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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to a file called crawler.log in the directory the application is run from, as specified by the Java environment variable user.dir. Second, an ActionListener is added to the Search button so that the actionSearch( ) method is called each time the button is clicked. Third, the font for each label that is used to display results is updated with a call to setFont( ). The setFont( ) call is used to turn off the bolding of the label fonts so that they are distinguished in the interface. Following the Search and Stats sections of the interface is the Matches section that consists of the matches table, which contains the URLs containing the search string. The matches table is instantiated with a new DefaultTableModel subclass passed to its constructor. Typically a fully qualified subclass of DefaultTableModel is used to customize the data model used by a JTable; however, in this case only the isCellEditable( ) method needs to be implemented. The isCellEditable( ) method instructs the table that no cells should be editable by returning false, regardless of the row and column specified. Once the matches table is initialized, it is added to the Matches panel. Finally, the Search panel and Matches panel are added to the interface.
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The actionSearch( ) Method
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The actionSearch( ) method is invoked each time the Search (or Stop) button is clicked. The actionSearch( ) method starts with these lines of code:
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// If stop button clicked, turn crawling flag off. if (crawling) { crawling = false; return; }
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Since the Search button in the interface doubles as both the Search button and the Stop button, it s necessary to know which of the two buttons was clicked. When crawling is underway, the crawling flag is set to true. Thus if the crawling flag is true when the actionsearch( ) method is invoked, the Stop button was clicked. In this scenario, the crawling flag is set to false and actionSearch( ) returns so that the rest of the method is not executed. Next, an ArrayList variable, errorList, is initialized:
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ArrayList errorList = new ArrayList();
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The errorList is used to hold any error messages generated by the next several lines of code that validate all required search fields have been entered. It goes without saying that the Search Crawler will not function without a URL that specifies the location at which to start crawling. The following code verifies that a starting URL has been entered and that the URL is valid:
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// Validate that the start URL has been entered. String startUrl = startTextField.getText().trim(); if (startUrl.length() < 1) { errorList.add("Missing Start URL.");
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} // Verify start URL. else if (verifyUrl(startUrl) == null) { errorList.add("Invalid Start URL."); }
If either of these checks fails, an error message is added to the error list. Next, the Max URLs to Crawl combo box value is validated:
// Validate that Max URLs is either empty or is a number. int maxUrls = -1; String max = ((String) maxComboBox.getSelectedItem()).trim(); if (max.length() > 0) { try { maxUrls = Integer.parseInt(max); } catch (NumberFormatException e) { } if (maxUrls < 1) { errorList.add("Invalid Max URLs value."); } }
Validating the maximum number of URLs to crawl is a bit more involved than the other validations in this method. This is because the Max URLs to Crawl field can either contain a positive number that indicates the maximum number of URLs to crawl or can be left blank to indicate that no maximum should be used. Initially, maxUrls is defaulted to 1 to indicate no maximum. If the user enters something into the Max URLs to Crawl field, it is validated as being a valid numeric value with a call to Integer.parseInt( ). Integer.parseInt( ) converts a String representation of an integer into an int value. If the String representation cannot be converted to an integer, a NumberFormatException is thrown and the maxUrls value is not set. Next, maxUrls is checked to see if it is less than 1. If so, an error is added to the error list. Next, the Matches Log File and Search String fields are validated:
// Validate that the matches log file has been entered. String logFile = logTextField.getText().trim(); if (logFile.length() < 1) { errorList.add("Missing Matches Log File."); } // Validate that the search string has been entered. String searchString = searchTextField.getText().trim(); if (searchString.length() < 1) { errorList.add("Missing Search String."); }
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