Entity to DTO mapping in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this DTO mapping method?

Given the following Spring Boot entity and DTO classes, what will be the value of userDTO.getFullName() after calling mapToDTO(user)?

Spring Boot

public class User {
    private String firstName;
    private String lastName;
    private int age;

    // getters and setters
    public String getFirstName() { return firstName; }
    public void setFirstName(String firstName) { this.firstName = firstName; }
    public String getLastName() { return lastName; }
    public void setLastName(String lastName) { this.lastName = lastName; }
    public int getAge() { return age; }
    public void setAge(int age) { this.age = age; }
}

public class UserDTO {
    private String fullName;
    private int age;

    // getters and setters
    public String getFullName() { return fullName; }
    public void setFullName(String fullName) { this.fullName = fullName; }
    public int getAge() { return age; }
    public void setAge(int age) { this.age = age; }
}

public UserDTO mapToDTO(User user) {
    UserDTO dto = new UserDTO();
    dto.setFullName(user.getFirstName() + " " + user.getLastName());
    dto.setAge(user.getAge());
    return dto;
}

// Usage:
User user = new User();
user.setFirstName("Jane");
user.setLastName("Doe");
user.setAge(30);
UserDTO userDTO = mapToDTO(user);

A "JaneDoe"

B "Jane Doe"

C "Doe Jane"

D "Doe"

💡 Hint

Look at how the fullName is constructed by concatenating firstName and lastName with a space.

The mapToDTO method concatenates user.getFirstName() + " " + user.getLastName(), so the fullName will be "Jane Doe".

📝 Syntax

intermediate

Which option correctly maps a list of entities to a list of DTOs using Java streams?

You want to convert a List to List using Java streams in Spring Boot. Which code snippet correctly performs this mapping?

Spring Boot

List<User> users = ...; // list of User entities

// Map to List<UserDTO>

A List<UserDTO> dtos = users.stream().map(user -> mapToDTO(user)).collect(Collectors.toList());

B List<UserDTO> dtos = users.map(user -> mapToDTO(user)).toList();

C List<UserDTO> dtos = users.stream().map(mapToDTO).collect(Collectors.toList());

D List<UserDTO> dtos = users.stream().map(user -> mapToDTO).collect(Collectors.toList());

💡 Hint

Remember that List does not have a map method, streams do.

Option A correctly uses users.stream(), then maps each user with mapToDTO(user), and collects to a list. Option A is invalid because List has no map method. Option A is invalid because mapToDTO is a method, not a variable or function reference without parentheses. Option A passes the method itself instead of calling it.

🔧 Debug

advanced

Why does this mapping code throw a NullPointerException?

Consider this mapping method in a Spring Boot service: public UserDTO mapToDTO(User user) { UserDTO dto = new UserDTO(); dto.setFullName(user.getFirstName().toUpperCase() + " " + user.getLastName().toUpperCase()); dto.setAge(user.getAge()); return dto; } If user.getFirstName() is null, what happens when this method runs?

A It throws a NullPointerException because calling toUpperCase() on null causes an error.

B It compiles but skips setting fullName.

C It returns a UserDTO with fullName as " " (empty string).

D It returns a UserDTO with fullName as "null null".

💡 Hint

What happens if you call a method on a null object in Java?

Calling toUpperCase() on a null string causes a NullPointerException at runtime.

🧠 Conceptual

advanced

What is the main benefit of using DTOs instead of exposing entities directly in Spring Boot APIs?

Why do developers often map entities to DTOs before sending data in API responses?

A To make the API responses larger and more detailed by including all entity fields.

B To increase coupling between database schema and API contracts.

C To avoid writing any mapping code and use entities directly everywhere.

D To control and limit the data exposed to clients, improving security and decoupling internal models.

💡 Hint

Think about why you might not want to share your database structure directly with users.

DTOs help control what data is sent out, hide sensitive fields, and allow internal models to change without breaking API contracts.

❓ state_output

expert

What is the value of dto.getAddress().getCity() after mapping?

Given these classes and mapping method, what will dto.getAddress().getCity() return? public class User { private String name; private Address address; // getters/setters } public class Address { private String city; private String street; // getters/setters } public class UserDTO { private String name; private AddressDTO address; // getters/setters } public class AddressDTO { private String city; // getters/setters } public UserDTO mapToDTO(User user) { UserDTO dto = new UserDTO(); dto.setName(user.getName()); AddressDTO addrDto = new AddressDTO(); addrDto.setCity(user.getAddress().getCity()); dto.setAddress(addrDto); return dto; } // Usage: User user = new User(); user.setName("Alice"); Address addr = new Address(); addr.setCity("Springfield"); addr.setStreet("Main St"); user.setAddress(addr); UserDTO dto = mapToDTO(user);

A "Main St"

B null

C "Springfield"

D Throws NullPointerException

💡 Hint

Check how the city field is copied from Address to AddressDTO.

The mapToDTO method copies the city from user.getAddress() to AddressDTO. The street is not copied, so dto.getAddress().getCity() returns "Springfield".