That would be a very clunky approach to put it mildly - the explicit design intent of BufferedReader is to let you read lines without caring what the line separator is - it's a lossy transformation. You also can't subclass String. You'd be adding something very unwieldy for an edge case that is directly contrary to the purpose of the API. The 'mistake' (such as it is) of the author here is simply using the wrong API - there are shorter, more direct ways to read a file into a string without any of these contortions.

    data = new String(Files.readAllBytes(Paths.get(filename)));

That still comes down to an understanding of the API.

I don't think that appending a SingleLineString to a String object would obviously add a newline. The String/StringBuffer/StringBuilder classes in Java certainly don't work that way.

If you were adding to some other object like an instance of some text representation class, this could be different. But it would still not be 100% safe to deduce just by the API itself without looking at the documentation.