/**
 * Download Manager
 *
 * Save-Listing1 as Download.java, Listing 2 as DownloadManager.java,
   Listing 3 as DownloadsTableModel.java, Listing 4 as ProgressRenderer.java.
   Then Compile javac DownloadManager.java DownloadsTableModel.java ProgressRenderer.java Download.java
  & then javaw DownloadManager
 */
listing 1
import java.io.*;
import java.net.*;
import java.util.*;

// This class downloads a file from a URL.
class Download extends Observable implements Runnable {
  // Max size of download buffer.
  private static final int MAX_BUFFER_SIZE = 1024;

  // These are the status names.
  public static final String STATUSES[] = {"Downloading",
    "Paused", "Complete", "Cancelled", "Error"};

  // These are the status codes.
  public static final int DOWNLOADING = 0;
  public static final int PAUSED = 1;
  public static final int COMPLETE = 2;
  public static final int CANCELLED = 3;
  public static final int ERROR = 4;

  private URL url; // download URL
  private int size; // size of download in bytes
  private int downloaded; // number of bytes downloaded
  private int status; // current status of download

  // Constructor for Download.
  public Download(URL url) {
    this.url = url;
    size = -1;
    downloaded = 0;
    status = DOWNLOADING;

    // Begin the download.
    download();
  }

  // Get this download's URL.
  public String getUrl() {
    return url.toString();
  }

  // Get this download's size.
  public int getSize() {
    return size;
  }

  // Get this download's progress.
  public float getProgress() {
    return ((float) downloaded / size) * 100;
  }

  // Get this download's status.
  public int getStatus() {
    return status;
  }

  // Pause this download.
  public void pause() {
    status = PAUSED;
    stateChanged();
  }

  // Resume this download.
  public void resume() {
    status = DOWNLOADING;
    stateChanged();
    download();
  }

  // Cancel this download.
  public void cancel() {
    status = CANCELLED;
    stateChanged();
  }

  // Mark this download as having an error.
  private void error() {
    status = ERROR;
    stateChanged();
  }

  // Start or resume downloading.
  private void download() {
    Thread thread = new Thread(this);
    thread.start();
  }

  // Get file name portion of URL.
  private String getFileName(URL url) {
    String fileName = url.getFile();
    return fileName.substring(fileName.lastIndexOf('/') + 1);
  }

  // Download file.
  public void run() {
    RandomAccessFile file = null;
    InputStream stream = null;

    try {
      // Open connection to URL.
      HttpURLConnection connection =
        (HttpURLConnection) url.openConnection();

      // Specify what portion of file to download.
      connection.setRequestProperty("Range",
        "bytes=" + downloaded + "-");

      // Connect to server.
      connection.connect();

      // Make sure response code is in the 200 range.
      if (connection.getResponseCode() / 100 != 2) {
        error();
      }

      // Check for valid content length.
      int contentLength = connection.getContentLength();
      if (contentLength < 1) {
        error();
      }

      /* Set the size for this download if it
         hasn't been already set. */
      if (size == -1) {
        size = contentLength;
        stateChanged();
      }

      // Open file and seek to the end of it.
      file = new RandomAccessFile(getFileName(url), "rw");
      file.seek(downloaded);

      stream = connection.getInputStream();
      while (status == DOWNLOADING) {
        /* Size buffer according to how much of the
           file is left to download. */
        byte buffer[];
        if (size - downloaded > MAX_BUFFER_SIZE) {
          buffer = new byte[MAX_BUFFER_SIZE];
        } else {
          buffer = new byte[size - downloaded];
        }

        // Read from server into buffer.
        int read = stream.read(buffer);
        if (read == -1)
          break;

        // Write buffer to file.
        file.write(buffer, 0, read);
        downloaded += read;
        stateChanged();
      }

      /* Change status to complete if this point was
         reached because downloading has finished. */
      if (status == DOWNLOADING) {
        status = COMPLETE;
        stateChanged();
      }
    } catch (Exception e) {
      error();
    } finally {
      // Close file.
      if (file != null) {
        try {
          file.close();
        } catch (Exception e) {}
      }

      // Close connection to server.
      if (stream != null) {
        try {
          stream.close();
        } catch (Exception e) {}
      }
    }
  }

  // Notify observers that this download's status has changed.
  private void stateChanged() {
    setChanged();
    notifyObservers();
  }
}

listing 2
import java.awt.*;
import java.awt.event.*;
import java.net.*;
import java.util.*;
import javax.swing.*;
import javax.swing.event.*;

