BAEL-591 : New Stream Collectors in Java 9 (#1046)

* String to char and vice versa initial article code

* Junit test case for String Conversion. Deleting normal class.

* Changes based on review.

* BAEL-591 : New Java9 Collectors code snippet

* Deleting String conversion test

core-java-9/src/test/java/com/baeldung/java9/language/stream/CollectorImprovementTest.java

@@ -0,0 +1,81 @@
+package com.baeldung.java9.language.stream;
+
+import org.junit.Test;
+
+import java.util.*;
+import java.util.stream.Collectors;
+import java.util.function.Function;
+
+import static org.junit.Assert.assertEquals;
+
+public class CollectorImprovementTest {
+    private static class Blog {
+        private String authorName;
+        private List<String> comments;        
+        
+        public Blog(String authorName) {
+            this.authorName = authorName;
+            this.comments = new LinkedList<String>();
+        }
+        
+        public String getAuthorName() {
+            return this.authorName;
+        }
+        
+        public List<String> getComments() {
+            return new LinkedList<String>(this.comments);
+        }
+        
+        public void addComment(String comment) {
+            this.comments.add(comment);
+        }
+    }
+    
+    @Test 
+    public void testFiltering() {
+        List<Integer> numbers = List.of(1, 2, 3, 5, 5);
+ 
+        Map<Integer, Long> result = numbers.stream()
+          .filter(val -> val > 3).collect(Collectors.groupingBy(
+          Function.identity(), Collectors.counting()));
+     
+        assertEquals(1, result.size());
+     
+        result = numbers.stream().collect(Collectors.groupingBy(
+          Function.identity(), Collectors.filtering(val -> val > 3, 
+          Collectors.counting())));
+     
+        assertEquals(4, result.size());
+    }
+    
+    @Test
+    public void testFlatMapping() {
+        Blog blog1 = new CollectorImprovementTest.Blog("1");
+        blog1.addComment("Nice");
+        blog1.addComment("Very Nice");
+        Blog blog2 = new CollectorImprovementTest.Blog("2");
+        blog2.addComment("Disappointing");
+        blog2.addComment("Ok");
+        blog2.addComment("Could be better");
+        List<Blog> blogs = List.of(blog1, blog2);
+             
+        Map<String,  List<List<String>>> authorComments1 = 
+          blogs.stream().collect(Collectors.groupingBy(
+          Blog::getAuthorName, Collectors.mapping(
+          Blog::getComments, Collectors.toList())));
+            
+        assertEquals(2, authorComments1.size());
+        assertEquals(2, authorComments1.get("1").get(0).size());
+        assertEquals(3, authorComments1.get("2").get(0).size());
+     
+        Map<String, List<String>> authorComments2 = 
+          blogs.stream().collect(Collectors.groupingBy(
+          Blog::getAuthorName, Collectors.flatMapping(
+          blog -> blog.getComments().stream(), 
+          Collectors.toList())));
+     
+        assertEquals(2, authorComments2.size());
+        assertEquals(2, authorComments2.get("1").size());
+        assertEquals(3, authorComments2.get("2").size());
+    }
+}