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

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Kai.Yuan 2024-03-23 17:23:38 +01:00
parent 317496fa87
commit 47e4316d01
4 changed files with 158 additions and 0 deletions
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1
core-java-modules/core-java-collections-maps-8/README.md Normal file
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## Relevant Articles:

29
core-java-modules/core-java-collections-maps-8/pom.xml Normal file
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<?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>

127
core-java-modules/core-java-collections-maps-8/src/test/java/com/baeldung/map/valuetokeyset/ConvertMapKeyValueToMapValueKeySetUnitTest.java Normal file
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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);
}
}

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core-java-modules/pom.xml
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<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>