computeIfAbsent的使用 键不存在于映射中时

V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)

import java.util.HashMap;
import java.util.Map;
import java.util.function.Function;

public class CacheExample {
    private final Map<String, User> userCache = new HashMap<>();

    public User getUserById(String userId) {
        // 使用 computeIfAbsent 确保只在必要时从数据库加载用户信息
        Function<String, User> mappingFunction = id -> loadUserFromDatabase(id);
        return userCache.computeIfAbsent(userId, mappingFunction);
    }

    private User loadUserFromDatabase(String userId) {
        // 模拟从数据库加载用户信息的过程
        System.out.println("Loading user with ID " + userId + " from database.");
        // 这里应该是实际从数据库加载用户信息的代码
        return new User(userId, "Username for " + userId);
    }

    static class User {
        String id;
        String name;

        public User(String id, String name) {
            this.id = id;
            this.name = name;
        }

        @Override
        public String toString() {
            return "User{id='" + id + "', name='" + name + "'}";
        }
    }

    public static void main(String[] args) {
        CacheExample example = new CacheExample();
        System.out.println(example.getUserById("123"));
        // 第二次调用，应该直接从缓存中获取
        System.out.println(example.getUserById("123"));
    }
}