Why DTOs matter in Spring Boot - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a simple DTO class with a private field and a getter.

Spring Boot

public class UserDTO {
    private String [1];

    public String getName() {
        return name;
    }
}

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

Aname

Bage

Cemail

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field name that does not match the getter method.

Forgetting to make the field private.

The field name matches the getter getName(), which is typical in DTOs.

2fill in blank

medium

Complete the code to create a constructor for the DTO that sets the name field.

Spring Boot

public class UserDTO {
    private String name;

    public UserDTO([1]) {
        this.name = name;
    }
}

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

AString name

Bint age

Cboolean active

Ddouble salary

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter type that does not match the field.

Using a different parameter name than the field.

The constructor parameter must match the field type and name to set the value correctly.

3fill in blank

hard

Fix the error in the DTO setter method to correctly set the field.

Spring Boot

public void setName(String name) {
    [1] = name;
}

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

AsetName

Bname

CUserDTO.name

Dthis.name

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the parameter to itself instead of the field.

Using the class name instead of this.

Using this.name refers to the field, avoiding confusion with the parameter name.

4fill in blank

hard

Fill both blanks to create a DTO from an entity object.

Spring Boot

public UserDTO([1] user) {
    this.name = user.[2]();
}

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

AUser

BgetName

CsetName

DUserDTO

Attempts:

3 left

💡 Hint

Common Mistakes

Using the DTO class as parameter type instead of the entity.

Calling a setter method instead of a getter on the entity.

The constructor takes a User entity and calls its getName() method to set the DTO's name.

5fill in blank

hard

Fill all three blanks to convert a list of entities to a list of DTOs using streams.

Spring Boot

List<UserDTO> dtos = users.stream()
    .map(user -> new UserDTO(user.[1]()))
    .[2](Collectors.[3]());

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

AgetName

Bcollect

CtoList

Dstream

Attempts:

3 left

💡 Hint

Common Mistakes

Using a setter method instead of a getter in the map.

Forgetting to collect the stream into a list.

Using incorrect collector methods.

The stream maps each user by calling getName(), then collects the results into a list using collect(Collectors.toList()).

Practice

1. Why do we use DTOs (Data Transfer Objects) in a Spring Boot application?

easy

A. To make the application slower by adding extra layers

B. To increase the size of data sent over the network

C. To replace the database entities completely

D. To carry only the data we want to share and hide internal details

Solution

1. Step 1: Understand the purpose of DTOs

   DTOs are designed to carry only the necessary data between processes or layers, avoiding exposure of internal details.

2. Step 2: Analyze the options

   To carry only the data we want to share and hide internal details correctly states the purpose of DTOs. The other options describe incorrect or harmful uses.

3. Final Answer:

   To carry only the data we want to share and hide internal details -> Option D

4. Quick Check:

   DTOs protect and simplify data transfer = D [OK]

Hint: DTOs share only needed data, hiding internals [OK]

Common Mistakes:

• Thinking DTOs replace entities fully
• Assuming DTOs increase data size
• Believing DTOs slow down the app

2. Which of the following is the correct way to define a simple DTO class in Spring Boot?

easy

A. public class UserDTO { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } }

B. @Entity public class UserDTO { private String name; private int age; }

C. public interface UserDTO { String getName(); int getAge(); }

D. @Repository public class UserDTO { private String name; private int age; }

Solution

1. Step 1: Identify correct DTO structure

   A DTO is a simple class with private fields and public getters/setters, without annotations like @Entity or @Repository.

2. Step 2: Evaluate each option

   public class UserDTO { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } } correctly defines a DTO class with fields and accessors. @Entity public class UserDTO { private String name; private int age; } wrongly uses @Entity, which is for database entities. public interface UserDTO { String getName(); int getAge(); } defines an interface, not a DTO class. @Repository public class UserDTO { private String name; private int age; } wrongly uses @Repository, which is for data access layers.

3. Final Answer:

   public class UserDTO { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } } -> Option A

4. Quick Check:

   DTO = simple class with getters/setters = B [OK]

Hint: DTOs are plain classes with getters/setters, no @Entity [OK]

Common Mistakes:

• Adding @Entity annotation to DTO
• Using interfaces instead of classes for DTO
• Marking DTO as @Repository

3. Given this Spring Boot controller method, what will be the output JSON when calling /user?

@GetMapping("/user")
public UserDTO getUser() {
    UserDTO dto = new UserDTO();
    dto.setName("Alice");
    dto.setAge(30);
    return dto;
}

medium

A. {"name":"Alice"}

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

C. {"name":"Alice","age":30}

D. Error: Cannot serialize UserDTO

Solution

1. Step 1: Understand default JSON serialization in Spring Boot

   Spring Boot uses Jackson to convert returned objects to JSON, serializing all public getters by default.

2. Step 2: Analyze the returned UserDTO object

   UserDTO has name and age fields with getters, so JSON will include both as simple key-value pairs.

3. Final Answer:

   {"name":"Alice","age":30} -> Option C

4. Quick Check:

   DTO fields serialize as JSON keys = A [OK]

Hint: Returned DTO converts to JSON with all getters [OK]

Common Mistakes:

• Expecting nested JSON with class name
• Assuming partial fields serialize
• Thinking serialization causes error

4. What is wrong with this DTO class that causes a runtime error when Spring Boot tries to deserialize it?

public class ProductDTO {
    private String name;
    private int price;
    public ProductDTO(String name, int price) {
        this.name = name;
        this.price = price;
    }
}

medium

A. Fields should be public, not private

B. Missing default no-argument constructor

C. Constructor parameters should be annotated with @Autowired

D. Class should be annotated with @Entity

Solution

1. Step 1: Identify deserialization requirements

   Jackson requires a default no-argument constructor to create an instance before setting fields via setters or reflection.

2. Step 2: Check the DTO class

   This class only has a parameterized constructor and no default constructor, causing Jackson to fail during deserialization.

3. Final Answer:

   Missing default no-argument constructor -> Option B

4. Quick Check:

   Jackson needs no-arg constructor = A [OK]

Hint: DTOs need no-arg constructor for JSON serialization [OK]

Common Mistakes:

• Thinking fields must be public
• Adding @Autowired to constructor parameters
• Confusing DTO with entity needing @Entity

5. You have an entity class User with many fields, but you want to expose only id and email in your API response. How should you use a DTO to achieve this cleanly?

hard

A. Create a UserDTO with only id and email fields, map User to UserDTO before returning

B. Return the User entity directly and ignore unwanted fields in the frontend

C. Add @JsonIgnore to all unwanted fields in the User entity

D. Use the User entity but rename unwanted fields to empty strings

Solution

1. Step 1: Understand data exposure risks

   Returning the full User entity exposes all fields, risking sensitive data leaks.

2. Step 2: Use DTO to control data

   Creating a UserDTO with only id and email fields and mapping User to UserDTO ensures only desired data is sent.

3. Step 3: Evaluate other options

   Return the User entity directly and ignore unwanted fields in the frontend risks exposing all data. Add @JsonIgnore to all unwanted fields in the User entity mixes entity with serialization concerns and can be error-prone. Use the User entity but rename unwanted fields to empty strings is a bad practice and confusing.

4. Final Answer:

   Create a UserDTO with only id and email fields, map User to UserDTO before returning -> Option A

5. Quick Check:

   DTOs control exposed data = C [OK]

Hint: Use DTO to expose only needed fields safely [OK]

Common Mistakes:

• Returning full entity directly
• Using @JsonIgnore on entity fields
• Modifying entity fields to hide data