This PR is for BAEL-6788 (#14563)

This PR aims to add java classes for the HashMap multi values operations with same key

core-java-modules/core-java-string-conversions-3/src/main/java/com/baeldung/hashmapmultivalues/MultiValueHashMap.java

@@ -0,0 +1,29 @@
+package com.baeldung.hashmapmultivalues;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+
+public class MultiValueHashMap<K, V> {
+    private final HashMap<K, ArrayList<V>> map;
+
+    // Constructor
+    public MultiValueHashMap() {
+        map = new HashMap<>();
+    }
+
+    public void put(K key, V value) {
+        map.computeIfAbsent(key, k -> new ArrayList<>()).add(value);
+    }
+
+    public List<V> get(K key) {
+        return map.getOrDefault(key, new ArrayList<>());
+    }
+
+    public void remove(K key, V value) {
+        map.computeIfPresent(key, (k, v) -> {
+            v.remove(value);
+            return v;
+        });
+    }
+}

core-java-modules/core-java-string-conversions-3/src/test/java/com/baeldung/hashmapmultivalues/MultiValueHashMapUnitTest.java

@@ -0,0 +1,53 @@
+package com.baeldung.hashmapmultivalues;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.jupiter.api.Assertions.*;
+
+public class MultiValueHashMapUnitTest {
+    @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));
+    }
+
+}