Merge pull request #16209 from sk1418/map-to-value->Set-of-keys

[map-to-value->Set-of-keys] Map<K,V> -> Map<V,Set<K>>

core-java-modules/core-java-collections-maps-8/README.md

@@ -0,0 +1 @@
+## Relevant Articles:

core-java-modules/core-java-collections-maps-8/pom.xml

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-collections-maps-8</artifactId>
+    <name>core-java-collections-maps-8</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <artifactId>core-java-modules</artifactId>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <configuration>
+                    <argLine>
+                        --add-opens java.base/java.util=ALL-UNNAMED
+                    </argLine>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+</project>

core-java-modules/core-java-collections-maps-8/src/test/java/com/baeldung/map/valuetokeyset/ConvertMapKeyValueToMapValueKeySetUnitTest.java

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+package com.baeldung.map.valuetokeyset;
+
+import static java.util.stream.Collectors.collectingAndThen;
+import static java.util.stream.Collectors.groupingBy;
+import static java.util.stream.Collectors.mapping;
+import static java.util.stream.Collectors.toSet;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+import org.junit.jupiter.api.Test;
+
+import com.google.common.collect.HashMultimap;
+import com.google.common.collect.Multimaps;
+import com.google.common.collect.SetMultimap;
+
+public class ConvertMapKeyValueToMapValueKeySetUnitTest {
+
+    private static final Map<String, String> INPUT_MAP = Map.of(
+        // @formatter:off
+        "Kai", "Linux",
+        "Eric", "MacOS",
+        "Kevin", "Windows",
+        "Liam", "MacOS",
+        "David", "Linux",
+        "Saajan", "Windows",
+        "Loredana", "MacOS"
+        // @formatter:on
+    );
+
+    private static final Map<String, Set<String>> EXPECTED = Map.of(
+        // @formatter:off
+        "Linux", Set.of("Kai", "David"),
+        "Windows", Set.of("Saajan", "Kevin"),
+        "MacOS", Set.of("Eric", "Liam", "Loredana")
+        // @formatter:on
+    );
+
+    private static final Map<String, String> INPUT_MAP_WITH_NULLS = new HashMap<String, String>(INPUT_MAP) {{
+        put("Tom", null);
+        put("Jerry", null);
+        put(null, null);
+    }};
+
+    private static final Map<String, Set<String>> EXPECTED_WITH_NULLS = new HashMap<String, Set<String>>(EXPECTED) {{
+        put(null, new HashSet<String>() {{
+            add("Tom");
+            add("Jerry");
+            add(null);
+        }});
+    }};
+
+    public static <K, V> Map<V, Set<K>> transformMap(Map<K, V> input) {
+        Map<V, Set<K>> resultMap = new HashMap<>();
+        for (Map.Entry<K, V> entry : input.entrySet()) {
+            if (!resultMap.containsKey(entry.getValue())) {
+                resultMap.put(entry.getValue(), new HashSet<>());
+            }
+            resultMap.get(entry.getValue())
+                .add(entry.getKey());
+        }
+        return resultMap;
+    }
+
+    @Test
+    void whenUsingClassicLoopBasedSolution_thenGetExpectedResult() {
+        Map<String, Set<String>> result = transformMap(INPUT_MAP);
+        assertEquals(EXPECTED, result);
+
+        Map<String, Set<String>> result2 = transformMap(INPUT_MAP_WITH_NULLS);
+        assertEquals(EXPECTED_WITH_NULLS, result2);
+    }
+
+    @Test
+    void whenUsingJava8StreamGroupingBy_thenGetExpectedResult() {
+        Map<String, Set<String>> result = INPUT_MAP.entrySet()
+            .stream()
+            .collect(groupingBy(Map.Entry::getValue, mapping(Map.Entry::getKey, toSet())));
+        assertEquals(EXPECTED, result);
+
+        assertThrows(NullPointerException.class, () -> INPUT_MAP_WITH_NULLS.entrySet()
+            .stream()
+            .collect(groupingBy(Map.Entry::getValue, mapping(Map.Entry::getKey, toSet()))));
+    }
+
+    @Test
+    void whenUsingJava8ForEach_thenGetExpectedResult() {
+        Map<String, Set<String>> result = new HashMap<>();
+        INPUT_MAP.forEach((key, value) -> result.computeIfAbsent(value, k -> new HashSet<>())
+            .add(key));
+        assertEquals(EXPECTED, result);
+
+        Map<String, Set<String>> result2 = new HashMap<>();
+        INPUT_MAP_WITH_NULLS.forEach((key, value) -> result2.computeIfAbsent(value, k -> new HashSet<>())
+            .add(key));
+        assertEquals(EXPECTED_WITH_NULLS, result2);
+    }
+
+    @Test
+    void whenUsingGuavaMultiMapCollector_thenGetExpectedResult() {
+        Map<String, Set<String>> result = INPUT_MAP.entrySet()
+            .stream()
+            .collect(collectingAndThen(Multimaps.toMultimap(Map.Entry::getValue, Map.Entry::getKey, HashMultimap::create), Multimaps::asMap));
+        assertEquals(EXPECTED, result);
+
+        Map<String, Set<String>> result2 = INPUT_MAP_WITH_NULLS.entrySet()
+            .stream()
+            .collect(collectingAndThen(Multimaps.toMultimap(Map.Entry::getValue, Map.Entry::getKey, HashMultimap::create), Multimaps::asMap));
+        assertEquals(EXPECTED_WITH_NULLS, result2);
+    }
+
+    @Test
+    void whenUsingGuavaInvertFromAndForMap_thenGetExpectedResult() {
+        SetMultimap<String, String> multiMap = Multimaps.invertFrom(Multimaps.forMap(INPUT_MAP), HashMultimap.create());
+        Map<String, Set<String>> result = Multimaps.asMap(multiMap);
+        assertEquals(EXPECTED, result);
+
+        SetMultimap<String, String> multiMapWithNulls = Multimaps.invertFrom(Multimaps.forMap(INPUT_MAP_WITH_NULLS), HashMultimap.create());
+        Map<String, Set<String>> result2 = Multimaps.asMap(multiMapWithNulls);
+        assertEquals(EXPECTED_WITH_NULLS, result2);
+    }
+
+}

core-java-modules/pom.xml

@@ -99,6 +99,7 @@
         <module>core-java-collections-maps-2</module>
         <module>core-java-collections-maps-3</module>
         <module>core-java-collections-maps-7</module>
+        <module>core-java-collections-maps-8</module>
         <module>core-java-compiler</module>
         <module>core-java-concurrency-2</module>
         <module>core-java-concurrency-advanced</module>