1+ package com .ak .app ;
2+
3+ import org .junit .jupiter .api .Test ;
4+
5+ import java .util .*;
6+ import java .util .concurrent .ConcurrentHashMap ;
7+ import java .util .concurrent .ConcurrentMap ;
8+ import java .util .function .BiConsumer ;
9+ import java .util .function .Consumer ;
10+ import java .util .function .Function ;
11+ import java .util .stream .Collectors ;
12+ import java .util .stream .Stream ;
13+
14+ import static org .assertj .core .api .Assertions .assertThat ;
15+ import static org .assertj .core .api .Assertions .entry ;
16+
17+ class CollectionsAndCollectorsTest {
18+ record Employee (String firstName , String lastName , String position , int salary ) {
19+ }
20+
21+ private final List <Employee > employees = List .of (
22+ new Employee ("AAA" , "BBB" , "developer" , 10_000 ),
23+ new Employee ("AAB" , "BBC" , "architect" , 15_000 ),
24+ new Employee ("AAC" , "BBD" , "developer" , 13_000 ),
25+ new Employee ("AAD" , "BBE" , "tester" , 7_000 ),
26+ new Employee ("AAE" , "BBF" , "tester" , 9_000 )
27+ );
28+
29+ /**
30+ * <a href='https://piotrminkowski.com/2024/04/25/interesting-facts-about-java-streams-and-collections/'>
31+ * Interesting Facts About Java Streams and Collections
32+ * </a>
33+ */
34+ @ Test
35+ void groupingAndAggregations () {
36+ var m = employees .stream ().collect (
37+ Collectors .groupingBy (
38+ Employee ::position ,
39+ Collectors .summingInt (Employee ::salary )
40+ )
41+ );
42+
43+ assertThat (m ).hasSize (3 )
44+ .contains (entry ("developer" , 23000 ))
45+ .contains (entry ("architect" , 15000 ))
46+ .contains (entry ("tester" , 16000 ));
47+
48+ var p = employees .stream ().collect (Collectors .partitioningBy (e -> e .salary > 10_000 ));
49+ assertThat (p ).hasSize (2 )
50+ .hasEntrySatisfying (Boolean .TRUE , es -> assertThat (es ).hasSize (2 ))
51+ .hasEntrySatisfying (Boolean .FALSE , es -> assertThat (es ).hasSize (3 ));
52+ }
53+
54+ /**
55+ * <p>
56+ * <a href="https://stackabuse.com/guide-to-java-8-collectors-groupingbyconcurrent/">Guide to Java 8 Collectors: groupingByConcurrent()</a>
57+ * </p>
58+ *
59+ * <p>
60+ * <a href="https://habr.com/ru/articles/684912//">Практические примеры использования Stream API</a>
61+ * </p>
62+ */
63+ @ Test
64+ void groupingByConcurrent () {
65+ record Book (String title , String author , int releaseYear ) {
66+ }
67+
68+ String georgeOrwell = "George Orwell" ;
69+ String tolkien = "J.R.R. Tolkien" ;
70+ String williamGolding = "William Golding" ;
71+ List <Book > books = Arrays .asList (
72+ new Book ("The Lord of the Rings" , tolkien , 1954 ),
73+ new Book ("The Hobbit" , tolkien , 1937 ),
74+ new Book ("Animal Farm" , georgeOrwell , 1945 ),
75+ new Book ("Nineteen Eighty-Four" , georgeOrwell , 1949 ),
76+ new Book ("The Road to Wigan Pier" , georgeOrwell , 1937 ),
77+ new Book ("Lord of the Flies" , williamGolding , 1954 )
78+ );
79+
80+ ConcurrentMap <String , List <String >> booksByAuthor = books .parallelStream ()
81+ .collect (
82+ Collectors .groupingByConcurrent (Book ::author , ConcurrentHashMap ::new , Collectors .mapping (Book ::title , Collectors .toList ()))
83+ );
84+ assertThat (booksByAuthor )
85+ .hasEntrySatisfying (georgeOrwell , strings -> assertThat (strings ).hasSize (3 ))
86+ .hasEntrySatisfying (tolkien , strings -> assertThat (strings ).hasSize (2 ))
87+ .hasEntrySatisfying (williamGolding , strings -> assertThat (strings ).hasSize (1 ));
88+ }
89+
90+ /**
91+ * <a href="https://stackoverflow.com/questions/19031213/java-get-most-common-element-in-a-list">Java-get most common element in a list</a>
92+ */
93+ @ Test
94+ void mostCommonElement () {
95+ var v = Stream .of (1 , 3 , 4 , 3 , 2 , 3 , 3 , 3 , 3 , 4 )
96+ .collect (Collectors .groupingBy (Function .identity (), Collectors .counting ()))
97+ .entrySet ().stream ().max (Map .Entry .comparingByValue ()).orElseThrow ();
98+ assertThat (v .getKey ()).isEqualTo (3 );
99+ assertThat (v .getValue ()).isEqualTo (6 );
100+ }
101+
102+ @ Test
103+ void maxElementKey () {
104+ var map = Map .of ("1" , 1 , "2" , 2 , "3" , 3 );
105+ String maxKey = Collections .max (map .entrySet (), Map .Entry .comparingByValue ()).getKey ();
106+ assertThat (maxKey ).isEqualTo ("3" );
107+ }
108+
109+ /**
110+ * <a href="https://blogs.oracle.com/javamagazine/post/the-hidden-gems-in-java-16-and-java-17-from-streammapmulti-to-hexformat">Hidden gems in Java 16 and Java 17, from Stream.mapMulti to HexFormat</a>
111+ */
112+ @ Test
113+ void mapMulti () {
114+ List <String > strings = Stream .of ("Java" , "Valhalla" , "Panama" , "Loom" , "Amber" )
115+ .mapMulti ((BiConsumer <String , Consumer <String >>) (s , mapper ) -> {
116+ if (s .length () >= 5 ) {
117+ mapper .accept (s );
118+ }
119+ })
120+ .toList ();
121+ assertThat (strings ).containsExactly ("Valhalla" , "Panama" , "Amber" );
122+ }
123+
124+ /**
125+ * <a href='https://piotrminkowski.com/2024/04/25/interesting-facts-about-java-streams-and-collections/'>
126+ * Interesting Facts About Java Streams and Collections
127+ * </a>
128+ */
129+ @ Test
130+ void mapMerge () {
131+ var map = new HashMap <Integer , Integer >();
132+ var nums = List .of (2 , 3 , 4 , 2 , 3 , 5 , 1 , 3 , 4 , 4 );
133+ nums .forEach (num -> map .merge (num , 1 , Integer ::sum ));
134+ assertThat (map ).hasSize (5 ).contains (entry (4 , 3 ));
135+ }
136+
137+ @ Test
138+ void mapMerge2 () {
139+ var map = new HashMap <String , Integer >();
140+ employees .forEach (emp -> map .merge (emp .position (), emp .salary (), Integer ::sum ));
141+ assertThat (map ).hasSize (3 ).contains (entry ("developer" , 23000 ));
142+ }
143+ }
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