Low-level design (LLD) for a blockchain:

1. Data structure: The blockchain will need to store the data that it is tracking, such as transactions or records. This will likely involve using a data structure such as a linked list or a tree to store the data in a sequential and tamper-evident manner.

2. Consensus mechanism: The blockchain will need to ensure that all participants in the network agree on the state of the data. This will involve implementing a consensus mechanism such as proof of work or proof of stake to validate new data and reach agreement on the current state of the blockchain.

3. Cryptographic hashes: The blockchain will need to use cryptographic hashes to secure the data and provide tamper-evident properties. This will involve using a cryptographic hash function such as SHA-256 to generate a unique hash for each block of data.

4. P2P network: The blockchain will need to communicate with other nodes on the network in order to exchange data and reach consensus. This will involve implementing a peer-to-peer (P2P) network protocol to allow nodes to connect and communicate with each other.

5. Block creation and validation: The blockchain will need to create new blocks of data and validate them using the consensus mechanism and cryptographic hashes. This will involve creating functions to generate new blocks, validate the data, and add the blocks to the chain.

6. User interface: Finally, we will need to design the user interface for the blockchain, including any necessary views or tools for interacting with the data and monitoring the network. 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.*;
import java.security.MessageDigest;

// Factory Pattern for creating blocks
interface BlockFactory {
    Block createBlock(int index, String previousHash, List<Transaction> transactions);
}

class ConcreteBlockFactory implements BlockFactory {
    @Override
    public Block createBlock(int index, String previousHash, List<Transaction> transactions) {
        return new Block(index, System.currentTimeMillis(), transactions, previousHash);
    }
}

// Observer Pattern for Node Synchronization
interface NodeObserver {
    void update(Block block);
}

class Node implements NodeObserver {
    private String address;
    private List<Block> localBlockchain;

    public Node(String address) {
        this.address = address;
        this.localBlockchain = new ArrayList<>();
    }

    public String getAddress() {
        return address;
    }

    @Override
    public void update(Block block) {
        System.out.println("Node " + address + " received new block: " + block.getHash());
        localBlockchain.add(block);
    }

    public List<Block> getLocalBlockchain() {
        return localBlockchain;
    }
}

// Subject Interface for Blockchain node synchronization
interface BlockchainSubject {
    void addNode(Node node);
    void removeNode(Node node);
    void notifyNodes(Block block);
}

class Blockchain implements BlockchainSubject {
    private List<Block> blocks;
    private List<Transaction> currentTransactions;
    private Set<Node> nodes;
    private BlockFactory blockFactory;
    private ConsensusStrategy consensusStrategy;

    public Blockchain(ConsensusStrategy consensusStrategy) {
        this.blocks = new ArrayList<>();
        this.currentTransactions = new ArrayList<>();
        this.nodes = new HashSet<>();
        this.blockFactory = new ConcreteBlockFactory();
        this.consensusStrategy = consensusStrategy;

        // Create the Genesis block
        Block genesisBlock = createBlock(0, "1", new ArrayList<>(), "Genesis block");
        blocks.add(genesisBlock);
    }

    public Block createBlock(int index, String previousHash, List<Transaction> transactions, String data) {
        Block block = blockFactory.createBlock(index, previousHash, transactions);
        blocks.add(block);
        currentTransactions.clear();
        notifyNodes(block);
        return block;
    }

    public void addTransaction(Transaction transaction) {
        // Transaction validation can be extended with more rules
        if (validateTransaction(transaction)) {
            currentTransactions.add(transaction);
        }
    }

    public boolean validateTransaction(Transaction transaction) {
        // Validation logic (e.g., checking sender's balance)
        return true;
    }

    @Override
    public void addNode(Node node) {
        nodes.add(node);
    }

    @Override
    public void removeNode(Node node) {
        nodes.remove(node);
    }

    @Override
    public void notifyNodes(Block block) {
        for (Node node : nodes) {
            node.update(block);
        }
    }

    public boolean validateChain() {
        return consensusStrategy.validateChain(blocks);
    }

    public Block getLastBlock() {
        return blocks.get(blocks.size() - 1);
    }
}

// Consensus Strategy Pattern to allow different consensus algorithms
interface ConsensusStrategy {
    boolean validateChain(List<Block> chain);
}

class ProofOfWorkStrategy implements ConsensusStrategy {
    private int difficulty;

    public ProofOfWorkStrategy(int difficulty) {
        this.difficulty = difficulty;
    }

    @Override
    public boolean validateChain(List<Block> chain) {
        for (int i = 1; i < chain.size(); i++) {
            Block currentBlock = chain.get(i);
            Block previousBlock = chain.get(i - 1);

            // Validate hash integrity and difficulty
            if (!currentBlock.getHash().equals(currentBlock.calculateHash()) ||
                !currentBlock.getPreviousHash().equals(previousBlock.getHash()) ||
                !currentBlock.getHash().substring(0, difficulty).equals("0".repeat(difficulty))) {
                return false;
            }
        }
        return true;
    }
}

// Block class to represent a blockchain block
class Block {
    private int index;
    private long timestamp;
    private List<Transaction> transactions;
    private String previousHash;
    private String hash;

    public Block(int index, long timestamp, List<Transaction> transactions, String previousHash) {
        this.index = index;
        this.timestamp = timestamp;
        this.transactions = transactions;
        this.previousHash = previousHash;
        this.hash = calculateHash();
    }

    public String calculateHash() {
        try {
            String input = index + previousHash + timestamp + transactions.toString();
            MessageDigest digest = MessageDigest.getInstance("SHA-256");
            byte[] hashBytes = digest.digest(input.getBytes());
            StringBuilder hexString = new StringBuilder();
            for (byte b : hashBytes) {
                hexString.append(String.format("%02x", b));
            }
            return hexString.toString();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    public int getIndex() {
        return index;
    }

    public long getTimestamp() {
        return timestamp;
    }

    public List<Transaction> getTransactions() {
        return transactions;
    }

    public String getPreviousHash() {
        return previousHash;
    }

    public String getHash() {
        return hash;
    }
}

// Transaction class to represent a blockchain transaction
class Transaction {
    private String sender;
    private String recipient;
    private double amount;

    public Transaction(String sender, String recipient, double amount) {
        this.sender = sender;
        this.recipient = recipient;
        this.amount = amount;
    }

    public String getSender() {
        return sender;
    }

    public String getRecipient() {
        return recipient;
    }

    public double getAmount() {
        return amount;
    }

    @Override
    public String toString() {
        return sender + " -> " + recipient + " : " + amount;
    }
}

// Example usage
public class Main {
    public static void main(String[] args) {
        // Create blockchain with Proof of Work consensus strategy
        ConsensusStrategy powStrategy = new ProofOfWorkStrategy(4);
        Blockchain blockchain = new Blockchain(powStrategy);

        // Register nodes
        Node node1 = new Node("http://node1.com");
        Node node2 = new Node("http://node2.com");
        blockchain.addNode(node1);
        blockchain.addNode(node2);

        // Add transactions
        blockchain.addTransaction(new Transaction("Alice", "Bob", 50));
        blockchain.addTransaction(new Transaction("Bob", "Charlie", 20));

        // Create new block and notify nodes
        blockchain.createBlock(1, blockchain.getLastBlock().getHash(), blockchain.currentTransactions, "Data Block");

        // Validate blockchain
        boolean isValid = blockchain.validateChain();
        System.out.println("Blockchain is valid: " + isValid);
    }
}