visual basic 6.0 barcode generator 4: Creating a Download Manager in Java in Java

Paint PDF417 in Java 4: Creating a Download Manager in Java

4: Creating a Download Manager in Java
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The Download class employs the Observer pattern by extending Java s built-in Observable utility class. Extending the Observable class allows classes that implement Java s Observer interface to register themselves with the Download class to receive change notifications. Each time the Download class needs to notify its registered Observers of a change, the stateChanged( ) method is invoked. The stateChanged( ) method first calls the Observable class setChanged( ) method to flag the class as having been changed. Next, the stateChanged( ) method calls Observable s notifyObservers( ) method, which broadcasts the change notification to the registered Observers.
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Action and Accessor Methods
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The Download class has numerous action and accessor methods for controlling a download and getting data from it. Each of the pause( ), resume( ), and cancel( ) action methods simply does as its name implies: pauses, resumes, or cancels the download, respectively. Similarly, the error( ) method marks the download as having an error. The getUrl( ), getSize( ), getProgress( ), and getStatus( ) accessor methods each return their current respective values.
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The ProgressRenderer Class
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The ProgressRenderer class is a small utility class that is used to render the current progress of a download listed in the GUI s Downloads JTable instance. Normally, a JTable instance renders each of its table cell s data as text. However, often it s particularly useful to render a cell s data as something other than text. In the Download Manager s case, we want to render each of the table s Progress column cells as progress bars. The ProgressRenderer class shown here makes that possible. Notice that it extends JProgressBar and implements TableCellRenderer:
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import java.awt.*; import javax.swing.*; import javax.swing.table.*; // This class renders a JProgressBar in a table cell. class ProgressRenderer extends JProgressBar implements TableCellRenderer { // Constructor for ProgressRenderer. public ProgressRenderer(int min, int max) { super(min, max); } /* Returns this JProgressBar as the renderer for the given table cell. */ public Component getTableCellRendererComponent( JTable table, Object value, boolean isSelected,
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boolean hasFocus, int row, int column) { // Set JProgressBar's percent complete value. setValue((int) ((Float) value).floatValue()); return this; } }
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The ProgressRenderer class takes advantage of the fact that Swing s JTable class has a rendering system that can accept plug-ins for rendering table cells. To plug into this rendering system, first, the ProgressRenderer class has to implement Swing s TableCellRenderer interface. Second, a ProgressRenderer instance has to be registered with a JTable instance; doing so instructs the JTable instance as to which cells should be rendered with the plug-in. Implementing the TableCellRenderer interface requires the class to override the getTableCellRendererComponent( ) method. The getTableCellRendererComponent( ) method is invoked each time a JTable instance goes to render a cell for which this class has been registered. This method is passed several variables, but in this case only the value variable is used. The value variable holds the data for the cell being rendered and is passed to JProgressBar s setValue( ) method. The getTableCellRendererComponent( ) method wraps up by returning a reference to its class. This works because the ProgressRenderer class is a subclass of JProgessbar, which is a descendent of the AWT Component class.
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The DownloadsTableModel Class
The DownloadsTableModel class houses the Download Manager s list of downloads and is the backing data source for the GUI s Downloads JTable instance. The DownloadsTableModel class is shown here. Notice that it extends AbstractTableModel and implements the Observer interface:
import java.util.*; import javax.swing.*; import javax.swing.table.*; // This class manages the download table's data. class DownloadsTableModel extends AbstractTableModel implements Observer { // These are the names for the table's columns. private static final String[] columnNames = {"URL", "Size", "Progress", "Status"}; // These are the classes for each column's values. private static final Class[] columnClasses = {String.class, String.class, JProgressBar.class, String.class};
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