Interface Stream

Interface Stream

Stream: A sequence of elements supporting sequential and parallel aggregate operations.

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int sum = widgets.stream()
              .filter(w -> w.getColor() == RED)
              .mapToInt(w -> w.getWeight())
              .sum();

In this example, widgets is a Collection<Widget>. We create a stream of Widget objects via Collection.stream(), filter it to produce a stream containing only the red widgets, and then transform it into a stream of int values representing the weight of each red widget. Then this stream is summed to produce a total weight.

Aggregate methods:

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collect()

concat()

count()

distinct()

filter()

flatMap()

forEach()

...

flatMap: ~ unzip in python:

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{ {1,2}, {3,4}, {5,6} } -> flatMap -> {1,2,3,4,5,6}

{ {'a','b'}, {'c','d'}, {'e','f'} } -> flatMap -> {'a','b','c','d','e','f'}

In Java 8, Stream can hold different data types, for examples:

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Stream<String[]>	
Stream<Set<String>>	
Stream<List<String>>	
Stream<List<Object>>

But, the Stream operations (filter, sum, distinct…) and collectors do not support it, so, we need flatMap() to do the following conversion :

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Stream<String[]>		-> flatMap ->	Stream<String>
Stream<Set<String>>	-> flatMap ->	Stream<String>
Stream<List<String>>	-> flatMap ->	Stream<String>
Stream<List<Object>>	-> flatMap ->	Stream<Object>

parallelStream():

Collection.stream() creates a sequential stream, and Collection.parallelStream() creates a parallel one.

也就是说, .stream()和.parallelStream()都是collections的方法: Collection.parallelStream()

e.g. print a to z:

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import java.util.ArrayList;
import java.util.List;

public class ParallelExample2 {

    public static void main(String[] args) {

        System.out.println("Normal...");

        List<String> alpha = getData();
        alpha.stream().forEach(System.out::println);

        System.out.println("Parallel...");

        List<String> alpha2 = getData();
        alpha2.parallelStream().forEach(System.out::println);
        
    }

    private static List<String> getData() {

        List<String> alpha = new ArrayList<>();

        int n = 97;  // 97 = a , 122 = z
        while (n <= 122) {
            char c = (char) n;
            alpha.add(String.valueOf(c));
            n++;
        }

        return alpha;

    }

}