Add parallel-collectors samples (#7187)

* Add ParallelCollectorsTest

* Add dependency

* Fix naming

libraries-2/pom.xml

@@ -30,6 +30,11 @@
 	</repositories>
 
 	<dependencies>
+		<dependency>
+			<groupId>com.pivovarit</groupId>
+			<artifactId>parallel-collectors</artifactId>
+			<version>1.1.0</version>
+		</dependency>
 		<dependency>
 			<groupId>org.assertj</groupId>
 			<artifactId>assertj-core</artifactId>

libraries-2/src/test/java/com/baeldung/parallel_collectors/ParallelCollectorsUnitTest.java

@@ -0,0 +1,168 @@
+package com.baeldung.parallel_collectors;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.TreeMap;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.stream.Collectors;
+
+import static com.pivovarit.collectors.ParallelCollectors.parallel;
+import static com.pivovarit.collectors.ParallelCollectors.parallelOrdered;
+import static com.pivovarit.collectors.ParallelCollectors.parallelToCollection;
+import static com.pivovarit.collectors.ParallelCollectors.parallelToList;
+import static com.pivovarit.collectors.ParallelCollectors.parallelToMap;
+import static com.pivovarit.collectors.ParallelCollectors.parallelToStream;
+
+public class ParallelCollectorsUnitTest {
+
+    @Test
+    public void shouldProcessInParallelWithStreams() {
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        List<String> results = ids.parallelStream()
+          .map(i -> fetchById(i))
+          .collect(Collectors.toList());
+
+        System.out.println(results);
+    }
+
+    @Test
+    public void shouldProcessInParallelWithParallelCollectors() {
+        ExecutorService executor = Executors.newFixedThreadPool(10);
+
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        CompletableFuture<List<String>> results = ids.stream()
+          .collect(parallelToList(i -> fetchById(i), executor, 4));
+
+        System.out.println(results.join());
+    }
+
+    @Test
+    public void shouldCollectToList() {
+        ExecutorService executor = Executors.newFixedThreadPool(10);
+
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        List<String> results = ids.stream()
+          .collect(parallelToList(i -> fetchById(i), executor, 4))
+          .join();
+
+        System.out.println(results); // [user-1, user-2, user-3]
+    }
+
+    @Test
+    public void shouldCollectToCollection() {
+        ExecutorService executor = Executors.newFixedThreadPool(10);
+
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        List<String> results = ids.stream()
+          .collect(parallelToCollection(i -> fetchById(i), LinkedList::new, executor, 4))
+          .join();
+
+        System.out.println(results); // [user-1, user-2, user-3]
+    }
+
+    @Test
+    public void shouldCollectToStream() {
+        ExecutorService executor = Executors.newFixedThreadPool(10);
+
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        Map<Integer, List<String>> results = ids.stream()
+          .collect(parallelToStream(i -> fetchById(i), executor, 4))
+          .thenApply(stream -> stream.collect(Collectors.groupingBy(i -> i.length())))
+          .join();
+
+        System.out.println(results); // [user-1, user-2, user-3]
+    }
+
+    @Test
+    public void shouldStreamInCompletionOrder() {
+        ExecutorService executor = Executors.newFixedThreadPool(10);
+
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        ids.stream()
+          .collect(parallel(i -> fetchByIdWithRandomDelay(i), executor, 4))
+          .forEach(System.out::println);
+    }
+
+    @Test
+    public void shouldStreamInOriginalOrder() {
+        ExecutorService executor = Executors.newFixedThreadPool(10);
+
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        ids.stream()
+          .collect(parallelOrdered(i -> fetchByIdWithRandomDelay(i), executor, 4))
+          .forEach(System.out::println);
+    }
+
+    @Test
+    public void shouldCollectToMap() {
+        ExecutorService executor = Executors.newFixedThreadPool(10);
+
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        Map<Integer, String> results = ids.stream()
+          .collect(parallelToMap(i -> i, i -> fetchById(i), executor, 4))
+          .join();
+
+        System.out.println(results); // {1=user-1, 2=user-2, 3=user-3}
+    }
+
+    @Test
+    public void shouldCollectToTreeMap() {
+        ExecutorService executor = Executors.newFixedThreadPool(10);
+
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        Map<Integer, String> results = ids.stream()
+          .collect(parallelToMap(i -> i, i -> fetchById(i), TreeMap::new,  executor, 4))
+          .join();
+
+        System.out.println(results); // {1=user-1, 2=user-2, 3=user-3}
+    }
+
+    @Test
+    public void shouldCollectToTreeMapAndResolveClashes() {
+        ExecutorService executor = Executors.newFixedThreadPool(10);
+
+        List<Integer> ids = Arrays.asList(1, 2, 3);
+
+        Map<Integer, String> results = ids.stream()
+          .collect(parallelToMap(i -> i, i -> fetchById(i), TreeMap::new, (s1, s2) -> s1,   executor, 4))
+          .join();
+
+        System.out.println(results); // {1=user-1, 2=user-2, 3=user-3}
+    }
+
+    private static String fetchById(int id) {
+        try {
+            Thread.sleep(100);
+        } catch (InterruptedException e) {
+            // ignore shamelessly
+        }
+
+        return "user-" + id;
+    }
+
+    private static String fetchByIdWithRandomDelay(int id) {
+        try {
+            Thread.sleep(ThreadLocalRandom.current().nextInt(1000));
+        } catch (InterruptedException e) {
+            // ignore shamelessly
+        }
+
+        return "user-" + id;
+    }
+}