Fix inconsistent behavior when parallel=1

src/main/java/com/pivovarit/collectors/AsyncParallelCollector.java

@@ -135,7 +135,13 @@ static void requireValidParallelism(int parallelism) {
     }
 
     static <T, R, RR> Collector<T, ?, CompletableFuture<RR>> asyncCollector(Function<T, R> mapper, Executor executor, Function<Stream<R>, RR> finisher) {
-        return collectingAndThen(toList(), list -> supplyAsync(() -> finisher.apply(list.stream().map(mapper)), executor));
+        return collectingAndThen(toList(), list -> supplyAsync(() -> {
+            List<R> acc = new ArrayList<>(list.size());
+            for (T t : list) {
+                acc.add(mapper.apply(t));
+            }
+            return finisher.apply(acc.stream());
+        }, executor));
     }
 
     static final class BatchingCollectors {

src/main/java/com/pivovarit/collectors/ParallelCollectors.java

@@ -138,6 +138,8 @@ private ParallelCollectors() {
      * A convenience {@link Collector} used for executing parallel computations on a custom {@link Executor}
      * and returning a {@link Stream} instance returning results in completion order
      *
+     * For the parallelism of 1, the stream is executed by the calling thread.
+     *
      * <br><br>
      * The max parallelism level defaults to {@code Runtime.availableProcessors() - 1}
      *
@@ -169,6 +171,8 @@ private ParallelCollectors() {
      * A convenience {@link Collector} used for executing parallel computations on a custom {@link Executor}
      * and returning a {@link Stream} instance returning results as they arrive.
      *
+     * For the parallelism of 1, the stream is executed by the calling thread.
+     *
      * <br>
      * Example:
      * <pre>{@code
@@ -195,6 +199,8 @@ private ParallelCollectors() {
      * A convenience {@link Collector} used for executing parallel computations on a custom {@link Executor}
      * and returning a {@link Stream} instance returning results as they arrive while maintaining the initial order.
      *
+     * For the parallelism of 1, the stream is executed by the calling thread.
+     *
      * <br><br>
      * The max parallelism level defaults to {@code Runtime.availableProcessors() - 1}
      *
@@ -225,6 +231,8 @@ private ParallelCollectors() {
      * A convenience {@link Collector} used for executing parallel computations on a custom {@link Executor}
      * and returning a {@link Stream} instance returning results as they arrive while maintaining the initial order.
      *
+     * For the parallelism of 1, the stream is executed by the calling thread.
+     *
      * <br>
      * Example:
      * <pre>{@code
@@ -350,6 +358,8 @@ private Batching() {
          * A convenience {@link Collector} used for executing parallel computations on a custom {@link Executor}
          * and returning a {@link Stream} instance returning results as they arrive.
          *
+         * For the parallelism of 1, the stream is executed by the calling thread.
+         *
          * <br>
          * Example:
          * <pre>{@code
@@ -376,6 +386,8 @@ private Batching() {
          * A convenience {@link Collector} used for executing parallel computations on a custom {@link Executor}
          * and returning a {@link Stream} instance returning results as they arrive while maintaining the initial order.
          *
+         * For the parallelism of 1, the stream is executed by the calling thread.
+         *
          * <br>
          * Example:
          * <pre>{@code

src/test/java/com/pivovarit/collectors/blackbox/CountingExecutor.java

@@ -0,0 +1,25 @@
+package com.pivovarit.collectors.blackbox;
+
+import java.util.concurrent.Executor;
+import java.util.concurrent.atomic.AtomicInteger;
+
+class CountingExecutor implements Executor {
+
+    private final AtomicInteger counter = new AtomicInteger();
+
+    private final Executor delegate;
+
+    public CountingExecutor(Executor delegate) {
+        this.delegate = delegate;
+    }
+
+    @Override
+    public void execute(Runnable command) {
+        counter.incrementAndGet();
+        delegate.execute(command);
+    }
+
+    public Integer getInvocations() {
+        return counter.get();
+    }
+}

src/test/java/com/pivovarit/collectors/blackbox/FunctionalTest.java

@@ -1,4 +1,4 @@
-package com.pivovarit.collectors.test;
+package com.pivovarit.collectors.blackbox;
 
 import com.pivovarit.collectors.ParallelCollectors.Batching;
 import org.junit.jupiter.api.DynamicTest;
@@ -7,6 +7,7 @@
 
 import java.time.Duration;
 import java.time.LocalTime;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashSet;
 import java.util.LinkedList;
@@ -98,6 +99,29 @@ void shouldCollectInCompletionOrder() {
         assertThat(result).isSorted();
     }
 
+    @Test
+    void shouldExecuteEagerlyOnProvidedThreadPool() {
+        ExecutorService executor = Executors.newFixedThreadPool(2);
+        CountingExecutor countingExecutor = new CountingExecutor(executor);
+        AtomicInteger executions = new AtomicInteger();
+        try {
+            List<String> list = Arrays.asList("A", "B");
+
+            Stream<String> stream = list.stream()
+              .collect(parallel(s -> {
+                  System.out.println("Running on " + Thread.currentThread().getName());
+                  executions.incrementAndGet();
+                  return s;
+              }, countingExecutor, 1))
+              .join();
+        } finally {
+            executor.shutdown();
+        }
+
+        assertThat(countingExecutor.getInvocations()).isEqualTo(1);
+        assertThat(executions.get()).isEqualTo(2);
+    }
+
     private static <R extends Collection<Integer>> Stream<DynamicTest> tests(CollectorSupplier<Function<Integer, Integer>, Executor, Integer, Collector<Integer, ?, CompletableFuture<R>>> collector, String name, boolean maintainsOrder) {
         return of(
           shouldCollect(collector, name, 1),