Convert List<Long> to Map<Long, Long> that counts occurrences

collectors, dictionary, java, java-8, java-stream

Solution

If you want to group the elements, you have to use `groupingBy`:

import static java.util.stream.Collectors.*;

Map<Long, Long> primeFactorCount = primeFactors.stream()
        .collect(groupingBy(p -> p, counting()));

Problem

I'm playing around with Java 8 and I know that this must be possible by reading the documentation, I just cannot find out how to do it. I have the following working code: ``` long factorProduct = LongStream.rangeClosed(1, maxFactor) .filter(this::isOptimalFactor) .reduce((i, j) -> i * j) .getAsLong(); List<Long> primeFactors = primeFactors(factorProduct); ``` Important part is that I have a `List<Long>` that may have duplicates on some `Long` numbers. Now I want to convert it to a `Map<Long, Long>` with as key the element and as value the occurences. I thought that: ``` Map<Long, Long> primeFactorCount = primeFactors.stream() .collect(Collectors.counting()); ``` would work, but it doesn't. I looked up the examples in the java.util.stream.Collectors documentation. How do I need to use these features?

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