MapStruct for automatic mapping in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this MapStruct mapping?

Given the following MapStruct mapper interface and classes, what will be the value of target.name after mapping?

Spring Boot

public class Source {
    private String name;
    public Source(String name) { this.name = name; }
    public String getName() { return name; }
}

public class Target {
    private String name;
    public void setName(String name) { this.name = name; }
    public String getName() { return name; }
}

@Mapper
public interface MyMapper {
    MyMapper INSTANCE = Mappers.getMapper(MyMapper.class);
    Target sourceToTarget(Source source);
}

// Usage:
Source source = new Source("Alice");
Target target = MyMapper.INSTANCE.sourceToTarget(source);
String result = target.getName();

A "Alice"

B null

C Compilation error

D "source.name"

💡 Hint

MapStruct automatically maps properties with the same name and compatible types.

MapStruct generates an implementation that copies the value of the 'name' property from Source to Target. Since the source has 'Alice', the target's name will be 'Alice'.

📝 Syntax

intermediate

Which MapStruct annotation is required to define a mapper interface?

You want to create a MapStruct mapper interface to convert between two classes. Which annotation must you use on the interface?

A @Mapper

B @Component

C @Service

D @Repository

💡 Hint

MapStruct uses a special annotation to generate mapper implementations.

The @Mapper annotation tells MapStruct to generate an implementation for the interface.

🔧 Debug

advanced

Why does this MapStruct mapping fail to compile?

Consider this mapper interface: @Mapper public interface UserMapper { UserDto toDto(User user); } User and UserDto have fields with different names: User has 'firstName', UserDto has 'givenName'. No additional configuration is provided. Why does MapStruct fail to generate the implementation?

A MapStruct requires all fields to be public for mapping.

B MapStruct only supports mapping between classes with identical field types.

C MapStruct cannot map fields with different names without explicit @Mapping annotations.

D MapStruct needs a default constructor in the mapper interface.

💡 Hint

MapStruct matches fields by name by default.

MapStruct matches fields by name. If names differ, you must specify mappings with @Mapping annotations.

❓ state_output

advanced

What is the output of this MapStruct mapping with nested objects?

Given these classes: public class Address { private String city; public Address(String city) { this.city = city; } public String getCity() { return city; } } public class User { private String name; private Address address; public User(String name, Address address) { this.name = name; this.address = address; } public String getName() { return name; } public Address getAddress() { return address; } } public class UserDto { private String name; private String city; public void setName(String name) { this.name = name; } public void setCity(String city) { this.city = city; } public String getName() { return name; } public String getCity() { return city; } } @Mapper public interface UserMapper { UserMapper INSTANCE = Mappers.getMapper(UserMapper.class); @Mapping(source = "address.city", target = "city") UserDto userToUserDto(User user); } // Usage: User user = new User("Bob", new Address("Paris")); UserDto dto = UserMapper.INSTANCE.userToUserDto(user); String output = dto.getCity();

A Compilation error

B null

C "address.city"

D "Paris"

💡 Hint

MapStruct supports nested property mapping with dot notation.

The @Mapping annotation tells MapStruct to map user.getAddress().getCity() to dto.setCity(). The value is "Paris".

🧠 Conceptual

expert

Which statement about MapStruct's generated mapper implementations is true?

Choose the correct statement about how MapStruct generates mapper implementations in a Spring Boot project.

A MapStruct requires manual implementation of all mapping methods despite the interface definition.

B MapStruct generates implementations at compile time, creating simple, fast code without reflection.

C MapStruct uses runtime reflection to map fields dynamically, which slows down performance.

D MapStruct only works with XML configuration files to define mappings.

💡 Hint

Think about when MapStruct creates the code for mapping.

MapStruct generates Java code during compilation, avoiding runtime reflection and improving performance.