if (!map.containsKey(key)) {
    // Выполнить дорогостоящую операцию только если ключ отсутствует
    Value value = computeExpensiveValue(key);
    map.put(key, value);
}

// Вместо:
if (map.containsKey(key)) {
    Value value = map.get(key);
    // обработка value
}

// Можно использовать:
Value value = map.get(key);
if (value != null) {
    // обработка value
}

import java.util.HashMap;
import java.util.Map;

public class Task121125 {
    public static void main(String[] args) {
        Map<Key121125, String> map = new HashMap<>();
        
        Key121125 k1 = new Key121125(1);
        Key121125 k2 = new Key121125(1);
        
        map.put(k1, "value");
        
        System.out.println(map.containsKey(k1));
        System.out.println(map.containsKey(k2));
        
        k1.id = 2;
        
        System.out.println(map.containsKey(k1));
        System.out.println(map.containsKey(k2));
    }
    
    static class Key121125 {
        int id;
        Key121125(int id) { this.id = id; }
        
        public boolean equals(Object o) {
            return o instanceof Key121125 && this.id == ((Key121125) o).id;
        }
        
        public int hashCode() {
            return id;
        }
    }
}

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);
    }
}