Java streams forming a map issue

Question

I am working on a framework where we are trying to convert our traditional loops to streams. My problem is I wrote two separate logics to get price and colors but I would like to merge both together so it will be presentable

Code to get the price values

List<Double> productPrices = product.getUpcs()
            .stream()
            .map(e -> e.getUpcDetails().getPrice().getRetail().getPriceValue())
            .distinct()
            .sorted(Comparator.reverseOrder())
            .collect(Collectors.toList());

Code to get the colors under prices

      product.getUpcs()
            .stream()
            .filter(e -> e.getUpcDetails().getPrice().getRetail().getPriceValue() == 74.5)
            .flatMap(e -> e.getUpcDetails().getAttributes().stream())
            .filter(e2 -> e2.getName().contentEquals("COLOR"))
            .forEach(e3 -> System.out.println(e3.getValues().get(0).get("value")));

I harcoded price in the above section to obtain the colors, instead, i would like to get that as input from the list of price values and get an output in

Map<Double,List<colors>
output Map<75.4, {blue,black,orange}>

I tried merging these both without success, any help would be appriciated.

looks like some sort of groupingBy that you need... since you need a Map as a result — Eugene
– Eugene, Commented Aug 9, 2017 at 8:14
As a side note, never use double to express money values. You’ll find loads of articles on the net about it… — Holger
– Holger, Commented Aug 9, 2017 at 15:25

Alex Pakka · Accepted Answer · 2017-08-09 23:05:37Z

I would suggest you examine this or similar tutorial to get a bit of understanding how this works.

The key to the solution is to learn about Collectors.groupingBy() functionality. As a side note, there it also shows a better way of handling pricing information in Java.

But what you would need to do is something like this:

 Map<Double, Set<String>> productPrices = product
            .stream()
            .map(e -> e.getUpcDetails())
            .collect(
                    Collectors.groupingBy(Details::getPrice,
                    Collectors.mapping(Details::getColors, Collectors.collectingAndThen(
                            Collectors.toList(),
                            (set) -> set
                                    .stream()
                                    .flatMap(Collection::stream)
                                    .collect(Collectors.toSet())))

            ));

Since your question is a bit unclear about the details of classes involved, I assumed this simple class structure:

class Details {
    private double price;
    private List<String> colors;

    double getPrice() { return price; }
    List<String> getColors() { return colors; }
}

class Product {
    private Details details;

    Details getUpcDetails() { return details; }
}

```

It would be possible to optimize the code above but I specifically left the possibility to filter and map colours in the mapping collector.

Manos Nikolaidis · Accepted Answer · 2017-08-09 23:13:53Z

You can first turn your second stream into a method that gets a List of products (assumed to be filtered/grouped by price) and transforms it to a List of colors:

List<Color> productsToColors(final List<Product> products) {
    return products.stream()
        .flatMap(e -> e.getUpcDetails().getAttributes().stream())
        .filter(e2 -> e2.getName().contentEquals("COLOR"))
        .map(e3 -> e3.getValues().get(0).get("value"))
        .collect(toList());
}

You can use the groupingBy collector to gather all products by their price in a List and then with a second create a second stream and the productsToColors method get the map you want:

Map<Double, List<Color>> colors = product.getUpcs().stream()
    .collect(groupingBy(e -> e.getUpcDetails().getPrice().getRetail().getPriceValue())
    .entrySet().stream()
    .collect(toMap(Entry::getKey, e -> productsToColors(e.getValue())));

You can also have groupingBy create a TreeMap instead so that the colors map will be sorted by price.

As a side-note beware of comparing double values for equality like this. You may want to round them first. Or use long variables multiplied by 100 (i.e. cents).

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