Count how many times a value is referenced in a grid

Question

I've got list of points: List<Point> seedPoints. Each Point contains x and y coordinates pointing to certain double value (let's call it pointValue). I would like to display how many times each pointValue is referenced in seedPoints list. To do so I've created following lambda expression:

seedPoints.stream()
    .collect(Collectors.groupingBy(p ->  pointGrid.get(p.x, p.y)))
    .entrySet()
    .stream()
    .map(entry -> "value:" + entry.getKey() + "   count: " + entry.getValue().size())
    .forEach(System.out::println);

The solution works fine, I get the following output:

value:0.0   count: 1
value:3.0   count: 2
value:4.0   count: 1
value:5.0   count: 4
value:6.0   count: 2

I don't find it very clean however. This solution requires two call to the stream() method (1st and 4th line of code), as collect() method is a terminal operation. Does anybody has any idea how could I improve my lambda expression?

Answer 1

There is no intermediate operation that groups items by key.

But in your example there are some changes that can make code more readable.

• Instead of collectiing to Map<Double, List<Point>> you could collect directly to Map<Double, Long>

  .collect(groupingBy(p ->  pointGrid.get(p.x, p.y), counting()))
  

• Instead of creating stream and performing output just use forEach on a map

  .forEach((value, count) -> System.out.println("value :" + value + " count: " + count);
  

So together it looks like

seedPoints.stream()
    .collect(groupingBy(p ->  pointGrid.get(p.x, p.y), counting()))
    .forEach((value, count) -> System.out.println("value :" + value + " count: " + count);