Java 8 identity mapping lambda?

Question

I want to find the average area of a few rectangles using aggregate operations in Java 8.

Rectangle[] rects = new Rectangle[]{
    new Rectangle(5, 10, 20, 30),
    new Rectangle(10, 20, 30, 40),
    new Rectangle(20, 30, 5, 15)
};

System.out.println("Average area: "
    + Arrays.asList(rects)
    .parallelStream()
    .map((RectangularShape r) -> (r.getWidth() * r.getHeight()))
    .collect(Collectors.averagingDouble(o -> o)));
// I don't like this "o -> o"
System.out.println("Expected: 625");

However, I find the o -> o required by averagingDouble kind of silly. Is there a more intuitive replacement for this lambda (maybe even a stock identity lambda somewhere)?

FYI, o -> o is not exactly the identity here -- it is an unboxing function, actually equivalent to Double::doubleValue. — Louis Wasserman
– Louis Wasserman, Commented Apr 16, 2014 at 4:19

Louis Wasserman · Accepted Answer · 2014-04-15 17:32:10Z

9

No need:

System.out.println("Average area: "
     + Arrays.asList(rects)
       .parallelStream()
       .mapToDouble((RectangularShape r) -> (r.getWidth() * r.getHeight()))
       .average();

(Also, you may find Stream.of(rects).parallel()) preferable to Arrays.asList(rects).parallelStream().)

answered Apr 15, 2014 at 17:32

Louis Wasserman

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1 Comment

Alexis C. Over a year ago

.getAsDouble() if you want to get the double value in the Optional (if there is)

ajb · Accepted Answer · 2014-04-16 02:07:04Z

3

There are identity() methods in java.util.function.X where X = Function<T,R>, UnaryOperator<T>, IntUnaryOperator, LongUnaryOperator, DoubleUnaryOperator. The one in Function<T,R> uses T. So it looks like DoubleUnaryOperator::identity should work, although o -> o is a lot less typing. (Then again, o -> o looks more like an emoticon of some kind than an expression.......)

answered Apr 16, 2014 at 2:07

ajb

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Rohit Jain · Accepted Answer · 2014-04-15 17:53:50Z

2

In addition to the answer from @Louis, you might also want to add an area() method in Rectangle class, and then pass a method reference to mapToDouble() method:

System.out.println("Average area: "
            + Arrays.asList(rects)
            .parallelStream()
            .mapToDouble(Rectangle::area)
            .average());

answered Apr 15, 2014 at 17:53

Rohit Jain

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