs beneficial when services communicate?

A DTOs allow direct database sharing between microservices

B DTOs automatically encrypt data between microservices for security

C DTOs reduce data transfer size by including only needed fields, improving performance and decoupling services

D DTOs replace the need for API gateways in microservices

💡 Hint

Consider how data is shared efficiently and safely between independent services.

DTOs help by sending only necessary data, reducing payload size and keeping services independent by not exposing internal entities.