// The Download Manager.
public class DownloadManager extends JFrame
  implements Observer
{
  // Add download text field.
  private JTextField addTextField;

  // Download table's data model.
  private DownloadsTableModel tableModel;

  // Table showing downloads.
  private JTable table;

  // These are the buttons for managing the selected download.
  private JButton pauseButton, resumeButton;
  private JButton cancelButton, clearButton;

  // Currently selected download.
  private Download selectedDownload;

  // Flag for whether or not table selection is being cleared.
  private boolean clearing;

  // Constructor for Download Manager.
  public DownloadManager()
  {
    // Set application title.
    setTitle("Download Manager");

    // Set window size.
    setSize(640, 480);

    // Handle window closing events.
    addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent e) {
        actionExit();
      }
    });

    // Set up file menu.
    JMenuBar menuBar = new JMenuBar();
    JMenu fileMenu = new JMenu("File");
    fileMenu.setMnemonic(KeyEvent.VK_F);
    JMenuItem fileExitMenuItem = new JMenuItem("Exit",
      KeyEvent.VK_X);
    fileExitMenuItem.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        actionExit();
      }
    });
    fileMenu.add(fileExitMenuItem);
    menuBar.add(fileMenu);
    setJMenuBar(menuBar);

    // Set up add panel.
    JPanel addPanel = new JPanel();
    addTextField = new JTextField(30);
    addPanel.add(addTextField);
    JButton addButton = new JButton("Add Download");
    addButton.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        actionAdd();
      }
    });
    addPanel.add(addButton);

    // Set up Downloads table.
    tableModel = new DownloadsTableModel();
    table = new JTable(tableModel);
    table.getSelectionModel().addListSelectionListener(new
      ListSelectionListener() {
      public void valueChanged(ListSelectionEvent e) {
        tableSelectionChanged();
      }
    });
    // Allow only one row at a time to be selected.
    table.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

    // Set up ProgressBar as renderer for progress column.
    ProgressRenderer renderer = new ProgressRenderer(0, 100);
    renderer.setStringPainted(true); // show progress text
    table.setDefaultRenderer(JProgressBar.class, renderer);

    // Set table's row height large enough to fit JProgressBar.
    table.setRowHeight(
      (int) renderer.getPreferredSize().getHeight());

    // Set up downloads panel.
    JPanel downloadsPanel = new JPanel();
    downloadsPanel.setBorder(
      BorderFactory.createTitledBorder("Downloads"));
    downloadsPanel.setLayout(new BorderLayout());
    downloadsPanel.add(new JScrollPane(table),
      BorderLayout.CENTER);

    // Set up buttons panel.
    JPanel buttonsPanel = new JPanel();
    pauseButton = new JButton("Pause");
    pauseButton.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        actionPause();
      }
    });
    pauseButton.setEnabled(false);
    buttonsPanel.add(pauseButton);
    resumeButton = new JButton("Resume");
    resumeButton.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        actionResume();
      }
    });
    resumeButton.setEnabled(false);
    buttonsPanel.add(resumeButton);
    cancelButton = new JButton("Cancel");
    cancelButton.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        actionCancel();
      }
    });
    cancelButton.setEnabled(false);
    buttonsPanel.add(cancelButton);
    clearButton = new JButton("Clear");
    clearButton.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        actionClear();
      }
    });
    clearButton.setEnabled(false);
    buttonsPanel.add(clearButton);

    // Add panels to display.
    getContentPane().setLayout(new BorderLayout());
    getContentPane().add(addPanel, BorderLayout.NORTH);
    getContentPane().add(downloadsPanel, BorderLayout.CENTER);
    getContentPane().add(buttonsPanel, BorderLayout.SOUTH);
  }

  // Exit this program.
  private void actionExit() {
    System.exit(0);
  }

  // Add a new download.
  private void actionAdd() {
    URL verifiedUrl = verifyUrl(addTextField.getText());
    if (verifiedUrl != null) {
      tableModel.addDownload(new Download(verifiedUrl));
      addTextField.setText(""); // reset add text field
    } else {
      JOptionPane.showMessageDialog(this,
        "Invalid Download URL", "Error",
        JOptionPane.ERROR_MESSAGE);
    }
  }

