Java8Triads, Java - rextester

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.ArrayList;
import java.util.List;
import java.util.function.BiConsumer;
import java.util.stream.IntStream;
class Rextester
{  
    public static void main(String args[])
    {
final BiConsumer<List<List<String>>, String> accumulator = (acc, a) -> {
      StringBuilder stringBuilder = new StringBuilder();
      stringBuilder.append("Accumulator|");
      stringBuilder.append("Before: ").append(acc.toString());
      int accumulatorSize = acc.size();
      if (accumulatorSize == 0) {
        List<String> newList = new ArrayList<>();
        newList.add(a);
        acc.add(newList);
      } else {
        List<String> lastList = acc.get(accumulatorSize - 1);
        if (lastList.size() != 3) {
          lastList.add(a);
        } else {
          List<String> newList = new ArrayList<>();
          newList.add(a);
          acc.add(newList);
        }
      }
      stringBuilder.append("|After: ").append(acc.toString());
      stringBuilder.append("|a: ").append(a);
      System.out.println(stringBuilder.toString());
    };

final BiConsumer<List<String>, List<String>> fixSize = (l1, l2) -> {
      while(l1.size() != 3 && l2.size() > 0) {
        l1.add(l2.remove(0));
      }
    };

final BiConsumer<List<List<String>>, List<List<String>>> combiner = (l1, l2) -> {
      StringBuilder stringBuilder = new StringBuilder();
      stringBuilder.append("Combiner|");
      stringBuilder.append("Before, l1: ").append(l1).append(", l2: ").append(l2);
      if (l1.isEmpty()) {
        // l1 is empty
        l1.addAll(l2);
      } else {
        // l1 is not empty
        List<String> lastL1List = l1.get(l1.size() - 1);
        if (lastL1List.size() == 3) {
          l1.addAll(l2);
        } else {
          if (l2.isEmpty()) {
            // do nothing
          } else {
            List<List<String>> fixSizeList = new ArrayList<>(1 + l2.size());
            fixSizeList.add(lastL1List);
            fixSizeList.addAll(l2);
            for (int i = 0; i < fixSizeList.size() - 1; i++) {
              List<String> x = fixSizeList.get(i), y = fixSizeList.get(i + 1);
              fixSize.accept(x, y);
            }
            l2.stream().filter(l -> !l.isEmpty()).forEach(l1::add);
            // everything is now of size three except, may be last
          }
        }
      }
      stringBuilder.append("|After, l1: ").append(l1).append(", l2: ").append(l2);
      System.out.println(stringBuilder.toString());
    };

final List<List<String>> triads = IntStream.range(1, 14)
        .parallel()
        .boxed()
        .map(Object::toString)
        .collect(ArrayList::new, accumulator, combiner);

System.out.println(triads.toString());
    }
}

[ + ] Show input
Compilation time: 1.18 sec, absolute running time: 0.33 sec, cpu time: 0.35 sec, memory peak: 25 Mb, absolute service time: 1,52 sec