Nested DTOs in Spring Boot

Introduction

Nested DTOs help organize related data inside other data objects. They make it easier to send or receive complex information in a clear way.

When you want to send a user object that contains an address object inside it.

When your API needs to return a product with a list of reviews, each review being its own object.

When you want to group related fields together inside a bigger data transfer object.

When you want to keep your code clean by separating parts of data into smaller objects.

When you need to map complex JSON structures to Java objects in Spring Boot.

Syntax

Spring Boot

public class OuterDTO {
    private InnerDTO inner;

    // getters and setters

    public static class InnerDTO {
        private String field;

        // getters and setters
    }
}

Nested DTOs are usually static inner classes or separate classes referenced inside another DTO.

Use getters and setters or Lombok annotations to access nested fields.

Examples

This example shows a UserDTO containing an AddressDTO as a nested object.

Spring Boot

public class UserDTO {
    private String name;
    private AddressDTO address;

    // getters and setters
}

public class AddressDTO {
    private String street;
    private String city;

    // getters and setters
}

Here, OrderDTO has a list of ItemDTO objects to represent multiple items.

Spring Boot

import java.util.List;

public class OrderDTO {
    private int id;
    private List<ItemDTO> items;

    // getters and setters
}

public class ItemDTO {
    private String productName;
    private int quantity;

    // getters and setters
}

This shows a static nested DTO class inside another DTO class.

Spring Boot

public class OuterDTO {
    private InnerDTO inner;

    public static class InnerDTO {
        private String detail;

        // getters and setters
    }

    // getters and setters
}

Sample Program

This example defines a PersonDTO with a nested ContactDTO inside it. The main method creates a person with contact details and prints them.

Spring Boot

package com.example.demo.dto;

public class PersonDTO {
    private String name;
    private ContactDTO contact;

    public PersonDTO() {}

    public PersonDTO(String name, ContactDTO contact) {
        this.name = name;
        this.contact = contact;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public ContactDTO getContact() {
        return contact;
    }

    public void setContact(ContactDTO contact) {
        this.contact = contact;
    }

    public static class ContactDTO {
        private String email;
        private String phone;

        public ContactDTO() {}

        public ContactDTO(String email, String phone) {
            this.email = email;
            this.phone = phone;
        }

        public String getEmail() {
            return email;
        }

        public void setEmail(String email) {
            this.email = email;
        }

        public String getPhone() {
            return phone;
        }

        public void setPhone(String phone) {
            this.phone = phone;
        }
    }
}

// Usage example in a main method or test
class Main {
    public static void main(String[] args) {
        PersonDTO.ContactDTO contact = new PersonDTO.ContactDTO("[e-mail]", "123-456-7890");
        PersonDTO person = new PersonDTO("Alice", contact);

        System.out.println("Name: " + person.getName());
        System.out.println("Email: " + person.getContact().getEmail());
        System.out.println("Phone: " + person.getContact().getPhone());
    }
}

Important Notes

Nested DTOs help keep related data together and improve code organization.

Remember to provide getters and setters for nested objects to access their fields.

Using static nested classes avoids unnecessary references to the outer class instance.

Summary

Nested DTOs group related data inside other data objects.

They make it easier to handle complex data structures in Spring Boot.

Use static inner classes or separate classes to create nested DTOs.

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