Managing a Voting System using Java

import java.util.*;

public class VotingSystem {
    private Map<String, Integer> candidates;  // Stores candidate_id as key and votes as value
    private Map<String, VotingHistory> voters;  // Tracks each voter's voting history

    public VotingSystem() {
        this.candidates = new HashMap<>();  // Initialize candidates map
        this.voters = new HashMap<>();  // Initialize voters map
    }

    public boolean registerCandidate(String candidateId) {
        if (candidates.containsKey(candidateId)) {
            return false;  // Candidate is already registered
        }
        candidates.put(candidateId, 0);  // Initialize candidates with 0 votes
        return true;
    }

    public boolean vote(long timestamp, String voterId, String candidateId) {
        if (!candidates.containsKey(candidateId)) {
            return false;  // Return false if candidate is not registered
        }
        VotingHistory voterHistory = voters.computeIfAbsent(voterId, k -> new VotingHistory());
        voterHistory.getVotes().add(candidateId);  // Record the vote
        voterHistory.getTimestamps().add(timestamp);  // Record the time of the vote
        candidates.put(candidateId, candidates.get(candidateId) + 1);  // Increment vote count for the candidate
        return true;
    }

    public Integer getVotes(String candidateId) {
        return candidates.get(candidateId);  // Retrieve vote count for a candidate or null if not found
    }

    public List<String> topNCandidates(int n) {
        return candidates.entrySet().stream()
                .sorted((entry1, entry2) -> entry2.getValue().compareTo(entry1.getValue()))
                .limit(n)
                .map(Map.Entry::getKey)
                .toList();  // Return top n candidates based on votes
    }

    private static class VotingHistory {
        private final List<String> votes;
        private final List<Long> timestamps;

        public VotingHistory() {
            this.votes = new ArrayList<>();
            this.timestamps = new ArrayList<>();
        }

        public List<String> getVotes() {
            return votes;
        }

        public List<Long> getTimestamps() {
            return timestamps;
        }
    }
}