Bael 5066 aggregate over multiple fields (#11156)

* Commit source code to branch

* BAEL-5065 improvement of groupBy with complex key

* Aggregation of multiple attributes of a grouped result

core-java-modules/core-java-16/src/main/java/com/baeldung/java_16_features/groupingby/BlogPost.java

@@ -1,5 +1,8 @@
 package com.baeldung.java_16_features.groupingby;
 
+import java.util.IntSummaryStatistics;
+
+
 public class BlogPost {
     
     private String title;
@@ -7,6 +10,8 @@ public class BlogPost {
     private BlogPostType type;
     private int likes;
     record AuthPostTypesLikes(String author, BlogPostType type, int likes) {};
+    record PostcountTitlesLikesStats(long postCount, String titles, IntSummaryStatistics likesStats){};
+    record TitlesBoundedSumOfLikes(String titles, int boundedSumOfLikes) {};
     
     public BlogPost(String title, String author, BlogPostType type, int likes) {
         this.title = title;

core-java-modules/core-java-16/src/test/java/com/baeldung/java_16_features/groupingby/JavaGroupingByCollectorUnitTest.java

@@ -5,6 +5,8 @@ import static java.util.stream.Collectors.averagingInt;
 import static java.util.stream.Collectors.counting;
 import static java.util.stream.Collectors.groupingBy;
 import static java.util.stream.Collectors.groupingByConcurrent;
+import static java.util.stream.Collectors.collectingAndThen;
+import static java.util.stream.Collectors.toMap;
 import static java.util.stream.Collectors.joining;
 import static java.util.stream.Collectors.mapping;
 import static java.util.stream.Collectors.maxBy;
@@ -13,6 +15,7 @@ import static java.util.stream.Collectors.summingInt;
 import static java.util.stream.Collectors.toList;
 import static java.util.stream.Collectors.toSet;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.offset;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -203,7 +206,7 @@ public class JavaGroupingByCollectorUnitTest {
     public void givenAListOfPosts_whenGroupedByComplexMapPairKeyType_thenGetAMapBetweenPairAndList() {
 
         Map<Pair<BlogPostType, String>, List<BlogPost>> postsPerTypeAndAuthor = posts.stream()
-          .collect(groupingBy(post -> new ImmutablePair<>(post.getType(), post.getAuthor())));
+            .collect(groupingBy(post -> new ImmutablePair<>(post.getType(), post.getAuthor())));
 
         List<BlogPost> result = postsPerTypeAndAuthor.get(new ImmutablePair<>(BlogPostType.GUIDE, "Author 1"));
 
@@ -237,7 +240,7 @@ public class JavaGroupingByCollectorUnitTest {
     public void givenAListOfPosts_whenGroupedByRecord_thenGetAMapBetweenRecordAndList() {
 
         Map<BlogPost.AuthPostTypesLikes, List<BlogPost>> postsPerTypeAndAuthor = posts.stream()
-          .collect(groupingBy(post -> new BlogPost.AuthPostTypesLikes(post.getAuthor(), post.getType(), post.getLikes())));
+            .collect(groupingBy(post -> new BlogPost.AuthPostTypesLikes(post.getAuthor(), post.getType(), post.getLikes())));
 
         List<BlogPost> result = postsPerTypeAndAuthor.get(new BlogPost.AuthPostTypesLikes("Author 1", BlogPostType.GUIDE, 20));
 
@@ -251,4 +254,49 @@ public class JavaGroupingByCollectorUnitTest {
         assertThat(blogPost.getLikes()).isEqualTo(20);
     }
 
+    @Test
+    public void givenListOfPosts_whenGroupedByAuthor_thenGetAMapUsingCollectingAndThen() {
+
+        Map<String, BlogPost.PostcountTitlesLikesStats> postsPerAuthor = posts.stream()
+            .collect(groupingBy(BlogPost::getAuthor, collectingAndThen(toList(), list -> {
+                long count = list.stream()
+                    .map(BlogPost::getTitle)
+                    .collect(counting());
+                String titles = list.stream()
+                    .map(BlogPost::getTitle)
+                    .collect(joining(" : "));
+                IntSummaryStatistics summary = list.stream()
+                    .collect(summarizingInt(BlogPost::getLikes));
+                return new BlogPost.PostcountTitlesLikesStats(count, titles, summary);
+            })));
+
+        BlogPost.PostcountTitlesLikesStats result = postsPerAuthor.get("Author 1");
+        assertThat(result.postCount()).isEqualTo(3L);
+        assertThat(result.titles()).isEqualTo("News item 1 : Programming guide : Tech review 2");
+        assertThat(result.likesStats().getMax()).isEqualTo(20);
+        assertThat(result.likesStats().getMin()).isEqualTo(15);
+        assertThat(result.likesStats().getAverage()).isEqualTo(16.666d, offset(0.001d));
+    }
+
+    @Test
+    public void givenListOfPosts_whenGroupedByAuthor_thenGetAMapUsingToMap() {
+
+        int maxValLikes = 17;
+        Map<String, BlogPost.TitlesBoundedSumOfLikes> postsPerAuthor = posts.stream()
+            .collect(toMap(BlogPost::getAuthor, post -> {
+                int likes = (post.getLikes() > maxValLikes) ? maxValLikes : post.getLikes();
+                return new BlogPost.TitlesBoundedSumOfLikes(post.getTitle(), likes);
+            }, (u1, u2) -> {
+                int likes = (u2.boundedSumOfLikes() > maxValLikes) ? maxValLikes : u2.boundedSumOfLikes();
+                return new BlogPost.TitlesBoundedSumOfLikes(u1.titles()
+                    .toUpperCase() + " : "
+                    + u2.titles()
+                        .toUpperCase(),
+                    u1.boundedSumOfLikes() + likes);
+            }));
+        
+        BlogPost.TitlesBoundedSumOfLikes result = postsPerAuthor.get("Author 1");
+        assertThat(result.titles()).isEqualTo("NEWS ITEM 1 : PROGRAMMING GUIDE : TECH REVIEW 2");
+        assertThat(result.boundedSumOfLikes()).isEqualTo(47);
+    }
 }