[JAVA-6173] Difference Between parallelStream() and stream().parallel() (#13653)

* [JAVA-6173] stream.parallel and parallelStream

* [JAVA-6173] stream.parallel and parallelStream test cases

---------

Co-authored-by: Bhaskar <bhaskar.dastidar@freshworks.com>

core-java-modules/core-java-streams-4/src/test/java/parallelstream/ParallelStreamUnitTest.java

@@ -0,0 +1,47 @@
+package parallelstream;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import com.baeldung.streams.parallelstream.Book;
+import com.baeldung.streams.parallelstream.MyBookContainer;
+import com.baeldung.streams.parallelstream.ParallelStreamApplication;
+
+public class ParallelStreamUnitTest {
+
+    @Test
+    public void givenCollectionWhenCollectionsParallelIsUsedThenReturnCount() {
+        ParallelStreamApplication parallelStreamApplication = new ParallelStreamApplication();
+        Assert.assertEquals(parallelStreamApplication.usingCollectionsParallel(generateListOfBooks(), 1974), 2);
+    }
+
+    @Test
+    public void givenCollectionWhenStreamParallelIsUsedThenReturnCount() {
+        ParallelStreamApplication parallelStreamApplication = new ParallelStreamApplication();
+        Assert.assertEquals(parallelStreamApplication.usingStreamParallel(generateListOfBooks(), 1974), 2);
+    }
+
+    @Test
+    public void givenBookContainerWhenParallelStreamIsUsedThenReturnIncorrectCount() {
+        ParallelStreamApplication parallelStreamApplication = new ParallelStreamApplication();
+        Assert.assertNotEquals(parallelStreamApplication.usingWithCustomSpliterator(getBookContainer(), 1974), 2);
+    }
+
+    private List<Book> generateListOfBooks() {
+        Book book1 = new Book("The Blue Umbrella", "Ruskin Bond", 1974);
+        Book book2 = new Book("Carrie", "Stephen King", 1974);
+        Book book3 = new Book("The Psychology of money", "Morgan Housel", 2020);
+        List<Book> books = List.of(book1, book2, book3);
+        return books;
+    }
+
+    private MyBookContainer<Book> getBookContainer() {
+        MyBookContainer<Book> listOfBooks = new MyBookContainer<>(new Book[] { new Book("The Blue Umbrella", "Ruskin Bond", 1974),
+          new Book("Carrie", "Stephen King", 1974),
+          new Book("The Psychology of money", "Morgan Housel", 2020)});
+        return listOfBooks;
+    }
+}