Response DTO for output in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Spring Boot Response DTO?

Given this simple Response DTO class and controller method, what JSON will be returned when the endpoint is called?

Spring Boot

public record UserResponseDTO(String name, int age) {}

@RestController
public class UserController {
    @GetMapping("/user")
    public UserResponseDTO getUser() {
        return new UserResponseDTO("Alice", 30);
    }
}

A {"name":"Alice","age":30}

B {"name":"Alice","age":"30"}

C {"name":"Alice"}

D {"age":30}

💡 Hint

Remember that Java records serialize all components by default as JSON properties.

The record UserResponseDTO has two fields: name and age. Spring Boot uses Jackson to convert this record to JSON, including both fields with their exact types. So the JSON output includes both name as a string and age as a number.

📝 Syntax

intermediate

Which Response DTO code snippet will cause a compilation error?

Identify the code snippet that will NOT compile as a valid Spring Boot Response DTO.

A public record ProductResponseDTO(String id, double price) { public ProductResponseDTO() {} }

B public class ProductResponseDTO { private String id; private double price; public ProductResponseDTO(String id, double price) { this.id = id; this.price = price; } public String getId() { return id; } public double getPrice() { return price; } }

C public record ProductResponseDTO(String id, double price) {}

D public class ProductResponseDTO { public String id; public double price; }

💡 Hint

Records cannot have explicit no-argument constructors.

Option A tries to add a no-argument constructor to a record, which is not allowed in Java records. Records automatically generate a canonical constructor with all components. Adding a no-arg constructor causes a compilation error.

❓ state_output

advanced

What is the JSON output when a Response DTO has a nested object?

Consider this Response DTO with a nested Address record. What JSON is returned?

Spring Boot

public record Address(String street, String city) {}
public record UserResponseDTO(String name, Address address) {}

@RestController
public class UserController {
    @GetMapping("/user")
    public UserResponseDTO getUser() {
        Address addr = new Address("123 Main St", "Springfield");
        return new UserResponseDTO("Bob", addr);
    }
}

A {"name":"Bob","address":"123 Main St, Springfield"}

B {"name":"Bob","address":{"street":"123 Main St","city":"Springfield"}}

C {"name":"Bob","address":null}

D {"name":"Bob","address":{"street":"123 Main St"}}

💡 Hint

Nested records are serialized as nested JSON objects by default.

The nested Address record is serialized as a JSON object inside the address property. Both street and city fields appear in the nested JSON.

🔧 Debug

advanced

Why does this Response DTO return an empty JSON object?

Given this DTO and controller, why does the API return {} instead of the expected fields?

Spring Boot

public class EmptyResponseDTO {
    private String data;

    public String getData() {
        return data;
    }
}

@RestController
public class TestController {
    @GetMapping("/test")
    public EmptyResponseDTO getTest() {
        EmptyResponseDTO dto = new EmptyResponseDTO();
        dto.data = "Hello";
        return dto;
    }
}

A Because the field 'data' is not initialized in the constructor.

B Because the DTO class is missing the @JsonProperty annotation on the field.

C Because the controller method returns null instead of the DTO.

D Because the field 'data' is private and has no getter method, Jackson cannot serialize it.

💡 Hint

Jackson needs public getters or annotations to serialize private fields.

Jackson serializes Java objects to JSON by calling public getters or accessing fields with proper annotations. Since 'data' is private and no getter is provided, Jackson sees no properties to serialize, resulting in an empty JSON object.

🧠 Conceptual

expert

Which statement about Response DTOs in Spring Boot is TRUE?

Select the one correct statement about how Response DTOs work in Spring Boot.

A Response DTOs must always be immutable classes to be serialized correctly by Spring Boot.

B Response DTOs require explicit @ResponseBody annotation on every field to be included in the JSON output.

C Spring Boot automatically converts Response DTOs to JSON using Jackson if the controller method returns an object and the client accepts JSON.

D Spring Boot only supports Response DTOs implemented as Java records, not regular classes.

💡 Hint

Think about how Spring Boot handles controller return values and content negotiation.

Spring Boot uses the Jackson library by default to convert returned objects from controller methods into JSON when the client requests JSON. This happens automatically without needing extra annotations on fields.