Java 8 grouping by collector pull request (#1102)

* Char array to string and string to char array test cases added

* Minor code renames

* Added groupingBy collector unit tests

* Added test case for int summary calculation on grouped results

* Added the grouping by classes to the main source path

* Reverting char array to string test class

* Reverting char array to string test class

* Reverting char array to string test class

* Reverting char array to string test class

core-java/src/main/java/com/baeldung/java_8_features/groupingby/BlogPost.java

@@ -0,0 +1,36 @@
+package com.baeldung.java_8_features.groupingby;
+
+public class BlogPost {
+    private String title;
+    private String author;
+    private BlogPostType type;
+    private int likes;
+
+    public BlogPost(String title, String author, BlogPostType type, int likes) {
+        this.title = title;
+        this.author = author;
+        this.type = type;
+        this.likes = likes;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public String getAuthor() {
+        return author;
+    }
+
+    public BlogPostType getType() {
+        return type;
+    }
+
+    public int getLikes() {
+        return likes;
+    }
+
+    @Override
+    public String toString() {
+        return "BlogPost{" + "title='" + title + '\'' + ", type=" + type + ", likes=" + likes + '}';
+    }
+}

core-java/src/main/java/com/baeldung/java_8_features/groupingby/BlogPostType.java

@@ -0,0 +1,5 @@
+package com.baeldung.java_8_features.groupingby;
+
+public enum BlogPostType {
+    NEWS, REVIEW, GUIDE
+}