  // Verify download URL.
  private URL verifyUrl(String url) {
    // Only allow HTTP URLs.
    if (!url.toLowerCase().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 (verifiedUrl.getFile().length() < 2)
      return null;

    return verifiedUrl;
  }

  // Called when table row selection changes.
  private void tableSelectionChanged() {
    /* Unregister from receiving notifications
       from the last selected download. */
    if (selectedDownload != null)
      selectedDownload.deleteObserver(DownloadManager.this);

    /* If not in the middle of clearing a download,
       set the selected download and register to
       receive notifications from it. */
    if (!clearing && table.getSelectedRow() > -1) {
      selectedDownload =
        tableModel.getDownload(table.getSelectedRow());
      selectedDownload.addObserver(DownloadManager.this);
      updateButtons();
    }
  }

  // Pause the selected download.
  private void actionPause() {
    selectedDownload.pause();
    updateButtons();
  }

  // Resume the selected download.
  private void actionResume() {
    selectedDownload.resume();
    updateButtons();
  }

  // Cancel the selected download.
  private void actionCancel() {
    selectedDownload.cancel();
    updateButtons();
  }

  // Clear the selected download.
  private void actionClear() {
    clearing = true;
    tableModel.clearDownload(table.getSelectedRow());
    clearing = false;
    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 = selectedDownload.getStatus();
      switch (status) {
        case Download.DOWNLOADING:
          pauseButton.setEnabled(true);
          resumeButton.setEnabled(false);
          cancelButton.setEnabled(true);
          clearButton.setEnabled(false);
          break;
        case Download.PAUSED:
          pauseButton.setEnabled(false);
          resumeButton.setEnabled(true);
          cancelButton.setEnabled(true);
          clearButton.setEnabled(false);
          break;
        case Download.ERROR:
          pauseButton.setEnabled(false);
          resumeButton.setEnabled(true);
          cancelButton.setEnabled(false);
          clearButton.setEnabled(true);
          break;
        default: // COMPLETE or CANCELLED
          pauseButton.setEnabled(false);
          resumeButton.setEnabled(false);
          cancelButton.setEnabled(false);
          clearButton.setEnabled(true);
      }
    } else {
      // No download is selected in table.
      pauseButton.setEnabled(false);
      resumeButton.setEnabled(false);
      cancelButton.setEnabled(false);
      clearButton.setEnabled(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 && selectedDownload.equals(o))
      updateButtons();
  }

  // Run the Download Manager.
  public static void main(String[] args) {
    DownloadManager manager = new DownloadManager();
    manager.setVisible(true);
  }
}

listing 3
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};

  // The table's list of downloads.
  private ArrayList<Download> downloadList =
             new ArrayList<Download>();

  // Add a new download to the table.
  public void addDownload(Download download) {
    // Register to be notified when the download changes.
    download.addObserver(this);

    downloadList.add(download);

    // Fire table row insertion notification to table.
    fireTableRowsInserted(getRowCount() - 1, getRowCount() - 1);
  }

  // Get a download for the specified row.
  public Download getDownload(int row) {
    return (Download) downloadList.get(row);
  }

  // Remove a download from the list.
  public void clearDownload(int row) {
    downloadList.remove(row);

    // Fire table row deletion notification to table.
    fireTableRowsDeleted(row, row);
  }

  // Get table's column count.
  public int getColumnCount() {
    return columnNames.length;
  }

  // Get a column's name.
  public String getColumnName(int col) {
     return columnNames[col];
  }

  // Get a column's class.
  public Class getColumnClass(int col) {
    return columnClasses[col];
  }

  // Get table's row count.
  public int getRowCount() {
    return downloadList.size();
  }

  // Get value for a specific row and column combination.
  public Object getValueAt(int row, int col) {
    Download download = downloadList.get(row);
    switch (col) {
      case 0: // URL
        return download.getUrl();
      case 1: // Size
        int size = download.getSize();
        return (size == -1) ? "" : Integer.toString(size);
      case 2: // Progress
        return new Float(download.getProgress());
      case 3: // Status
        return Download.STATUSES[download.getStatus()];
    }
    return "";
  }

  /* Update is called when a Download notifies its
     observers of any changes */
  public void update(Observable o, Object arg) {
    int index = downloadList.indexOf(o);

    // Fire table row update notification to table.
    fireTableRowsUpdated(index, index);
  }
}

listing 4
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,
    boolean hasFocus, int row, int column)
  {
    // Set JProgressBar's percent complete value.
    setValue((int) ((Float) value).floatValue());
    return this;
  }
}