This commit is for the article BAEL-6788 (#14513)

This commit aims to add a test class (MultiValueHashMapTest) to test the HashMap that allows multiple values to be associated with the same key

core-java-modules/core-java-string-conversions-3/src/java.com.baeldung.multivluehashmap/MultiValueHashMapTest.java

@@ -0,0 +1,79 @@
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class MultiValueHashMapTest {
+    @Test
+    public void given_MultiValueHashMap_whenPuttingAndGettingSingleValue_thenValueIsRetrieved() {
+        MultiValueHashMap<String, Integer> map = new MultiValueHashMap<>();
+        map.put("key1", 10);
+        assertEquals(List.of(10), map.get("key1"));
+    }
+
+    @Test
+    public void given_MultiValueHashMap_whenPuttingAndGettingMultipleValues_thenAllValuesAreRetrieved() {
+        MultiValueHashMap<String, String> map = new MultiValueHashMap<>();
+        map.put("key2", "value1");
+        map.put("key2", "value2");
+        map.put("key2", "value3");
+
+        assertEquals(List.of("value1", "value2", "value3"), map.get("key2"));
+    }
+
+    @Test
+    public void given_MultiValueHashMap_whenGettingNonExistentKey_thenEmptyListIsReturned() {
+        MultiValueHashMap<String, Double> map = new MultiValueHashMap<>();
+        assertTrue(map.get("nonexistent").isEmpty());
+    }
+
+    @Test
+    public void given_MultiValueHashMap_whenRemovingValue_thenValueIsSuccessfullyRemoved() {
+        MultiValueHashMap<Integer, String> map = new MultiValueHashMap<>();
+        map.put(1, "one");
+        map.put(1, "uno");
+        map.put(1, "eins");
+
+        map.remove(1, "uno");
+        assertEquals(List.of("one", "eins"), map.get(1));
+    }
+
+    @Test
+    public void testRemoveNonExistentValue() {
+        MultiValueHashMap<Integer, String> map = new MultiValueHashMap<>();
+        map.put(1, "one");
+        map.remove(1, "nonexistent");
+        assertEquals(List.of("one"), map.get(1));
+    }
+
+    public class MultiValueHashMap<K, V> {
+        private HashMap<K, ArrayList<V>> map;
+
+        // Constructor
+        public MultiValueHashMap() {
+            map = new HashMap<>();
+        }
+
+        public void put(K key, V value) {
+            if (!map.containsKey(key)) {
+                map.put(key, new ArrayList<>());
+            }
+            map.get(key).add(value);
+        }
+
+        public List<V> get(K key) {
+            return map.getOrDefault(key, new ArrayList<>());
+        }
+
+        public void remove(K key, V value) {
+            if (map.containsKey(key)) {
+                map.get(key).remove(value);
+            }
+        }
+
+
+    }
+}