int sum = widgets.stream()
.filter(w -> w.getColor() == RED)
.mapToInt(w -> w.getWeight())
.sum();
こ、これは？
Java(Java Platform SE 8)なんです。
従来の書き方だとこんな感じでしょうか。
public class Widget {
private COLOR color;
private int weight;
public enum COLOR {
RED,
YELLOW,
BLUE
}
public Widget(COLOR color, int weight) {
this.color = color;
this.weight = weight;
}
public COLOR getColor() {
return color;
}
public int getWeight() {
return weight;
}
}
List<Widget> widgets = new ArrayList<Widget>() {{
add(new Widget(RED, 1));
add(new Widget(RED, 2));
add(new Widget(YELLOW, 3));
add(new Widget(BLUE, 4));
add(new Widget(RED, 5));
}};
Stream<Widget> stream = widgets.stream();
Predicate<Widget> predicate = new Predicate<Widget>() {
@Override
public boolean test(Widget w) {
return w.getColor() == RED;
}
};
stream = stream.filter(predicate);
ToIntFunction<Widget> tif = new ToIntFunction<Widget>() {
@Override
public int applyAsInt(Widget w) {
return w.getWeight();
}
};
IntStream is = stream.mapToInt(tif);
int sum = is.sum();
System.out.println("sum: " + sum);
-----------------------------------
Rubyで書くとこんな感じです。
class Widget
attr_accessor :color, :weight
def initialize(color, weight)
@color = color
@weight = weight
end
end
sum = [Widget.new(:RED, 1), Widget.new(:RED, 2), Widget.new(:YELLOW, 3), Widget.new(:BLUE, 4), Widget.new(:RED, 5)].select{ |w| w.color == :RED }.inject(0){ |memo, obj| memo += obj.weight }
p sum
