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From: Eric Sosman <esosman@comcast-dot-net.invalid> (Eric Sosman)
Newsgroups: comp.lang.java.programmer
Subject: Re: Can I put a filter into a lambda?
Date: Fri, 01 Dec 2017 16:28:04 +1200
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On 12/1/2017 3:52 PM, Eric Douglas wrote:
> I haven't gotten around to mastering the lambda syntax.
> I have a JTable with a custom row sorter building an index of all model rows:
>
> rowToModelIndex.clear();
> if (modelRowCount > 0) {
>       if (getRowFilter() != null) {
>            for (int r = 0; r < modelRowCount; r++) {
>                 if (include(r)) {
>                      rowToModelIndex.add(r);
>                 }
>            }
>       } else {
>            rowToModelIndex.addAll(Stream.iterate(firstRow, n -> n +
1).limit(modelRowCount).collect(Collectors.toList()));
>       }
> }
>
> If it's a straight up series it's a lambda.  Can the filter loop be also?  I
would guess that would be more efficient.

     It's not clear what your filter loop is actually doing:
the RowFilter seems never to be used, and I can't find any one-argument
include() method in any Java SE (8 or 9) class. (You also haven't shown what
rowToModelIndex is, but I'll guess it's a Collection<Integer>, possibly
List<Integer>). So I'll restate the problem without the table trappings:

     1) If getRowFilter() returns null, you want a List of
        all the values 0 through modelRowCount-1, in order.

     2) If getRowFilter() returns non-null, you want a List
        of the same values, excluding those for which the
        boolean-valued include(int) method returns false.

     If that's not the goal, read no further.  But if it is
(or is close enough for jazz), both cases can be written more simply:

        // Case 1:
        rowModelIndex = IntStream.range(0, modelRowCount)
            .mapToObj(Integer::valueOf)
            .collect(Collectors.toList());

        // Case 2:
        rowModelIndex = IntStream.range(0, modelRowCount)
            .filter((value) -> include(value))
            .mapToObj(Integer::valueOf)
            .collect(Collectors.toList());

     Final point: If you can't stop worrying about efficiency,
at the very least stop *guessing* about it!

--
esosman@comcast-dot-net.invalid
Eleven hundred forty-six days to go.