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

public class Task131125 {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("a", 1);
        map.put("b", 2);
        
        map.keySet().remove("a");
        map.values().remove(2);
        map.entrySet().removeIf(entry -> entry.getKey().equals("c"));
        
        System.out.println(map.size());
        System.out.println(map.get("a"));
        System.out.println(map.get("b"));
    }
}

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

Типы описывают структуру данных — аналог классов/DTO.

input CreateUserInput {
  name: String!
  age: Int
}

enum Role {
  ADMIN
  USER
  GUEST
}

interface Entity {
  id: ID!
}

type User implements Entity {
  id: ID!
  name: String!
}

type Post implements Entity {
  id: ID!
  title: String!
}

Клиент может запросить id для любого, кто реализует Entity.

union SearchResult = User | Post | Comment

schema {
  query: Query
  mutation: Mutation
}

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

type Post {
  id: ID!
  title: String!
  content: String
  author: User!
  comments: [Comment!]!
}

type Comment {
  id: ID!
  text: String!
  author: User!
}

Перечисления
enum Role {
  ADMIN
  USER
  GUEST
}

Входные данные
input CreatePostInput {
  title: String!
  content: String
}

Запросы
type Query {
  user(id: ID!): User
  users: [User!]!

  post(id: ID!): Post
  posts(limit: Int, authorId: ID): [Post!]!
}

Мутации
type Mutation {
  createPost(input: CreatePostInput!): Post!
  deletePost(id: ID!): Boolean!
}

Подписки
type Subscription {
  postCreated: Post!
}

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

@Component
public class UserResolver implements GraphQLQueryResolver {
    private final UserService service;

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

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

type User {
  id: ID!
  name: String!
  email: String # новое поле
}

type User {
  id: ID!
  name: String!

  # старое поле
  username: String @deprecated(reason: "Use 'name' instead")
}

Старое:
input CreatePostInput {
  title: String!
}

Новое:
input CreatePostInput {
  title: String!
  content: String @deprecated(reason: "Use body instead")
  body: String
}

import java.util.EnumMap;
import java.util.Map;

enum Color { RED, GREEN, BLUE }

public class Task171125 {
    public static void main(String[] args) {
        Map<Color, String> map = new EnumMap<>(Color.class);
        
        map.put(Color.RED, "red");
        map.put(Color.GREEN, null);
        map.put(null, "null value");
        
        System.out.println(map.get(Color.RED));
        System.out.println(map.get(Color.GREEN));
        System.out.println(map.get(null));
        System.out.println(map.size());
    }
}