Lisp for Java Users: Trivial Reader
We can now implement a very simple reader class for simplified Lisp lists. The class header and constructor look like this:
public class ListReader
{
private static final char OPEN_PAREN = '(';
private static final char CLOSE_PAREN = ')';
private final PeekStream stream;
public ListReader (final PeekStream stream)
{
this.stream = stream;
}
}
Here is the read method:
public Object read () throws IOException
{
stream.skipBlanks ();
final int chr = stream.peekChar ();
if (chr == OPEN_PAREN)
{
stream.readChar (OPEN_PAREN);
final List<Object> result = new ArrayList<> ();
while (!stream.peekChar (CLOSE_PAREN))
{
result.add (read ());
stream.skipBlanks ();
}
stream.readChar (CLOSE_PAREN);
return result;
}
final StringBuilder buffer = new StringBuilder ();
while (!Character.isWhitespace (stream.peekChar ())
&& stream.peekChar () != OPEN_PAREN
&& stream.peekChar () != CLOSE_PAREN)
{
buffer.append ((char)stream.readChar ());
}
return Double.parseDouble (buffer.toString ());
}
The first part of this method reads everything in a balanced pair of parentheses as a List,
using the read method recursively for the list content. The second part of this method reads
characters that are not blanks or parentheses into a StringBuilder buffer and parses that
as a double float number. You can download and run this code yourself, using a simple ReadDemo
class with a main method.
