Low-level design (LLD) for an online voting system:

1. Database design: We will need to store information about voters, candidates, and the votes themselves. This will likely include tables for voters, candidates, and votes, with foreign keys to link them together. We will also need to consider security and privacy concerns, such as ensuring that votes are anonymous and cannot be traced back to individual voters.

2. User authentication and authorization: Voters will need to be able to log in to the system in order to cast their votes. We will need to implement measures to ensure the security of voter accounts, such as password hashing and potentially two-factor authentication. We will also need to ensure that only eligible voters are able to log in and cast their votes.

3. Ballot design: The online voting system will need to allow voters to select their preferred candidates or options. This will likely involve creating forms for voters to select their choices, with fields for the candidates or options. We will need to consider usability and accessibility concerns when designing the ballot.

4. Vote tracking: The system will need to keep track of the votes as they are cast and provide a way for administrators to view the results. This will involve implementing logic to store and count the votes, as well as creating views for the administrators to view the results.

5. User interface: Finally, we will need to design the user interface for the voting system, including the layout, navigation, and any necessary graphics or styling. This will involve creating wireframes and mockups, as well as implementing the front-end code to bring the design to life.

Code

import java.util.*;

// Factory for creating core entities (Voter, Candidate)
interface VotingFactory {
    Voter createVoter(String name, String password);
    Candidate createCandidate(String name, String party);
}

// Concrete Factory for the Online Voting System
public class VotingSystemFactory implements VotingFactory {
    @Override
    public Voter createVoter(String name, String password) {
        return new Voter(name, password);
    }

    @Override
    public Candidate createCandidate(String name, String party) {
        return new Candidate(name, party);
    }
}

// Core classes for Voter, Candidate, and other entities
public class Voter {
    private String name;
    private String password;
    private boolean hasVoted = false;

    public Voter(String name, String password) {
        this.name = name;
        this.password = password;
    }

    public String getName() {
        return name;
    }

    public String getPassword() {
        return password;
    }

    public boolean hasVoted() {
        return hasVoted;
    }

    public void vote() {
        this.hasVoted = true;
    }
}

public class Candidate {
    private String name;
    private String party;
    private String encryptedVotes;

    public Candidate(String name, String party) {
        this.name = name;
        this.party = party;
    }

    public String getName() {
        return name;
    }

    public String getParty() {
        return party;
    }

    public String getEncryptedVotes() {
        return encryptedVotes;
    }

    public void setEncryptedVotes(String encryptedVotes) {
        this.encryptedVotes = encryptedVotes;
    }
}

// Authentication Strategy interface and concrete implementation
interface AuthenticationStrategy {
    boolean authenticate(Voter voter, VoterDatabase voterDatabase);
}

public class PasswordAuthenticationStrategy implements AuthenticationStrategy {
    @Override
    public boolean authenticate(Voter voter, VoterDatabase voterDatabase) {
        Voter storedVoter = voterDatabase.getVoter(voter.getName());
        if (storedVoter != null && storedVoter.getPassword().equals(voter.getPassword()) && !storedVoter.hasVoted()) {
            storedVoter.vote();
            return true;
        }
        return false;
    }
}

// Encryption Strategy interface and concrete implementation
interface EncryptionStrategy {
    String encrypt(int value);
}

public class BasicEncryptor implements EncryptionStrategy {
    @Override
    public String encrypt(int value) {
        // Implement encryption logic here
        return "encrypted_" + value;
    }
}

// Observer interface for vote notifications
interface Observer {
    void update(String message);
}

// Ballot observer to notify when a vote is cast
public class BallotObserver implements Observer {
    @Override
    public void update(String message) {
        System.out.println("Ballot Update: " + message);
    }
}

// Ballot class using Observer pattern for notifications
public class Ballot {
    private Map<Candidate, Integer> votes;
    private List<Observer> observers;

    public Ballot() {
        this.votes = new HashMap<>();
        this.observers = new ArrayList<>();
    }

    public void addObserver(Observer observer) {
        observers.add(observer);
    }

    private void notifyObservers(String message) {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }

    public void addVote(Candidate candidate) {
        votes.put(candidate, votes.getOrDefault(candidate, 0) + 1);
        notifyObservers("Vote added for candidate: " + candidate.getName());
    }

    public int getVotes(Candidate candidate) {
        return votes.getOrDefault(candidate, 0);
    }
}

