Link java to r
Every time our crawler visits a webpage, we want to collect all the URLs on that page and add them to the end of our big list of pages to visit. Recall that Lists have special methods that Sets ordinarily do not, such as adding an entry to the end of a list or adding an entry to the beginning of a list. When the crawler visits a page it collects all the URLs on that page and we just append them to this list. Why is pagesToVisit a List? This is just storing a bunch of URLs we have to visit next. We can enforce this idea by choosing the right data structure, in this case a set. All the pages we visit will be unique (or at least their URL will be unique). Why is pagesVisited a Set? Remember that a set, by definition, contains unique entries. Private List pagesToVisit = new LinkedList() Private Set pagesVisited = new HashSet() Private static final int MAX_PAGES_TO_SEARCH = 10 Let's sketch out the first draft of our Spider.java class: public class Spider Put a limit on the number of pages to search so this doesn't run for eternity.Keep track of pages that we've already visited.We'll end up back at the beginning again! So let's add a few more things our crawler needs to do: But what if Page B contains a bunch more links to other pages, and one of those pages links back to Page A? That's fine, we'll go to Page B next if we don't find the word we're looking for on Page A. Is that everything? What if we start at Page A and find that it contains links to Page B and Page C. See if the word we're looking for is contained in the list of words.Retrieve a web page (we'll call it a document) from a website.Let's think of all the things we need to do: But first, let's think how we'll separate out the logic and decide which classes are going to do what. Let's fire up Eclipse and start a new workspace.Īnd finally create our first class that we'll call Spider.java. There are only two classes, so even a text editor and a command line will work.
I'll be using Eclipse along the way, but any editor will suffice. Pretty simple, right? There are a few small edge cases we need to take care of, like handling HTTP errors, or retrieving something from the web that isn't HTML, and avoid accidentally visiting pages we've already visited, but those turn out to be pretty simple to implement. If the word isn't found on that page, it will go to the next page and repeat. The spider will go to that web page and collect all of the words on the page as well as all of the URLs on the page. You give it a URL to a web page and word to search for.
LINK JAVA TO R CODE
It turns out I was able to do it in about 150 lines of code spread over two classes.
A year or two after I created the dead simple web crawler in Python, I was curious how many lines of code and classes would be required to write it in Java.