新版的下载类

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 < size ="="" size =" contentLength;" file =" new" stream =" connection.getInputStream();" status ="=""> 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();
}

}

想扩展一下这个类实现多线程下载，FTP下载和断点续传。