create 2d barcode vb.net Creating a Download Manager in Java in Java

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Creating a Download Manager in Java
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selectedDownload = null; updateButtons(); } /* Update each button's state based off of the currently selected download's status */ private void updateButtons() { if (selectedDownload != null) { int status = selectedDownloadgetStatus(); switch (status) { case DownloadDOWNLOADING: pauseButtonsetEnabled(true); resumeButtonsetEnabled(false); cancelButtonsetEnabled(true); clearButtonsetEnabled(false); break; case DownloadPAUSED: pauseButtonsetEnabled(false); resumeButtonsetEnabled(true); cancelButtonsetEnabled(true); clearButtonsetEnabled(false); break; case DownloadERROR: pauseButtonsetEnabled(false); resumeButtonsetEnabled(true); cancelButtonsetEnabled(false); clearButtonsetEnabled(true); break; default: // COMPLETE or CANCELLED pauseButtonsetEnabled(false); resumeButtonsetEnabled(false); cancelButtonsetEnabled(false); clearButtonsetEnabled(true); } } else { // No download is selected in table pauseButtonsetEnabled(false); resumeButtonsetEnabled(false); cancelButtonsetEnabled(false); clearButtonsetEnabled(false); } } /* Update is called when a Download notifies its observers of any changes */ public void update(Observable o, Object arg) { // Update buttons if the selected download has changed if (selectedDownload != null && selectedDownloadequals(o)) updateButtons(); } // Run the Download Manager public static void main(String[] args) {
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SwingUtilitiesinvokeLater(new Runnable() { public void run() { DownloadManager manager = new DownloadManager(); managersetVisible(true); } }); } }
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The DownloadManager Variables
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DownloadManager starts off by declaring several instance variables, most of which hold references to the GUI controls The selectedDownload variable holds a reference to the Download object represented by the selected row in the table Finally, the clearing instance variable is a boolean flag that tracks whether or not a download is currently being cleared from the Downloads table
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When the DownloadManager is instantiated, all of the GUI s controls are initialized inside its constructor The constructor contains a lot of code, but most of it is straightforward The following discussion gives an overview First, the window s title is set with a call to setTitle( ) Next, the setSize( ) call establishes the window s width and height in pixels After that, a window listener is added by calling addWindowListener( ), passing a WindowAdapter object that overrides the windowClosing( ) event handler This handler calls the actionExit( ) method when the application s window is closed Next, a menu bar with a File menu is added to the application s window Then the Add panel, which has the Add Text field and button, is set up An ActionListener is added to the Add Download button so that the actionAdd( ) method is called each time the button is clicked The downloads table is constructed next A ListSelectionListener is added to the table so that each time a row is selected in the table, the tableSelectionChanged( ) method is invoked The table s selection mode is also updated to ListSelectionModelSINGLE_SELECTION so that only one row at a time can be selected in the table Limiting row selection to only one row at a time simplifies the logic for determining which buttons should be enabled in the GUI when a row in the download table is selected Next, a ProgressRenderer class is instantiated and registered with the table to handle the Progress column The table s row height is updated to the ProgressRenderer s height by calling tablesetRowHeight( ) After the table has been assembled and tweaked, it is wrapped in a JScrollPane to make it scrollable and then added to a panel Finally, the buttons panel is created The buttons panel has Pause, Resume, Cancel, and Clear buttons Each of the buttons adds an ActionListener that invokes its respective action method when it is clicked After creating the buttons panel, all of the panels that have been created are added to the window
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The verifyUrl( ) Method
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The verifyUrl( ) method is called by the actionAdd( ) method each time a download is added to the Download Manager The verifyUrl( ) method is shown here:
33:
Creating a Download Manager in Java
// Verify download URL private URL verifyUrl(String url) { // Only allow HTTP URLs if (!urltoLowerCase()startsWith("http://")) return null; // Verify format of URL URL verifiedUrl = null; try { verifiedUrl = new URL(url); } catch (Exception e) { return null; } // Make sure URL specifies a file if (verifiedUrlgetFile()length() < 2) return null; return verifiedUrl; }
This method first verifies that the URL entered is an HTTP URL since only HTTP is supported Next, the URL being verified is used to construct a new URL class instance If the URL is malformed, the URL class constructor will throw an exception Finally, this method verifies that a file is actually specified in the URL
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