设计文件分享系统

// Solution for Leetcode Problem 1500: Design a File Sharing System using Java Streams
// Similar to the standard Java solution, but leveraging Java Streams for operations.

import java.util.*;
import java.util.stream.Collectors;

class FileSharingStream {
    private Map<Integer, Set<Integer>> fileToUsers;
    private Map<Integer, Set<Integer>> userToFiles;
    private int fileIdCounter;

    public FileSharingStream() {
        fileToUsers = new HashMap<>();
        userToFiles = new HashMap<>();
        fileIdCounter = 0;
    }

    public int uploadFile() {
        int fileId = ++fileIdCounter;
        fileToUsers.put(fileId, new HashSet<>());
        return fileId;
    }

    public void downloadFile(int userId, int fileId) {
        fileToUsers.computeIfAbsent(fileId, k -> new HashSet<>()).add(userId);
        userToFiles.computeIfAbsent(userId, k -> new HashSet<>()).add(fileId);
    }

    public void shareFile(int fromUserId, int toUserId, int fileId) {
        if (userToFiles.getOrDefault(fromUserId, Collections.emptySet()).contains(fileId)) {
            downloadFile(toUserId, fileId);
        }
    }

    public Set<Integer> listUserFiles(int userId) {
        return userToFiles.getOrDefault(userId, Collections.emptySet());
    }

    public Set<Integer> listFileUsers(int fileId) {
        return fileToUsers.getOrDefault(fileId, Collections.emptySet());
    }

    public List<Integer> getAllFileIds() {
        return fileToUsers.keySet().stream().collect(Collectors.toList());
    }
}