Add conversion from Reader to InputStream

It appears there are use cases where one wants to convert a java.io.Reader to a java.io.InputStream using a given charset, see this Stack Overflow question or this Baeldung article.

Guava already has this functionality implemented as ReaderInputStream, but only exposes it for CharSource (therefore not for Reader), see also https://github.com/google/guava/issues/642#issuecomment-380502353.

Would it make sense to expose it also for Reader, e.g. as com.google.common.io.CharStreams.asInputStream(Reader, Charset)?

Considerations:

• Users might want to specify buffer size?
• Users very likely want to specify behavior on unencodable chars, currently ReaderInputStream uses CodingErrorAction.REPLACE for them

[eamonnmcmanus]

It is possible for anyone to do this with the existing API, like this:

public static InputStream asInputStream(Reader reader, Charset charset) throws IOException {
  return new CharSource() {
    @Override public Reader openStream() {
      return reader;
    }
  }.asByteSource(charset).openStream();
}

Not very elegant, but the com.google.common.io API does put the emphasis on reusable sources and sinks rather than one-shot streams like Reader and InputStream.

[eamonnmcmanus]

The Guava team has talked this over and we don’t think there’s enough evidence of demand for such a method. The com.google.common.io package has existed at Google for 15 years and we seem to have got by just fine all that time without the method. People who do need it for some reason can do what I suggested above.