10th july

The DownloadManager Class

The DownloadManager class is responsible for creating and running the Download Manager’s GUI. This class has a main( ) method declared, so on execution it will be invoked first. The main( ) method instantiates a new DownloadManager class instance and then calls its show( ) method, which causes it to be displayed.

The DownloadManager Variables- 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.

The DownloadManager Constructor- When the DownloadManager is instantiated, all of the GUI’s controls are initialized inside its constructor.  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 aWindowAdapter object that overrides the windowClosing( ) event handler. This handler calls the actionExit( ) method when the application’s window is closed. 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 ListSelectionModel.SINGLE_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 table.setRowHeight( ). 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-

The verifyUrl( ) method is called by the actionAdd( ) method each time a download is added to the Download Manager. 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.

The tableSelectionChanged( ) Method-This method starts by seeing if there is already a row currently selected by checking if the selectedDownload variable is null. If the selectedDownload variable is not null, DownloadManager removes itself as an observer of the download so that it no longer receives change notifications.

The updateButtons( ) Method-

The updateButtons( ) method updates the state of all the buttons on the button panel basedon the state of the selected download.

Compiling and Running the Download Manager

Compile DownloadManager like this:

javac DownloadManager.java DownloadsTableModel.java ProgressRenderer.java Download.java

Run DownloadManager like this:

java DownloadManager