[map-stream-to-map] merge a stream of maps to a single map

core-java-modules/core-java-streams-maps/src/test/java/com/baeldung/streams/mapstreamtomap/MapStreamToMapUnitTest.java

@@ -0,0 +1,153 @@
+package com.baeldung.streams.mapstreamtomap;
+
+import static java.lang.Math.max;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.Test;
+
+import com.google.common.collect.Lists;
+
+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);
+    }};
+
+    static Stream<Map<String, Integer>> getMapStream(Map<String, Integer>... playerMaps) {
+        List<Map<String, Integer>> theList = Lists.newArrayList(playerMaps);
+        Collections.shuffle(theList);
+        return theList.stream();
+    }
+
+    @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 = getMapStream(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, () -> getMapStream(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 = getMapStream(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, () -> getMapStream(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<>();
+        getMapStream(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);
+        }};
+        Stream<Map<String, Integer>> mapStream = getMapStream(playerMap1, playerMap2, playerMap3, playerMap4, playerMap5);
+        Map<String, Integer> mergedMap = mapStream.reduce(new HashMap<>(), (resultMap, theMap) -> {
+            theMap.forEach((k, v) -> {
+                if (resultMap.containsKey(k)) {
+                    resultMap.put(k, maxInteger(resultMap.get(k), v));
+                } else {
+                    resultMap.put(k, v);
+                }
+            });
+            return resultMap;
+        });
+
+        assertEquals(expectedMap, mergedMap);
+
+    }
+}