Java Stream与Collectors实现单键多值映射：策略与实践

// 这是一个错误的示例，Collectors.toMap 不支持三个参数来映射键和两个值
Map<UserId, String> userIdToEmailAndNameMap = futureList.stream()
                     .map(CompletableFuture::join)
                     .filter(Optional::isPresent)
                     .map(userProfile -> userProfile.get())
                     .collect(Collectors.toMap(UserProfile::getUserId, UserProfile::getEmail, UserProfile::getName));

import java.util.Map;
import java.util.Optional;
import java.util.concurrent.CompletableFuture;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.List;

// 假设存在 UserId, UserProfile 等实体类
// class UserId { /* ... */ }
// class UserProfile {
//     private UserId userId;
//     private String name;
//     private String email;
//     // Getters and constructor
//     public UserId getUserId() { return userId; }
//     public String getName() { return name; }
//     public String getEmail() { return email; }
// }

public class UserProfileMapper {

    public static Map<UserId, UserProfile> mapUserIdToUserProfile(List<CompletableFuture<Optional<UserProfile>>> futureList) {
        return futureList.stream()
            // 1. 等待并获取 CompletableFuture 的结果
            .map(CompletableFuture::join)
            // 2. 过滤掉空的 Optional 对象
            .filter(Optional::isPresent)
            // 3. 从 Optional 中提取 UserProfile 对象
            .map(Optional::get)
            // 4. 使用 Collectors.toMap 进行映射
            //    键映射器: UserProfile::getUserId (获取 UserId 作为键)
            //    值映射器: Function.identity() (将 UserProfile 对象本身作为值)
            .collect(Collectors.toMap(
                UserProfile::getUserId, // Key Mapper
                Function.identity()     // Value Mapper: 返回当前流中的元素本身
            ));
    }

    public static void main(String[] args) {
        // 示例数据构建 (实际应用中可能来自数据库或服务调用)
        UserId user1Id = new UserId("user1");
        UserId user2Id = new UserId("user2");

        UserProfile profile1 = new UserProfile(user1Id, "Alice", "alice@example.com");
        UserProfile profile2 = new UserProfile(user2Id, "Bob", "bob@example.com");

        List<CompletableFuture<Optional<UserProfile>>> futures = List.of(
            CompletableFuture.completedFuture(Optional.of(profile1)),
            CompletableFuture.completedFuture(Optional.of(profile2)),
            CompletableFuture.completedFuture(Optional.empty()) // 模拟一个空结果
        );

        Map<UserId, UserProfile> userIdToProfileMap = mapUserIdToUserProfile(futures);

        // 访问映射后的数据
        UserProfile retrievedProfile1 = userIdToProfileMap.get(user1Id);
        if (retrievedProfile1 != null) {
            System.out.println("User ID: " + user1Id.getId() + ", Name: " + retrievedProfile1.getName() + ", Email: " + retrievedProfile1.getEmail());
        }

        UserProfile retrievedProfile2 = userIdToProfileMap.get(user2Id);
        if (retrievedProfile2 != null) {
            System.out.println("User ID: " + user2Id.getId() + ", Name: " + retrievedProfile2.getName() + ", Email: " + retrievedProfile2.getEmail());
        }

        // 尝试获取不存在的用户
        UserId user3Id = new UserId("user3");
        UserProfile retrievedProfile3 = userIdToProfileMap.get(user3Id);
        System.out.println("User ID " + user3Id.getId() + " exists: " + (retrievedProfile3 != null));
    }
}

// 辅助实体类定义 (为使示例完整可运行)
class UserId {
    private String id;
    public UserId(String id) { this.id = id; }
    public String getId() { return id; }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        UserId userId = (UserId) o;
        return id.equals(userId.id);
    }
    @Override
    public int hashCode() {
        return id.hashCode();
    }
    @Override
    public String toString() {
        return "UserId{" + "id='" + id + '\'' + '}';
    }
}

class UserProfile {
    private UserId userId;
    private String name;
    private String email;

    public UserProfile(UserId userId, String name, String email) {
        this.userId = userId;
        this.name = name;
        this.email = email;
    }

    public UserId getUserId() { return userId; }
    public String getName() { return name; }
    public String getEmail() { return email; }

    @Override
    public String toString() {
        return "UserProfile{" +
               "userId=" + userId +
               ", name='" + name + '\'' +
               ", email='" + email + '\'' +
               '}';
    }
}

UserProfile userProfile = userIdToProfileMap.get(someUserId);
if (userProfile != null) {
    String name = userProfile.getName();
    String email = userProfile.getEmail();
    // ... 使用 name 和 email
}