Nested DTOs in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a nested DTO class inside the main DTO.

Spring Boot

public class UserDTO {
    private String name;
    private AddressDTO [1];

    public static class AddressDTO {
        private String city;
        private String zipCode;
    }
}

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

AaddressDto

BaddressDTO

Caddress

DaddressData

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name as the field name without camelCase.

Using unrelated names that do not represent the nested DTO.

The field name for the nested DTO should be 'address' to represent the AddressDTO object inside UserDTO.

2fill in blank

medium

Complete the constructor to initialize the nested DTO field.

Spring Boot

public UserDTO(String name, AddressDTO [1]) {
    this.name = name;
    this.address = [1];
}

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

Aaddress

Baddr

CaddressDTO

Dlocation

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and the field.

Forgetting to assign the parameter to the field.

The constructor parameter and assignment should use the same field name 'address' to initialize the nested DTO.

3fill in blank

hard

Fix the error in the getter method for the nested DTO field.

Spring Boot

public AddressDTO [1]() {
    return address;
}

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

Aaddress

BgetAddress

CfetchAddress

DretrieveAddress

Attempts:

3 left

💡 Hint

Common Mistakes

Naming the getter method the same as the field.

Using non-standard prefixes like 'fetch' or 'retrieve'.

The getter method for a field named 'address' should be named 'getAddress' following Java bean conventions.

4fill in blank

hard

Fill both blanks to create a nested DTO with Lombok annotations for getters and setters.

Spring Boot

@Data
public class UserDTO {
    private String name;
    private [1] [2];

    @Data
    public static class AddressDTO {
        private String city;
        private String zipCode;
    }
}

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

AAddressDTO

Baddress

CString

Dcity

Attempts:

3 left

💡 Hint

Common Mistakes

Using primitive types instead of the nested DTO type.

Using incorrect field names that do not match the nested DTO.

The field type is 'AddressDTO' and the field name is 'address' to represent the nested DTO with Lombok @Data annotation.

5fill in blank

hard

Fill all three blanks to map nested DTO fields in a Spring Boot controller method.

Spring Boot

@PostMapping("/user")
public ResponseEntity<String> createUser(@RequestBody UserDTO [1]) {
    String city = [2].get[3]().getCity();
    // process city
    return ResponseEntity.ok("User from " + city);
}

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

AuserDTO

BAddress

CAddressDTO

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent parameter names.

Incorrect getter method names for nested DTO.

The method parameter is 'userDTO', then we call 'userDTO.getAddress()' to access the nested DTO and get the city.

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