Java是一種廣泛使用的編程語言，可以用于開發各種類型的應用程序，包括文件上傳和下載的功能。在Java中，可以使用HttpURLConnection類或Apache HttpClient庫來實現文件上傳和下載操作。

// 上傳文件
URL url = new URL("http://example.com/upload");
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setDoOutput(true);
conn.setRequestMethod("POST");
File file = new File("path/to/file");
FileInputStream inputStream = new FileInputStream(file);
byte[] buffer = new byte[4096];
int bytesRead = -1;
OutputStream outputStream = conn.getOutputStream();
while ((bytesRead = inputStream.read(buffer)) != -1) {
outputStream.write(buffer, 0, bytesRead);
}
inputStream.close();
outputStream.close();
// 下載文件
URL url = new URL("http://example.com/download/file.txt");
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream()));
String inputLine;
StringBuilder content = new StringBuilder();
while ((inputLine = in.readLine()) != null) {
content.append(inputLine);
}
in.close();
File file = new File("path/to/save/file.txt");
FileOutputStream outputStream = new FileOutputStream(file);
outputStream.write(content.toString().getBytes());
outputStream.close();

以上代碼僅作為演示，實際應用中還需要加入文件名、文件類型等信息的處理，以及異常處理和安全性考慮。