你可以使用Java的URL和URLConnection類來下載文件到指定文件夾。以下是一個簡單的示例代碼：

import java.io.*;
import java.net.URL;
import java.net.URLConnection;

public class DownloadFile {

    public static void main(String[] args) {
        String fileURL = "http://example.com/file.txt";
        String saveDir = "/path/to/save/directory";

        try {
            URL url = new URL(fileURL);
            URLConnection conn = url.openConnection();
            InputStream inputStream = conn.getInputStream();

            File saveDirFile = new File(saveDir);
            if (!saveDirFile.exists()) {
                saveDirFile.mkdirs();
            }

            String fileName = fileURL.substring(fileURL.lastIndexOf("/") + 1);
            String filePath = saveDir + File.separator + fileName;

            FileOutputStream outputStream = new FileOutputStream(filePath);

            int bytesRead;
            byte[] buffer = new byte[4096];
            while ((bytesRead = inputStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, bytesRead);
            }

            outputStream.close();
            inputStream.close();

            System.out.println("File downloaded to: " + filePath);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

在上面的代碼中，你需要將fileURL替換為要下載的文件的URL，將saveDir替換為要保存文件的目錄路徑。然后運行該程序即可將文件下載到指定的文件夾中。