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How does reduce() method work with parallel streams in Java 8?


How does reduce() method work with parallel streams in Java 8?

By : Ivan Tolstov
Date : October 25 2020, 07:10 PM
I hope this helps you . The problem is you use Stream::reduce for Mutable reduction, which has been Stream::collect specifically designed for. It can be either used for the safe parallel storing. Read about the differences also at official Oracle documentation Streams: Reduction. Unlike the reduce method, which always creates new value when it processes an element, the collect method modifies or mutates an existing value.
Notice the differences between both methods from their JavaDoc:
code :
StringBuilder concat = Arrays.stream(grades)
    .parallel()
    .collect(StringBuilder::new, StringBuilder::append, StringBuilder::append);


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Java 8 parallel streams don't appear to actually be working in parallel

Java 8 parallel streams don't appear to actually be working in parallel


By : user2328191
Date : March 29 2020, 07:55 AM
wish of those help When executing in parallel, there is overhead of decomposing the input set, creating tasks to represent the different portions of the calculation, distributing the actions across threads, waiting for results, combining results, etc. This is over and above the work of actually solving the problem. If a parallel framework were to always decompose problems down to a granularity of one element, for most problems, these overheads would overwhelm the actual computation and parallelism would result in a slower execution. So parallel frameworks have some latitude to decide how finely to decompose the input, and that's what's happening here.
In your case, your input set is simply too small to be decomposed. So the library chooses to execute sequentially.
code :
IntStream.range(0, 100_000).sum()
IntStream.range(0, 100_000).parallel().sum()
Java 8 Streams, perform part of the work as parallel stream and the other as sequential stream

Java 8 Streams, perform part of the work as parallel stream and the other as sequential stream


By : A.P. Yellowman
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , There is no need to turn a Stream to sequential in order to perform a sequential terminal operation. See, for example, the documentation of Stream.forEachOrdered:
Difference between java 8 streams and parallel streams

Difference between java 8 streams and parallel streams


By : Jerzy Skalski
Date : March 29 2020, 07:55 AM
it helps some times I wrote code using java 8 streams and parallel streams for the same functionality with a custom collector to perform an aggregation function. When I see CPU usage using htop, it shows all CPU cores being used for both `streams' and 'parallel streams' version. So, it seems when list.stream() is used, it also uses all CPUs. Here, what is the precise difference between parallelStream() and stream() in terms of usage of multi-core. , Consider the following program:
code :
import java.util.ArrayList;
import java.util.List;

public class Foo {
    public static void main(String... args) {
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < 1000; i++) {
            list.add(i);
        }
        list.stream().forEach(System.out::println);
    }
}
Synchronization issues with parallel streams on reduce operations

Synchronization issues with parallel streams on reduce operations


By : user3396905
Date : March 29 2020, 07:55 AM
fixed the issue. Will look into that further When you do new StringBuffer(), you are creating a reference to a single buffer. When you do .parallel(), both of the parallel streams are passed this reference and thereby operate on this same mutable buffer. The empty buffer first gets reduced with "B", then with "A", and is then reduced onto itself, resulting in "BABA".
For doing something like this with mutable structures such as StringBuffers, try .collect() instead:
code :
StringBuffer concat = Arrays.stream(grades).parallel().collect(
    () -> new StringBuffer(),
    (sb, s) -> sb.append(s),
    (sb1, sb2) -> sb1.append(sb2));
How does the reduce() method work in Java 8?

How does the reduce() method work in Java 8?


By : Vignesh P
Date : March 29 2020, 07:55 AM
Does that help I try to understand how does the reduce() method work in . , You should read the documentation of reduce that says:
code :
Arrays.asList("lorem", "ipsum", "sit", "amet")
      .stream()
      .mapToInt(String::length)
      .sum();
strs.parallelStream()
    .reduce(0,
            (a, b) -> {
                  System.out.println("Accumulator, a = " + a + ", b = " + b);
                  return a + b.length();
            },
            (a, b) -> {
                  System.out.println("Combiner");
                  return a + b;
            });
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