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User mentioned (1): @VGR
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@VGR 's comment to not use URLEncoder was completely wrong.

It turns out that the solution is:

String q = URLEncoder.encode("mimeType=\"application/vnd.google-apps.presentation\"", StandardCharsets.UTF_8);                      String f = URLEncoder.encode("files(id,name,webViewLink)", StandardCharsets.UTF_8);             

Then to pass
String uri = "https://www.googleapis.com/drive/v3/files?q=" + q + "&fields=" + f + "&key=mykey"
to

HttpRequest httpreq = HttpRequest.newBuilder().uri(URI.create(uri)).header("Authorization", "Bearer " + result.getString("access_token")).build();

The key is the escaped " (double quotes) around the mimeType:

\"application/vnd.google-apps.presentation\"
and to URL encode all of it as well as the fields value.

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