visual basic 6.0 barcode generator 6: Crawling the Web with Java in Java

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6: Crawling the Web with Java
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The removeWwwFromUrl( ) Method
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The removeWwwFromUrl( ) method is a simple utility method used to remove the www portion of a URL s host. For example, take the URL: http://www.osborne.com This method removes the www. piece of the URL, yielding: http://osborne.com Because many Web sites intermingle URLs that do and don t start with www , the Search Crawler uses this technique to find the lowest common denominator URL. Effectively, both URLs are the same on most Web sites, and having the lowest common denominator allows the Search Crawler to skip over duplicate URLs that would otherwise be redundantly crawled. The removeWwwFromUrl( ) method is shown here:
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// Remove leading "www" from a URL's host if present. private String removeWwwFromUrl(String url) { int index = url.indexOf("://www."); if (index != -1) { return url.substring(0, index + 3) + url.substring(index + 7); } return (url); }
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The removeWwwFromUrl( ) method starts out by finding the index of "://www." inside the string passed to url. The "://" at the beginning of the string passed to the indexOf( ) method indicates that "www" should be found at the beginning of a URL where the protocol is defined (for example, http://www.osborne.com). This way, URLs that simply contain the string "www" are not tampered with. If url contains "://www.", the characters before and after "www." are concatenated and returned. Otherwise, the string passed to url is returned.
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The retrieveLinks( ) Method
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The retrieveLinks( ) method parses through the contents of a Web page and retrieves all the relevant links. The Web page for which links are being retrieved is stored in a large String object. To say the least, parsing through this string, looking for specific character sequences, would be quite cumbersome using the methods defined by the String class. Fortunately,
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beginning with Java 2, v1.4, Java comes standard with a regular expression API library that makes easy work of parsing through strings. The regular expression API is contained in java.util.regex. The topic of regular expressions is fairly large, and a complete discussion is beyond the scope of this book. However, because parsing regular expressions is key to Search Crawler, a brief overview is presented here.
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An Overview of Regular Expression Processing
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As the term is used here, a regular expression is a sequence of characters that describes a character sequence. This general description, called a pattern, can then be used to find matches in other character sequences. Regular expressions can specify wildcard characters, sets of characters, and various quantifiers. Thus, you can specify a regular expression that represents a general form that can match several different specific character sequences. There are two classes that support regular expression processing: Pattern and Matcher. You use Pattern to define a regular expression. To match the pattern against another sequence, use Matcher. The Pattern class defines no constructors. Instead, a pattern is created by calling the compile( ) factory method. The form used here is static Pattern compile(String pattern, int options) Here, pattern is the regular expression that you want to use, and options specifies one or more options that affect matching. The option used by Search Crawler is Pattern.CASE_INSENSITIVE, which causes the case of the strings to be ignored. The compile( ) method transforms the string in pattern into a pattern that can be used for pattern matching by the Matcher class. It returns a Pattern object that contains the pattern. Once you have created a Pattern object, you will use it to create a Matcher. This is done by calling the matcher( ) factory method defined by Pattern. It is shown here: Matcher matcher(CharSequence str) Here, str is the character sequence that the pattern will be matched against. This is called the input sequence. CharSequence is an interface that was added by Java 2, v1.4 and defines a read-only set of characters. It is implemented by the String class, among others. Thus, you can pass a string to matcher( ). You will use methods defined by Matcher to perform various pattern-matching operations. The ones used by retrieveLinks( ) are find( ) and group( ). The find( ) method determines if a subsequence of the input sequence matches the pattern. The version used by Search Crawler is shown here: boolean find( ) It returns true if there is a matching subsequence and false otherwise. This method can be called repeatedly, allowing it to find all matching subsequences. Each call to find( ) begins where the previous one left off.
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