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Wednesday, 14 December 2016

Chapter 12 Exercise 33, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

12.33 (Search Web)
Modify Listing 12.18 WebCrawler.java to search for the word Computer
Programming starting from the URL http://cs.armstrong.edu/liang.
Your program terminates once the word is found. Display the URL for the
page that contains the word.


import java.util.ArrayList;
import java.util.Scanner;
public class Exercise_33 {

    public static void main(String[] args) {
//        Scanner input = new Scanner(System.in);
//        System.out.print("Enter a URL: ");
//        String url = input.nextLine();
        crawler("http://cs.armstrong.edu/liang"); // Traverse the Web from the a starting url
    }

    public static void crawler(String startingURL) {
        ArrayList<String> listOfPendingURLs = new ArrayList<>();
        ArrayList<String> listOfTraversedURLs = new ArrayList<>();

        listOfPendingURLs.add(startingURL);
        while (!listOfPendingURLs.isEmpty() &&
                listOfTraversedURLs.size() <= 100) {
            String urlString = listOfPendingURLs.remove(0);
            if (!listOfTraversedURLs.contains(urlString)) {
                listOfTraversedURLs.add(urlString);
                System.out.println("Craw " + urlString);

                for (String s: getSubURLs(urlString)) {
                    if (!listOfTraversedURLs.contains(s))
                        listOfPendingURLs.add(s);
                }
            }
        }
    }

    public static ArrayList<String> getSubURLs(String urlString) {
        ArrayList<String> list = new ArrayList<>();

        try {
            java.net.URL url = new java.net.URL(urlString);
            Scanner input = new Scanner(url.openStream());
            int current = 0;
            while (input.hasNext()) {
                String line = input.nextLine();
                if (line.contains("Computer Programming")) {
                    System.out.println("Found the word \"Computer Programming\".");
                    System.out.println("URL: " + url);
                    System.exit(1);
                }
                current = line.indexOf("http:", current);
                while (current > 0) {
                    int endIndex = line.indexOf("\"", current);
                    if (endIndex > 0) { // Ensure that a correct URL is found
                        list.add(line.substring(current, endIndex));
                        current = line.indexOf("http:", endIndex);
                    }
                    else
                        current = -1;
                }
            }
        }
        catch (Exception ex) {
            System.out.println("Error: " + ex.getMessage());
        }

        return list;
    }
}

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