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Merge pull request #16165 from sk1418/map-stream-to-map

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[map-stream-to-map] merge a stream of maps to a single map
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Liam Williams 2024-03-26 13:28:48 +00:00 committed by GitHub
commit e1e94a088a
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core-java-modules/core-java-streams-maps/src/test/java/com/baeldung/streams/mapstreamtomap/MapStreamToMapUnitTest.java Normal file
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package com.baeldung.streams.mapstreamtomap;
import static java.lang.Math.max;
import static java.util.stream.Collectors.flatMapping;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.reducing;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import org.junit.jupiter.api.Test;
public class MapStreamToMapUnitTest {
Map<String, Integer> playerMap1 = new HashMap<String, Integer>() {{
put("Kai", 92);
put("Liam", 100);
}};
Map<String, Integer> playerMap2 = new HashMap<String, Integer>() {{
put("Eric", 42);
put("Kevin", 77);
}};
Map<String, Integer> playerMap3 = new HashMap<String, Integer>() {{
put("Saajan", 35);
}};
Map<String, Integer> playerMap4 = new HashMap<String, Integer>() {{
put("Kai", 76);
}};
Map<String, Integer> playerMap5 = new HashMap<String, Integer>() {{
put("Kai", null);
put("Jerry", null);
}};
@Test
void givenMapsStream_whenUsingFlatMapAndToMap_thenMultipleMapsMergedIntoOneMap() {
Map<String, Integer> expectedMap = new HashMap<String, Integer>() {{
put("Saajan", 35);
put("Liam", 100);
put("Kai", 92);
put("Eric", 42);
put("Kevin", 77);
}};
Map<String, Integer> mergedMap = Stream.of(playerMap1, playerMap2, playerMap3)
.flatMap(map -> map.entrySet()
.stream())
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
assertEquals(expectedMap, mergedMap);
}
@Test
void givenMapsWithDuplicateKeys_whenUsingFlatMapAndToMap_thenMultipleMapsMergedIntoOneMap() {
Map<String, Integer> expectedMap = new HashMap<String, Integer>() {{
put("Saajan", 35);
put("Liam", 100);
put("Kai", 92); // max of 76 and 92
put("Eric", 42);
put("Kevin", 77);
}};
assertThrows(IllegalStateException.class, () -> Stream.of(playerMap1, playerMap2, playerMap3, playerMap4)
.flatMap(map -> map.entrySet()
.stream())
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)), "Duplicate key Kai (attempted merging values 92 and 76)");
Map<String, Integer> mergedMap = Stream.of(playerMap1, playerMap2, playerMap3, playerMap4)
.flatMap(map -> map.entrySet()
.stream())
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, Integer::max));
assertEquals(expectedMap, mergedMap);
}
private Integer maxInteger(Integer int1, Integer int2) {
if (int1 == null) {
return int2;
}
if (int2 == null) {
return int1;
}
return max(int1, int2);
}
@Test
void givenMapsWithDuplicateKeysAndNullValues_whenUsingFlatMapWithForEach_thenMultipleMapsMergedIntoOneMap() {
Map<String, Integer> expectedMap = new HashMap<String, Integer>() {{
put("Saajan", 35);
put("Liam", 100);
put("Kai", 92); // max of 92, 76, and null
put("Eric", 42);
put("Kevin", 77);
put("Jerry", null);
}};
assertThrows(NullPointerException.class, () -> Stream.of(playerMap1, playerMap2, playerMap3, playerMap4, playerMap5)
.flatMap(map -> map.entrySet()
.stream())
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, Integer::max)));
Map<String, Integer> mergedMap = new HashMap<>();
Stream.of(playerMap1, playerMap2, playerMap3, playerMap4, playerMap5)
.flatMap(map -> map.entrySet()
.stream())
.forEach(entry -> {
String k = entry.getKey();
Integer v = entry.getValue();
if (mergedMap.containsKey(k)) {
mergedMap.put(k, maxInteger(mergedMap.get(k), v));
} else {
mergedMap.put(k, v);
}
});
assertEquals(expectedMap, mergedMap);
}
@Test
void givenMapsWithDuplicateKeysAndNullValues_whenUsingReduce_thenMultipleMapsMergedIntoOneMap() {
Map<String, Integer> expectedMap = new HashMap<String, Integer>() {{
put("Saajan", 35);
put("Liam", 100);
put("Kai", 92); // max of 92, 76, and null
put("Eric", 42);
put("Kevin", 77);
put("Jerry", null);
}};
Map<String, Integer> mergedMap = Stream.of(playerMap1, playerMap2, playerMap3, playerMap4, playerMap5)
.flatMap(x -> x.entrySet()
.stream())
.collect(groupingBy(Map.Entry::getKey, mapping(Map.Entry::getValue, reducing(null, this::maxInteger))));
assertEquals(expectedMap, mergedMap);
}
}