Remove Duplicate Values From HashMap in Java (#14519)

* Remove Duplicate Values From HashMap in Java

* fix the typo in the package name

core-java-modules/core-java-collections-maps-6/src/test/java/com/baeldung/map/removeduplicate/RemoveDuplicateValuesUnitTest.java

@@ -0,0 +1,83 @@
+package com.baeldung.map.removeuplicate;
+
+import static java.util.stream.Collectors.toMap;
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+import com.google.common.collect.ImmutableMap;
+
+public class RemoveDuplicateValuesUnitTest {
+    private Map<String, String> initDevMap() {
+        Map<String, String> devMap = new HashMap<>();
+        devMap.put("Tom", "Linux");
+        devMap.put("Kent", "Linux");
+
+        devMap.put("Bob", "MacOS");
+        devMap.put("Eric", "MacOS");
+
+        devMap.put("Peter", "Windows");
+        devMap.put("Saajan", "Windows");
+        devMap.put("Jan", "Windows");
+
+        devMap.put("Kevin", "FreeBSD");
+        return devMap;
+    }
+
+    @Test
+    void whenUsingReverseMap_thenDuplicateValuesAreRemoved() {
+        Map<String, String> devMap = initDevMap();
+        Map<String, String> tmpReverseMap = new HashMap<>();
+        Map<String, String> result = new HashMap<>();
+        for (String name : devMap.keySet()) {
+            tmpReverseMap.put(devMap.get(name), name);
+        }
+
+        for (String os : tmpReverseMap.keySet()) {
+            result.put(tmpReverseMap.get(os), os);
+        }
+
+        assertThat(result.values()).hasSize(4)
+          .containsExactlyInAnyOrder("Windows", "MacOS", "Linux", "FreeBSD");
+    }
+
+    @Test
+    void whenUsingStream_thenDuplicateValuesAreRemoved() {
+        Map<String, String> devMap = initDevMap();
+        Map<String, String> result = devMap.entrySet()
+          .stream()
+          .collect(toMap(Map.Entry::getValue, Map.Entry::getKey, (keyInMap, keyNew) -> keyInMap))
+          .entrySet()
+          .stream()
+          .collect(toMap(Map.Entry::getValue, Map.Entry::getKey));
+
+        assertThat(result.values()).hasSize(4)
+          .containsExactlyInAnyOrder("Windows", "MacOS", "Linux", "FreeBSD");
+    }
+
+    @Test
+    void whenUsingStream_thenDuplicateValuesAreRemovedAndOnlyLongestNamesExist() {
+        Map<String, String> devMap = initDevMap();
+
+        //@formatter:off
+        Map<String, String> expected = ImmutableMap.of(
+          "Eric", "MacOS",
+          "Kent", "Linux",
+          "Saajan", "Windows",
+          "Kevin", "FreeBSD");
+        //@formatter:on
+
+        Map<String, String> result = devMap.entrySet()
+          .stream()
+          .collect(toMap(Map.Entry::getValue, Map.Entry::getKey, (k1, k2) -> k1.length() > k2.length() ? k1 : k2))
+          .entrySet()
+          .stream()
+          .collect(toMap(Map.Entry::getValue, Map.Entry::getKey));
+
+        assertThat(result).hasSize(4)
+          .isEqualTo(expected);
+    }
+}