Yes, flows are objects. But typically you then wrap a flow in a buffered reader, which isn’t an object. So reading a character or line doesn’t involve a method call, unless the buffer is empty and it needs to refill it (see Buffering and Parsing).
Also, we sometimes optimise away the method calls. For example, you can ask to copy from any source flow to any sink, but if the source and sink are both wrapping Unix file descriptors then it will automatically extract them and do the bulk of the copy directly.