您正在使用

You are using a ByteArrayOutputStream object as the output of your download. If your program terminates without having saved the contents of this object somewhere, you will not be able to find this information in your computer's disk, as it is not written to it but rather saved in memory as a buffered byte-array (refer to the previous link for more information).

如果要将下载的输出保存到文件中，建议您使用

If you want to save the output of the download to the file, I suggest you use instead a FileOutputStream as the destination of your download. In order to do that, you have to modify your code as follows:

1. 添加适当的import声明:

import java.io.FileOutputStream;

OutputStream outputStream = new FileOutputStream('/tmp/downloadedfile');

outputStream.flush();
outputStream.close();

This will ensure that your file is being written to properly.

关于下载文件夹，您完全正确-首先需要获取要下载的文件夹及其每个子级.为了更好地理解操作方法，建议您检查以下答案:下载Google Drive API的文件夹

In regards to downloading a folder, you are completely right - you will first need to fetch the folder you want to download, and each of their children. In order to better understand how to do it, I suggest you check out the following answer: Download folder with Google Drive API

String destinationFolder = "/tmp/downloadedfiles/";
List<File> files = result.getFiles();
File newFile;
if (files == null || files.isEmpty()) {
  System.out.println("No files found.");
} else {
  System.out.println("Files:");
  for (File file : files) {
    System.out.printf("%s (%s)\n", file.getName(), file.getId());
    String fileId = file.getId();
    String fileName = file.getName();
    OutputStream outputstream = new FileOutputStream(destinationFolder + fileName);
    service.files().get(fileId)
           .executeMediaAndDownloadTo(outputstream);
    outputstream.flush();
    outputstream.close();
  }
}