CODEWITHSUNDEEP
filterjava-8java-stream
user3897533
user3897533

Reputation: 477

Stream and Filter operations on a Map

I have an EdmPortfolio type that has id (int), name (String) and tags which is a map as its members.

EdmPortfolio say has an id 1, and name as Portfolio1 and Map could have the following values, analyst, John region, US

I have a List, At the end i want to have a Map which has all the map values combined from all portfolios. Another EdmPortfolio might have the id as 2, name as Portfolio2 and Map with values analyst, Smith region , UK

I want to have a combined Map with Values region 'US','UK' analyst 'John','Smith'

analyst and region are the keys to the map. It has both the maps combined. I have the following code but i am a little lost

List<Map<Tag,String>> portfolioTagsList = new ArrayList<>();
            for (EdmPortfolio edmPortfolio : edmPortfolioList) {
              Map<Tag,String> portfolioTags =  edmPortfolio.getPortfolioTags().entrySet()
                        .stream()

                        .filter(e -> (e.getValue() != ""|| e.getValue() !=null))
                        .collect(Collectors.toMap(
                                Map.Entry::getKey,
                                Map.Entry::getValue
                        ));


                portfolioTagsList.add(portfolioTags);
            }
            Map<Tag,String> finalTags = portfolioTagsList.stream()
                    .flatMap(m -> m.entrySet().stream())
                    .collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.joining(",",Map.Entry::getValue)));

I am getting nonstatic method cannot be referenced from static context. How do i achieve this?

Thanks for your time

Upvotes: 2

Views: 1136

Answers (2)

Holger
Holger

Reputation: 298539

Unfortunately, “nonstatic method cannot be referenced from static context” is the error that javac often ends up when type inference fails, especially in the context of method references or lambda expressions. The actual cause could be a wrong generic type parameter, a forgotten import statement, the use of a lambda expression or method reference at an inappropriate place, or even a misplaced or forgotten brace somewhere.

In your case, it’s Collectors.joining(",",Map.Entry::getValue) as that collector has no parameter of a function type that could accept a method reference.
You should use Collectors.mapping(Map.Entry::getValue, Collectors.joining(",")) instead.

Besides that, follow BrunoJCM’s suggestion to use a single stream operation and pay attention to the filter condition. e.getValue() != "" || e.getValue() !=null makes no sense as a reference can never be both, a reference to "" and the null reference, hence, this condition is always fulfilled. You most likely wanted to express that the string must not be null and not an empty string. In this regard, you should not compare strings by reference but use equals. For testing for an empty string, there is the alternative to use isEmpty() instead of equals(""). So the condition should be e.getValue()!=null && !e.getValue().isEmpty(). Mind the order, as we can invoke isEmpty() only after having verified that it is not null.

Upvotes: 4

Bruno Medeiros
Bruno Medeiros

Reputation: 2399

I'd do something like:

Map<Tag,String> finalTags = edmPortfolioList.stream()
.flatMap(edmPortfolio -> edmPortfolio.getPortfolioTags().entrySet().stream())
.filter(e -> (e.getValue() != "" || e.getValue() !=null)) // this seems weird, but just keeping
.collect(Collectors.groupingBy(e -> e.getKey(), 
    Collectors.mapping(ev -> ev.getValue(), 
        Collectors.joining(",")));

BTW: Any reason to break that in two streams?

Upvotes: 4

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