List<Widget> filtered = filter(widgets, new Filter<Widget>() {
        @Override public boolean keep(Widget w) {
            return w.getColor() == RED;
        }
    });

    List<Integer> weights = mapToInt(filtered, new Mapper<Widget, Integer>() {
        @Override public Integer map(Widget w) {
            return w.getWeight();
        }
    });

    int sum = sum(weights);