// Command pattern for casting votes
interface Command {
    void execute();
}

// Command to cast a vote
public class CastVoteCommand implements Command {
    private Voter voter;
    private Candidate candidate;
    private Ballot ballot;
    private AuthenticationStrategy authenticationStrategy;
    private VoterDatabase voterDatabase;

    public CastVoteCommand(Voter voter, Candidate candidate, Ballot ballot, AuthenticationStrategy authStrategy, VoterDatabase voterDatabase) {
        this.voter = voter;
        this.candidate = candidate;
        this.ballot = ballot;
        this.authenticationStrategy = authStrategy;
        this.voterDatabase = voterDatabase;
    }

    @Override
    public void execute() {
        if (authenticationStrategy.authenticate(voter, voterDatabase)) {
            ballot.addVote(candidate);
        } else {
            System.out.println("Authentication failed or voter has already voted.");
        }
    }
}

// OnlineVotingSystem class using design patterns
public class OnlineVotingSystem {
    private VoterDatabase voterDatabase;
    private CandidateDatabase candidateDatabase;
    private Ballot ballot;
    private EncryptionStrategy encryptionStrategy;
    private AuthenticationStrategy authenticationStrategy;

    public OnlineVotingSystem(VotingFactory factory) {
        this.voterDatabase = new VoterDatabase();
        this.candidateDatabase = new CandidateDatabase();
        this.ballot = new Ballot();
        this.encryptionStrategy = new BasicEncryptor(); // Can be swapped for a different encryption strategy
        this.authenticationStrategy = new PasswordAuthenticationStrategy(); // Can be swapped for a different auth strategy
    }

    public void registerVoter(String name, String password) {
        Voter voter = new Voter(name, password);
        voterDatabase.addVoter(voter);
    }

    public void addCandidate(String name, String party) {
        Candidate candidate = new Candidate(name, party);
        candidateDatabase.addCandidate(candidate);
    }

    public void castVote(Voter voter, Candidate candidate) {
        Command castVote = new CastVoteCommand(voter, candidate, ballot, authenticationStrategy, voterDatabase);
        castVote.execute();
    }

    public List<Candidate> getResults() {
        List<Candidate> results = new ArrayList<>();
        for (Candidate candidate : candidateDatabase.getCandidates()) {
            int votes = ballot.getVotes(candidate);
            String encryptedVotes = encryptionStrategy.encrypt(votes);
            candidate.setEncryptedVotes(encryptedVotes);
            results.add(candidate);
        }
        return results;
    }
}

// VoterDatabase managing voter records
public class VoterDatabase {
    private List<Voter> voters;

    public VoterDatabase() {
        this.voters = new ArrayList<>();
    }

    public void addVoter(Voter voter) {
        voters.add(voter);
    }

    public Voter getVoter(String name) {
        for (Voter voter : voters) {
            if (voter.getName().equals(name)) {
                return voter;
            }
        }
        return null;
    }
}

// CandidateDatabase managing candidate records
public class CandidateDatabase {
    private List<Candidate> candidates;

    public CandidateDatabase() {
        this.candidates = new ArrayList<>();
    }

    public void addCandidate(Candidate candidate) {
        candidates.add(candidate);
    }

    public List<Candidate> getCandidates() {
        return candidates;
    }
}

// Main class demonstrating usage
public class Main {
    public static void main(String[] args) {
        VotingFactory factory = new VotingSystemFactory();
        OnlineVotingSystem votingSystem = new OnlineVotingSystem(factory);

        // Register voters
        votingSystem.registerVoter("Alice", "password1");
        votingSystem.registerVoter("Bob", "password2");

        // Add candidates
        votingSystem.addCandidate("John Doe", "Party A");
        votingSystem.addCandidate("Jane Smith", "Party B");

        // Create Voter and Candidate instances
        Voter alice = new Voter("Alice", "password1");
        Candidate johnDoe = new Candidate("John Doe", "Party A");

        // Cast vote using the Command Pattern
        votingSystem.castVote(alice, johnDoe);

        // Get and display election results
        List<Candidate> results = votingSystem.getResults();
        for (Candidate candidate : results) {
            System.out.println("Candidate: " + candidate.getName() + ", Encrypted Votes: " + candidate.getEncryptedVotes());
        }
    }
}