Appending to a list within a stream to a map

Question

I'm attempting to consolidate multiple unnecessary web requests into a map, with the key connected to a location's ID, and the value being a list of products at that location.

The idea is to reduce the amount of requests to my flask server by creating a single request for each location, with a list of required products mapped to it.

I have tried to find others who has faced a similar problem using Java 8's streaming functionality, but I cannot find anyone who is trying to append to a list within a map.

Example;

public class Product {
    public Integer productNumber();
    public Integer locationNumber();
}

List<Product> products = ... (imagine many products in this list)

Map<Integer, List<Integer>> results = products.stream()
    .collect(Collectors.toMap(p -> p.locationNumber, p -> Arrays.asList(p.productNumber));

Also, the second p parameter cannot access the current product in stream.

Because of this, I have been unable to test if I can append to a List when the location number matches a pre-existing list. I don't believe I can use Arrays.asList(), as I believe its immutable.

At the end, the map should have many product numbers in a list per location. Is it possible to append Integers to a pre-existing list within a map?

Answer 1

You may do it like so,

Map<Integer, List<Integer>> res = products.stream()
    .collect(Collectors.groupingBy(Product::locationNumber,
        Collectors.mapping(Product::productNumber, Collectors.toList())));

Answer 2

The java collectors API is pretty powerful and have lots of nice utility method to solve this.

public class Learn {

    static class Product {
        final Integer productNumber;
        final Integer locationNumber;

        Product(Integer productNumber, Integer locationNumber) {
            this.productNumber = productNumber;
            this.locationNumber = locationNumber;
        }

        Integer getProductNumber() {
            return productNumber;
        }

        Integer getLocationNumber() {
            return locationNumber;
        }
    }

    public static Product of(int i, int j){
        return new Product(i,j);
    }

    public static void main(String[] args) {


        List productList = Arrays.asList(of(1,1),of(2,1),of(3,1),
                of(7,2),of(8,2),of(9,2));

        Map> results = productList.stream().collect(Collectors.groupingBy(Product::getLocationNumber,
                Collectors.collectingAndThen(Collectors.toList(), pl->pl.stream().map(Product::getProductNumber).collect(Collectors.toList()))));

        System.out.println(results);
    }
}

So, what we are doing here is we are streaming the product list and grouping the stream by the location attribute but with the twist that we want to transform the collected list of products to list of product numbers.

Collectors.collectingAndThen is precisely the method for this which will let you specify a main collector toList() and a transformer function which is nothing but again a stream to map product to product numbers. IN java API doc the main collector and transformer are labeled as downstream collector and finisher.

Please go through the Collectors source code to have a complete understanding as to how all these different collectors are defined.