This commit is related to BAEL-7161 (#15132)

This commit aims to add a test class "StreamToMapAndMultiMapUnitTest" that provides several approaches to convert Java stream to Map or MultiMap.

core-java-modules/core-java-streams-5/src/test/java/com/baeldung/streamtomapandmultimap/StreamToMapAndMultiMapUnitTest.java

@@ -0,0 +1,100 @@
+package com.baeldung.streamtomapandmultimap;
+
+import com.google.common.collect.LinkedHashMultimap;
+import org.junit.Test;
+
+import java.util.*;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.junit.Assert.assertEquals;
+
+public class StreamToMapAndMultiMapUnitTest {
+    @Test
+    public void givenStringStream_whenConvertingToMapWithMerge_thenExpectedMapIsGenerated() {
+        Stream<String> stringStream = Stream.of("one", "two", "three", "two");
+
+        Map<String, String> mergedMap = stringStream.collect(
+                Collectors.toMap(s -> s, s -> s, (s1, s2) -> s1 + ", " + s2)
+        );
+
+        // Define the expected map
+        Map<String, String> expectedMap = Map.of(
+                "one", "one",
+                "two", "two, two",
+                "three", "three"
+        );
+
+        assertEquals(expectedMap, mergedMap);
+    }
+
+    @Test
+    public void givenStringStream_whenConvertingToMultimap_thenExpectedMultimapIsGenerated() {
+        Stream<String> stringStream = Stream.of("one", "two", "three", "two");
+
+        LinkedHashMultimap<Object, Object> multimap = LinkedHashMultimap.create();
+
+        stringStream.collect(Collectors.groupingBy(
+                s -> s,
+                Collectors.mapping(s -> s, Collectors.toList())
+        )).forEach((key, value) -> multimap.putAll(key, value));
+
+        LinkedHashMultimap<Object, Object> expectedMultimap = LinkedHashMultimap.create();
+        expectedMultimap.put("one", "one");
+        expectedMultimap.put("two", "two");
+        expectedMultimap.put("three", "three");
+
+        assertEquals(expectedMultimap, multimap);
+    }
+
+    @Test
+    public void givenStringStream_whenConvertingToMultimapWithStreamReduce_thenExpectedMultimapIsGenerated() {
+        Stream<String> stringStream = Stream.of("one", "two", "three", "two");
+
+        Map<String, List<String>> multimap = stringStream.reduce(
+                new HashMap<>(),
+                (map, element) -> {
+                    map.computeIfAbsent(element, k -> new ArrayList<>()).add(element);
+                    return map;
+                },
+                (map1, map2) -> {
+                    map2.forEach((key, value) -> map1.merge(key, value, (list1, list2) -> {
+                        list1.addAll(list2);
+                        return list1;
+                    }));
+                    return map1;
+                }
+        );
+
+        Map<String, List<String>> expectedMultimap = new HashMap<>();
+        expectedMultimap.put("one", Collections.singletonList("one"));
+        expectedMultimap.put("two", Arrays.asList("two", "two"));
+        expectedMultimap.put("three", Collections.singletonList("three"));
+
+        assertEquals(expectedMultimap, multimap);
+    }
+
+    @Test
+    public void givenStringStream_whenConvertingToMapWithStreamReduce_thenExpectedMapIsGenerated() {
+        Stream<String> stringStream = Stream.of("one", "two", "three", "two");
+
+        Map<String, String> resultMap = stringStream.reduce(
+                new HashMap<>(),
+                (map, element) -> {
+                    map.put(element, element);
+                    return map;
+                },
+                (map1, map2) -> {
+                    map1.putAll(map2);
+                    return map1;
+                }
+        );
+
+        Map<String, String> expectedMap = new HashMap<>();
+        expectedMap.put("one", "one");
+        expectedMap.put("two", "two");
+        expectedMap.put("three", "three");
+
+        assertEquals(expectedMap, resultMap);
+    }
+}