java create InputStreamReader object from InputStream-Collection of common programming errors

No, Java does not usually automagically transform one character set into another, especially if you tell it explicitly which character set to use.

The thing is, however that UTF-8 is ASCII-compatible. That means that every valid ASCII stream is automatically a valid UTF-8 stream as well and that text containing only ASCII-characters encoded in UTF-8 is also valid ASCII.

So if you plan to accept only ASCII and UTF-8 input, then treating it all as UTF-8 is perfectly valid. If you plan to support other encodings as well, then you’ll need some way to transmit the information about the actual encoding being used as well.