Request DTO for input in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a Spring Boot controller receives this Request DTO?

Consider this Spring Boot Request DTO and controller method:

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

@PostMapping("/user")
public ResponseEntity createUser(@RequestBody UserRequest request) {
    return ResponseEntity.ok("User " + request.name() + " is " + request.age() + " years old.");
}

If the JSON {"name":"Alice","age":30} is sent, what will the response body be?

A HTTP 400 Bad Request error

B "User null is 0 years old."

C "User Alice is 30 years old."

D "User Alice is null years old."

💡 Hint

Records automatically map JSON fields to constructor parameters by name.

The JSON fields match the record components exactly, so Spring Boot deserializes correctly. The response concatenates the values properly.

📝 Syntax

intermediate

Which Request DTO declaration is syntactically correct in Spring Boot using Java records?

Choose the correct Java record declaration for a Request DTO with fields email (String) and subscribed (boolean):

A public record SubscriptionRequest(String email, Boolean subscribed) {}

B public record SubscriptionRequest(String email; boolean subscribed) {}

C public record SubscriptionRequest { String email; boolean subscribed; }

D public record SubscriptionRequest(String email, boolean subscribed) {}

💡 Hint

Records use parentheses with comma-separated components.

Option D uses the correct syntax for Java records. Option D uses semicolon instead of comma. Option D is not a record syntax. Option D uses boxed Boolean which is valid but not the exact requested type.

🔧 Debug

advanced

Why does this Spring Boot controller fail to bind the JSON to the Request DTO?

Given this Request DTO and controller method:

public record ProductRequest(String productName, int quantity) {}

@PostMapping("/product")
public ResponseEntity addProduct(ProductRequest request) {
    return ResponseEntity.ok("Added " + request.productName() + ", qty: " + request.quantity());
}

When sending JSON {"productName":"Pen","quantity":10}, the server returns HTTP 400. Why?

A Missing @RequestBody annotation on the method parameter causes binding failure.

B Record fields must be public for binding to work.

C JSON keys do not match record component names.

D The controller method must return void for POST requests.

💡 Hint

Spring Boot needs an annotation to know where to get the data from.

Without @RequestBody, Spring tries to bind from form data or query parameters, not JSON body, causing HTTP 400 error.

❓ state_output

advanced

What is the value of the 'age' field after deserialization?

Given this Request DTO record and JSON input:

public record PersonRequest(String name, Integer age) {}

JSON sent: {"name":"Bob"}

What will be the value of age() in the deserialized object?

A Undefined, depends on JVM

B null

C 0

D Throws NullPointerException

💡 Hint

Consider the difference between primitive and wrapper types.

Since age is Integer (wrapper), missing JSON field results in null. If it were int (primitive), it would default to 0.

🧠 Conceptual

expert

Which statement about using Java records as Request DTOs in Spring Boot is true?

Choose the correct statement about Java records used as Request DTOs in Spring Boot:

A Records automatically provide immutable data carriers suitable for Request DTOs.

B Records require a no-argument constructor to be used as Request DTOs.

C Records cannot be used as Request DTOs because they lack setters.

D Records require explicit Jackson annotations on every field to deserialize JSON.

💡 Hint

Think about how records handle data and immutability.

Records are immutable data carriers with final fields and canonical constructors, making them ideal for Request DTOs without needing setters or no-arg constructors.