Nested DTOs in Spring Boot - Mini Project: Build & Apply

Nested DTOs in Spring Boot

📖 Scenario: You are building a simple Spring Boot application to manage books and their authors. Each book has a title and an author. The author has a name and an email.

🎯 Goal: Create nested DTO (Data Transfer Object) classes to represent a BookDTO that contains an AuthorDTO. This helps organize data cleanly when sending or receiving JSON in your application.

📋 What You'll Learn

Create a class AuthorDTO with fields name and email.

Create a class BookDTO with fields title and author of type AuthorDTO.

Use Java records to define these DTOs for simplicity.

Ensure the nested structure is correct for JSON serialization.

💡 Why This Matters

🌍 Real World

Nested DTOs are common in web APIs where objects contain other objects, like a book containing an author. This helps keep data organized and easy to manage.

💼 Career

Understanding nested DTOs is essential for backend developers working with Spring Boot to build clean, maintainable APIs that handle complex data structures.

1

Create the AuthorDTO record

Create a Java record called AuthorDTO with two fields: String name and String email.

Spring Boot

// Your code here

Hint

Use the record keyword to create a simple immutable data class with the specified fields.

2

Create the BookDTO record with nested AuthorDTO

Create a Java record called BookDTO with two fields: String title and AuthorDTO author. Use the previously created AuthorDTO as the type for the author field.

Spring Boot

public record AuthorDTO(String name, String email) {
}

// Your code here

Hint

Use the AuthorDTO type inside BookDTO to nest the author information.

3

Create an instance of AuthorDTO

Create a variable called author and assign it a new AuthorDTO instance with name set to "Jane Austen" and email set to "[e-mail]".

Spring Boot

public record AuthorDTO(String name, String email) {
}

public record BookDTO(String title, AuthorDTO author) {
}

// Your code here

Hint

Use the new keyword to create an instance of the AuthorDTO record with the exact values.

4

Create an instance of BookDTO using the nested AuthorDTO

Create a variable called book and assign it a new BookDTO instance with title set to "Pride and Prejudice" and author set to the previously created author variable.

Spring Boot

public record AuthorDTO(String name, String email) {
}

public record BookDTO(String title, AuthorDTO author) {
}

AuthorDTO author = new AuthorDTO("Jane Austen", "[e-mail]");

// Your code here

Hint

Use the author variable as the second argument when creating the BookDTO instance.

Practice

1. What is the main purpose of using Nested DTOs in Spring Boot applications?

easy

A. To handle HTTP requests without controllers

B. To group related data inside other data objects for better structure

C. To replace entity classes with simpler objects

D. To improve database query performance automatically

Solution

1. Step 1: Understand DTO role

   DTOs (Data Transfer Objects) are used to carry data between processes or layers.

2. Step 2: Identify Nested DTO purpose

   Nested DTOs group related data inside other DTOs to represent complex data structures clearly.

3. Final Answer:

   To group related data inside other data objects for better structure -> Option B

4. Quick Check:

   Nested DTOs = Group related data [OK]

Hint: Nested DTOs organize data inside other objects [OK]

Common Mistakes:

• Thinking nested DTOs improve database speed
• Confusing DTOs with entities
• Assuming nested DTOs replace controllers

2. Which of the following is the correct way to declare a nested DTO class inside a parent DTO in Spring Boot?

easy

A. public class ParentDTO { private class ChildDTO { private String name; } }

B. public class ParentDTO { class ChildDTO { private String name; } }

C. public class ParentDTO { public static class ChildDTO { private String name; } }

D. public class ParentDTO { static class ChildDTO { public String name; } }

Solution

1. Step 1: Check nested class modifiers

   Static nested classes are recommended for DTOs to avoid implicit reference to outer class.

2. Step 2: Validate access modifiers

   Public static nested class with private fields and getters/setters is standard practice.

