FIGURE 33-1
The Download Manager GUI inter face
The Download Class
The Download class is the workhorse of the Download Manager. Its primary purpose is to
download a file and save that file's contents to disk. Each time a new download is added
to the Download Manager, a new Download object is instantiated to handle the download.
The Download Manager has the ability to download multiple files at once. To achieve
this, it's necessary for each of the simultaneous downloads to run independently. It's also
necessary for each individual download to manage its own state so that it can be reflected
in the GUI. This is accomplished with the Download class.
The entire code for Download is shown here. Notice that it extends Observable and
implements Runnable. Each part is examined in detail in the sections that follow.
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;
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