public Record
nextRecord() throws IOException {
	if (current > end)
		return null;
	String namestr = substitute(namePattern, current);
	Name name = Name.fromString(namestr, origin);
	String rdata = substitute(rdataPattern, current);
	current += step;
	return Record.fromString(name, type, dclass, ttl, rdata, origin);
}
Constructs and returns the next record in the expansion.
@throws IOException The name or rdata was invalid after substitutions were
performed.
src/main/java/core/org/xbill/DNS/Generator.java