Miscellaneous Utilities - The Java Programming Language - page 960

that token. When hasNext is invoked it looks past the line separator and
sees that there is something there, so it returns true , leaving the cur-
rent input position where it was. When hasNext(COMMENT) is invoked, it too
ignores the line separator delimiter and sees a pattern on the next line
that matches the comment, so it also returns true , leaving the current
input position where it was. When nextLine is invoked its job is to ad-
vance the current position to the beginning of the next line and return
the input that was skipped. The current position is immediately before
the line separator, so moving it after the line separator is a very short
move and, other than the line separator, no input is skipped. Conse-
quently, nextLine returns an empty string. In our example this is not
a problem because we loop around again and match the comment a
second time, and this time we remove it properly. However, if your code
assumed that the comment itself was gone after nextLine was invoked,
then that assumption would be incorrect. We can fix this problem like
so:
Scanner in = new Scanner(source);
Pattern COMMENT = Pattern.compile("#.*");
String comment;
// ...
while (in.hasNext()) {
if (in.hasNext(COMMENT)) {
comment = in.findWithinHorizon(COMMENT, 0);
in.nextLine();
}
else {
// process other tokens
}
}
Now when hasNext(COMMENT) tells us that there is a comment ahead, we
use findWithinHorizon(COMMENT,0) to skip the line separator, find the ac-
tual comment, and return it. We don't need to set any horizon because
we know from hasNext that the comment is there. After findWithinHorizon
returns the comment, the current position is just before the line separ-