Attempts to read characters into the specified character buffer.

  Attempts to read characters into the specified character buffer.
The buffer is used as a repository of characters as-is: the only
changes made are the results of a put operation. No flipping or
rewinding of the buffer is performed.
Returns: @return The number of char values added to the buffer, or -1 if this source of characters is at its end
Parameters:
- cb - the buffer to read characters into
Throws:
- IOException - if an I/O error occurs
- NullPointerException - if cb is null
- ReadOnlyBufferException - if cb is a read only buffer