Get the charset detected by this decoder, this method is optional.
If implementing an auto-detecting charset, then this decoder returns the detected charset from this method when it is available. The returned charset will be the same for the rest of the decode operation.
If insuffient bytes have been read to determine the charset, IllegalStateException
will be throwed.
The default implementation alwyas throws UnsupportedOperationException
, so it should be overridden by subclass if needed.
@return the charset detected by this decoder, or null if it is not yetdetermined
@throws UnsupportedOperationException if this decoder does not implmenet an auto-detecting charset
@throws IllegalStateException if insuffient bytes have been read to determine the charset