A web crawler is a program that is used to scan websites and analyze their content. Here is a high-level design (LLD) for a web crawler:

1. URL Queue: The web crawler maintains a queue of URLs to be processed. It begins with a seed URL and adds new URLs to the queue as it discovers them.

2. HTTP Client: The web crawler uses an HTTP client to send requests to websites and receive responses. The HTTP client is responsible for handling the details of making the request, such as formatting the request and parsing the response.

3. Parser: The web crawler includes a parser that is used to extract relevant information from the HTML or other content returned by the HTTP client. This might include extracting links to other pages, extracting text content, or extracting metadata such as page titles or descriptions.

4. Data Store: The web crawler stores the extracted information in a data store, such as a database or file system. This allows the information to be queried or analyzed later.

5. Scheduler: The web crawler includes a scheduler that determines when to send requests for each URL in the queue. The scheduler might implement a delay between requests to avoid overwhelming the server, or it might prioritize certain URLs for faster processing.

6. User Interface: The web crawler may include a user interface that allows users to control the crawl process and view the results. This might include a graphical user interface or a command-line interface.

Code:

public class WebCrawler {
    private Queue<String> urlQueue;
    private HTTPClient httpClient;
    private Parser parser;
    private DataStore dataStore;
    private Scheduler scheduler;

    public WebCrawler() {
        urlQueue = new LinkedList<>();
        httpClient = new HTTPClient();
        parser = new Parser();
        dataStore = new DataStore();
        scheduler = new Scheduler();
    }

    public void crawl(String seedUrl) {
        urlQueue.add(seedUrl);
        while (!urlQueue.isEmpty()) {
            String url = urlQueue.poll();
            Response response = httpClient.get(url);
            Data data = parser.parse(response);
            dataStore.store(data);
            scheduler.delay();
            for (String link : parser.extractLinks(response)) {
                urlQueue.add(link);
            }
        }
    }
}

public class HTTPClient {
    public Response get(String url) {
        // Send HTTP GET request to the specified URL and return the response
    }
}

public class Parser {
    public Data parse(Response response) {
        // Extract relevant information from the response and return it
    }

    public List<String> extractLinks(Response response) {
        // Extract links to other pages from the response and return them as a list
    }
}

public class DataStore {
    public void store(Data data) {
        // Store the data in the data store (e.g. database or file system)
    }
}

public class Scheduler {
    public void delay() {
        // Implement a delay between requests (e.g. sleep for a few seconds)
    }
}

This code defines five classes: WebCrawler, HTTPClient, Parser, DataStore, and Scheduler. The WebCrawler class is the main class that manages the crawl process, including maintaining the URL queue, sending requests, extracting and storing data, and scheduling requests. The other classes are responsible for specific tasks, such as making HTTP requests, parsing the response, storing the data, and implementing a delay between requests.