@Component
public class UserQueryResolver implements GraphQLQueryResolver {

    private final UserService service;

    public UserQueryResolver(UserService service) {
        this.service = service;
    }

    public User userById(Long id) {
        return service.findById(id);
    }
}

type Query {
    user(id: ID!): User
}

type User {
    id: ID!
    name: String!
    posts: [Post!]!
}

@Component
public class UserResolver implements GraphQLResolver<User> {

    private final PostService postService;

    public UserResolver(PostService postService) {
        this.postService = postService;
    }

    public List<Post> posts(User user) {
        return postService.findByUserId(user.getId());
    }
}

GraphQL вызывает posts() только если клиент запросит поле posts.
Если клиент запросит только user { id name }, резолвер posts не будет вызван вообще.

/graphql
  /schema
    schema.graphqls

/resolvers
  QueryResolver.java
  MutationResolver.java
  UserResolver.java
  PostResolver.java

/services
  UserService.java
  PostService.java

/repositories
  UserRepository.java
  PostRepository.java

query {
    user(id: 10) {
        id
        name
        posts {
            id
            title
        }
    }
}

import java.util.List;

public class Task251125 {
    public static void main(String[] args) {
        var list = List.of(1, 2, 3);
        var number = 10;
        var result = process(list, number);
        System.out.println(result);
    }
    
    static var process(List<Integer> list, int n) {
        var sum = 0;
        for (var item : list) {
            sum += item;
        }
        return sum * n;
    }
}