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

type Post {
  id: ID!
  title: String!
  content: String
}

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

query {
  user(id: 42) {
    name
    posts {
      title
    }
  }
}

{
  "data": {
    "user": {
      "name": "Den",
      "posts": [
        { "title": "gRPC и Java" },
        { "title": "GraphQL под капотом" }
      ]
    }
  }
}

mutation {
  createUser(input: { name: "Alex" }) {
    id
    name
  }
}

subscription {
  userCreated {
    id
    name
  }
}

{
  "query": "query { user(id: 42) { name posts { title } } }"
}

GraphQL-запрос — это декларативное описание структуры данных, не код и не SQL.

GraphQLObjectType userType = newObject()
    .name("User")
    .field(field -> field
        .name("id")
        .type(Scalars.GraphQLID))
    .field(field -> field
        .name("name")
        .type(Scalars.GraphQLString))
    .field(field -> field
        .name("posts")
        .type(new GraphQLList(postType))
        .dataFetcher(env -> postService.getByUser(env.getSource())))
    .build();

{
  "data": {
    "user": {
      "name": "Den",
      "posts": [{ "title": "gRPC и Java" }]
    }
  }
}

{
  "data": { "user": null },
  "errors": [{ "message": "User not found" }]
}

@Component
public class UserResolver implements GraphQLQueryResolver {
    private final UserService userService;
    public UserResolver(UserService userService) { this.userService = userService; }

    public User user(Long id) {
        return userService.findById(id);
    }

    public List<User> allUsers() {
        return userService.findAll();
    }
}

import java.util.concurrent.ConcurrentHashMap;

public class Task131125 {
    public static void main(String[] args) {
        ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
        map.put("a", 1);
        map.put("b", 2);
        map.put("c", 3);
        
        int result = map.reduce(2, 
            (k, v) -> k.length() + v, 
            Integer::sum
        );
        
        System.out.println(result);
    }
}