3. Final Answer:

   public class ParentDTO { public static class ChildDTO { private String name; } } -> Option C

4. Quick Check:

   Static nested class with public modifier = public class ParentDTO { public static class ChildDTO { private String name; } } [OK]

Hint: Use public static nested class for nested DTOs [OK]

Common Mistakes:

• Using non-static nested classes causing memory leaks
• Declaring nested class as private making it inaccessible
• Using public fields instead of private with getters/setters

3. Given the following nested DTO classes, what will be the output of System.out.println(order.getCustomer().getName()); if order is initialized as below?

public class OrderDTO {
  private CustomerDTO customer;
  public CustomerDTO getCustomer() { return customer; }
  public void setCustomer(CustomerDTO customer) { this.customer = customer; }
  public static class CustomerDTO {
    private String name;
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
  }
}

OrderDTO order = new OrderDTO();
OrderDTO.CustomerDTO cust = new OrderDTO.CustomerDTO();
cust.setName("Alice");
order.setCustomer(cust);

medium

A. Alice

B. null

C. Compilation error

D. Runtime NullPointerException

Solution

1. Step 1: Analyze object initialization

   The customer object is created and its name is set to "Alice" before being assigned to order.

2. Step 2: Check method calls

   Calling order.getCustomer().getName() returns the name "Alice" as set previously.

3. Final Answer:

   Alice -> Option A

4. Quick Check:

   Nested DTO getter returns set value = Alice [OK]

Hint: Set nested DTO fields before accessing getters [OK]

Common Mistakes:

• Forgetting to set nested DTO before calling getter
• Confusing null with empty string
• Assuming compilation error due to nested class

4. Identify the error in the following nested DTO code snippet:

public class UserDTO {
  private AddressDTO address;
  public static class AddressDTO {
    private String city;
    public String getCity() { return city; }
    public void setCity(String city) { this.city = city; }
  }

  public AddressDTO getAddress() { return address; }
  public void setAddress(AddressDTO address) { this.address = address; }
}

UserDTO user = new UserDTO();
user.getAddress().setCity("Paris");

medium

A. Compilation error due to missing constructor

B. No error, code runs correctly

C. IllegalAccessError on accessing city field

D. NullPointerException because address is not initialized

Solution

1. Step 1: Check object initialization

   The address field in UserDTO is never initialized, so it is null by default.

2. Step 2: Analyze method call

   Calling user.getAddress().setCity("Paris") tries to call setCity on null, causing NullPointerException.

3. Final Answer:

   NullPointerException because address is not initialized -> Option D

4. Quick Check:

   Uninitialized nested DTO causes NullPointerException [OK]

Hint: Always initialize nested DTO before calling its methods [OK]

Common Mistakes:

• Assuming default constructor initializes nested DTO
• Thinking compilation error occurs
• Ignoring possibility of NullPointerException

5. You have a nested DTO structure where OrderDTO contains a list of ItemDTO objects. You want to convert this nested DTO into a flat list of item names using Java streams. Which code snippet correctly achieves this?

public class OrderDTO {
  private List<ItemDTO> items;
  public List<ItemDTO> getItems() { return items; }
  public void setItems(List<ItemDTO> items) { this.items = items; }
  public static class ItemDTO {
    private String name;
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
  }
}

OrderDTO order = ...; // initialized with items

hard

A. List<String> names = order.getItems().stream().map(OrderDTO.ItemDTO::getName).toList();

B. List<String> names = order.getItems().stream().flatMap(ItemDTO::getName).collect(Collectors.toList());

C. List<String> names = order.getItems().map(ItemDTO::getName).collect(Collectors.toList());

D. List<String> names = order.getItems().stream().map(item -> item.name).collect(Collectors.toList());

Solution

1. Step 1: Understand stream mapping

   To get a list of names, map each ItemDTO to its name using map(OrderDTO.ItemDTO::getName).

2. Step 2: Collect results

   Use toList() (Java 16+) or collect(Collectors.toList()) to gather results into a list.

3. Final Answer:

   List<String> names = order.getItems().stream().map(OrderDTO.ItemDTO::getName).toList(); -> Option A

4. Quick Check:

   Stream map + toList() = List<String> names = order.getItems().stream().map(OrderDTO.ItemDTO::getName).toList(); [OK]

Hint: Use stream().map(...).toList() to extract nested DTO fields [OK]

Common Mistakes:

• Using flatMap instead of map for simple field extraction
• Calling map on List directly without stream()
• Accessing private fields without getter