Entity to DTO mapping in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a DTO class with private fields and public getters.

Spring Boot

public class UserDTO {
    private String [1];

    public String getUsername() {
        return username;
    }
}

Drag options to blanks, or click blank then click option'

Ausername

Buser_id

Cname

DuserName

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field name that does not match the getter method.

Using underscores in variable names in Java.

The field name must match the getter method name. Here, the getter is getUsername(), so the field should be username.

2fill in blank

medium

Complete the code to map an entity's field to a DTO field in the constructor.

Spring Boot

public UserDTO(UserEntity entity) {
    this.username = entity.[1]();
}

Drag options to blanks, or click blank then click option'

AgetUsername

Bget_name

CgetuserName

DgetUserName

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect casing in method names.

Using underscores in method names.

The entity's getter method should follow Java naming conventions. Usually, it is getUsername() for a field named username.

3fill in blank

hard

Fix the error in the mapping method that converts an entity to a DTO.

Spring Boot

public static UserDTO fromEntity(UserEntity entity) {
    return new UserDTO([1]);
}

Drag options to blanks, or click blank then click option'

Aentity.convertToDTO()

Bentity.getUsername()

Centity.toDTO()

Dentity

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a field value instead of the whole entity to the constructor.

Calling non-existent methods on the entity.

The constructor expects a UserEntity object, so pass entity directly to fix the error of passing a method result like a String from getUsername().

4fill in blank

hard

Complete the code to complete the method that converts a list of entities to a list of DTOs using streams.

Spring Boot

public static List<UserDTO> fromEntityList(List<UserEntity> entities) {
    return entities.stream()
        .map(entity -> new UserDTO(entity))
        .collect(Collectors.[1]());
}

Drag options to blanks, or click blank then click option'

BtoList

CtoSet

DtoCollection

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a method on the entity inside the constructor call.

Using Collectors.toSet() when a list is expected.

The constructor expects the whole entity, so no method call is needed inside the parentheses (blank 1 is empty). The stream collects to a list, so Collectors.toList() is correct.

5fill in blank

hard

Fill all three blanks to complete the code for a manual mapping method from DTO to entity.

Spring Boot

public UserEntity toEntity() {
    UserEntity entity = new UserEntity();
    entity.set[1](this.[2]);
    entity.setEmail(this.[3]);
    return entity;
}

Drag options to blanks, or click blank then click option'

AUsername

Busername

Cemail

DEmail

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect casing in setter method names.

Using wrong field names from the DTO.

The setter method uses PascalCase for the field name: setUsername. The DTO field is username (camelCase). The email field is accessed as email in the DTO and set via setEmail in the entity.

Practice

1. What is the main purpose of mapping an Entity to a DTO in Spring Boot?

easy

A. To speed up database queries

B. To separate database structure from data sent to clients

C. To store data in a different database

D. To automatically generate database tables

Solution

1. Step 1: Understand Entity and DTO roles

   Entity represents database data, DTO is for data transfer outside the app.

2. Step 2: Identify the purpose of mapping

   Mapping hides database details and controls what data is sent to clients.

3. Final Answer:

   To separate database structure from data sent to clients -> Option B

4. Quick Check:

   Entity to DTO mapping = data separation [OK]

Hint: Think: Entity is internal, DTO is external data format [OK]

Common Mistakes:

• Confusing DTO with database storage
• Thinking mapping speeds up queries
• Assuming DTO changes database schema

2. Which of the following is the correct way to manually map an Entity field name to a DTO field fullName in Java?

easy

A. entity.getName(dto.setFullName());

B. entity.setName(dto.getFullName());

C. dto.getFullName(entity.setName());

D. dto.setFullName(entity.getName());

Solution

1. Step 1: Identify source and target objects

   Entity is source, DTO is target for mapping.

2. Step 2: Use getter on entity and setter on DTO

   Correct syntax is calling entity.getName() and passing to dto.setFullName().

3. Final Answer:

   dto.setFullName(entity.getName()); -> Option D

4. Quick Check:

   Getter from entity, setter on DTO [OK]

Hint: Getter from entity, setter on DTO for mapping [OK]

Common Mistakes:

• Reversing source and target in mapping
• Using setter as getter or vice versa
• Calling methods with wrong parameters

3. Given the following code snippet, what will be the output of dto.getAge() after mapping?

public class UserEntity {
  private int age = 30;
  public int getAge() { return age; }
}

public class UserDTO {
  private int age;
  public void setAge(int age) { this.age = age; }
  public int getAge() { return age; }
}

UserEntity entity = new UserEntity();
UserDTO dto = new UserDTO();
dto.setAge(entity.getAge());
System.out.println(dto.getAge());

medium

A. 30

B. 0

C. null

D. Compilation error

Solution

1. Step 1: Check initial value in Entity

   Entity's age is set to 30 by default.

2. Step 2: Map Entity age to DTO and print

   DTO's age is set to entity.getAge(), so dto.getAge() returns 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Entity age 30 mapped to DTO age 30 [OK]

Hint: Mapping copies values exactly unless changed [OK]

Common Mistakes:

• Assuming default int is null
• Confusing getter/setter roles
• Expecting compilation error without syntax issues

4. Identify the error in this manual mapping method:

public UserDTO mapToDTO(UserEntity entity) {
  UserDTO dto = new UserDTO();
  dto.setName(entity.getFullName());
  dto.setEmail(entity.getEmail());
  return dto;
}

Assuming UserEntity has a getName() method but no getFullName() method.

medium

A. Setter methods used incorrectly

B. Missing return statement

C. Calling non-existent method getFullName() on entity

D. DTO object not created

Solution

1. Step 1: Check entity methods used

   Code calls entity.getFullName(), but entity only has getName().

2. Step 2: Identify cause of error

   Calling a method that does not exist causes a compile-time error.

3. Final Answer:

   Calling non-existent method getFullName() on entity -> Option C

4. Quick Check:

   Method must exist on entity for mapping [OK]

Hint: Verify entity methods before calling in mapping [OK]

Common Mistakes:

• Assuming method names match automatically
• Ignoring compile errors from wrong method calls
• Confusing getter names between classes

5. You want to map a list of UserEntity objects to a list of UserDTO objects using Java streams in Spring Boot. Which code snippet correctly performs this mapping assuming a method mapToDTO(UserEntity entity) exists?

hard

A. List<UserDTO> dtos = entities.stream().map(this::mapToDTO).collect(Collectors.toList());

B. List<UserDTO> dtos = entities.map(entity -> mapToDTO(entity));

C. List<UserDTO> dtos = entities.forEach(entity -> mapToDTO(entity));

D. List<UserDTO> dtos = entities.stream().forEach(this::mapToDTO);

Solution

1. Step 1: Use stream() to process list

   entities.stream() creates a stream to transform elements.

2. Step 2: Use map() to convert each entity to DTO

   map(this::mapToDTO) applies the mapping method to each element.

3. Step 3: Collect results into a list

   collect(Collectors.toList()) gathers mapped DTOs into a list.

4. Final Answer:

   List<UserDTO> dtos = entities.stream().map(this::mapToDTO).collect(Collectors.toList()); -> Option A

5. Quick Check:

   Stream map + collect to list = correct mapping [OK]

Hint: Use stream().map(...).collect(toList()) for list mapping [OK]

Common Mistakes:

• Using forEach instead of map for transformation
• Calling map on list directly without stream()
• Not collecting results after mapping