Defines the interface used internally so that different types of source
text can be accessed in the same way. Built-in concrete classes provide
support for String, StringBuffer, InputStream and char types.
A class that is CharIndexed supports the notion of a cursor within a
block of text. The cursor must be able to be advanced via the move()
method. The charAt() method returns the character at the cursor position
plus a given offset.
Returns the character at the given offset past the current cursor
position in the input. The index of the current position is zero.
It is possible for this method to be called with a negative index.
This happens when using the '^' operator in multiline matching mode
or the '\b' or '\<' word boundary operators. In any case, the lower
bound is currently fixed at -2 (for '^' with a two-character newline).
index - the offset position in the character field to examine
the character at the specified index, or the OUT_OF_BOUNDS
character defined by this interface.