[BAEL-9414] Add Values to Array List Used as Value in HashMap in Java

core-java-modules/core-java-collections-maps-9/pom.xml

@@ -39,12 +39,18 @@
             <artifactId>fastutil</artifactId>
             <version>${fastutil.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-core</artifactId>
+            <version>${spring-core.version}</version>
+        </dependency>
     </dependencies>
 
     <properties>
         <eclipse-collections.version>8.2.0</eclipse-collections.version>
         <hppc.version>0.7.2</hppc.version>
         <fastutil.version>8.1.0</fastutil.version>
+        <spring-core.version>6.1.2</spring-core.version>
     </properties>
 
 </project>

core-java-modules/core-java-collections-maps-9/src/main/java/com/baeldung/map/arraylistinhashmap/ArrayListInHashMap.java

@@ -0,0 +1,45 @@
+package com.baeldung.map.arraylistinhashmap;
+
+import java.util.HashMap;
+import java.util.ArrayList;
+import java.util.Map;
+import java.util.List;
+import java.util.Arrays;
+import org.apache.commons.collections4.MultiValuedMap;
+import org.apache.commons.collections4.multimap.ArrayListValuedHashMap;
+import org.springframework.util.MultiValueMap;
+import org.springframework.util.LinkedMultiValueMap;
+import com.google.common.collect.Multimap;
+import com.google.common.collect.ArrayListMultimap;
+
+public class ArrayListInHashMap {
+
+    public static Map<String, List<String>> addKeyManually(Map<String, List<String>> map, String key, String value) {
+        if (!map.containsKey(key)) {
+            map.put(key, new ArrayList<String>());
+        }
+        map.get(key).add(value);
+        return map;
+    }
+
+    public static Map<String, List<String>> addKeyWithComputeIfAbsent(Map<String, List<String>> map, String key, String value) {
+        map.computeIfAbsent(key, k -> new ArrayList<String>()).add(value);
+        return map;
+    }
+
+    public static MultiValuedMap<String, String> addKeyToApacheMultiValuedMap(MultiValuedMap<String, String> map, String key, String value) {
+        map.put(key, value);
+        return map;
+    }
+
+    public static MultiValueMap<String, String> addKeyToSpringLinkedMultiValueMap(MultiValueMap<String, String> map, String key, String value) {
+        map.add(key, value);
+        return map;
+    }
+
+    public static Multimap<String, String> addKeyToGuavaMultimap(Multimap<String, String> map, String key, String value) {
+        map.put(key, value);
+        return map;
+    }
+
+}

core-java-modules/core-java-collections-maps-9/src/test/java/com/baeldung/map/arraylistinhashmap/ArrayListInHashMapUnitTest.java

@@ -0,0 +1,78 @@
+package com.baeldung.map.arraylistinhashmap;
+
+import java.util.HashMap;
+import java.util.ArrayList;
+import java.util.Map;
+import java.util.List;
+import java.util.Arrays;
+import org.apache.commons.collections4.MultiValuedMap;
+import org.apache.commons.collections4.multimap.ArrayListValuedHashMap;
+import org.springframework.util.MultiValueMap;
+import org.springframework.util.LinkedMultiValueMap;
+import com.google.common.collect.Multimap;
+import com.google.common.collect.ArrayListMultimap;
+
+import org.junit.Test;
+import java.util.Collection;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+class ArrayListInHashMapUnitTest {
+
+    private static final String K1 = "key1";
+    private static final String K2 = "key2";
+    private static final String K1_V1 = "key1_value1";
+    private static final String K1_V2 = "key1_value2";
+    private static final String K2_V1 = "key2_value1";
+
+    private static void verifyMap(Map<String, ? extends Collection<String>> testMap) {
+        assertEquals(Map.of(K1, List.of(K1_V1, K1_V2), K2, List.of(K2_V1)), testMap);
+    }
+
+    @Test
+    void whenUsingAddKeyManually_thenMapMatches() {
+        Map<String, List<String>> map = new HashMap<>();
+        ArrayListInHashMap.addKeyManually(map, K1, K1_V1);
+        ArrayListInHashMap.addKeyManually(map, K1, K1_V2);
+        ArrayListInHashMap.addKeyManually(map, K2, K2_V1);
+        verifyMap(map);
+    }
+
+    @Test
+    void whenUsingComputeIfAbsent_thenMapMatches() {
+        Map<String, List<String>> map = new HashMap<>();
+        ArrayListInHashMap.addKeyWithComputeIfAbsent(map, K1, K1_V1);
+        ArrayListInHashMap.addKeyWithComputeIfAbsent(map, K1, K1_V2);
+        ArrayListInHashMap.addKeyWithComputeIfAbsent(map, K2, K2_V1);
+        verifyMap(map);
+    }
+
+    @Test
+    void whenUsingApacheMultiValuedMap_thenMapMatches() {
+        MultiValuedMap<String, String> map = new ArrayListValuedHashMap<>();
+        ArrayListInHashMap.addKeyToApacheMultiValuedMap(map, K1, K1_V1);
+        ArrayListInHashMap.addKeyToApacheMultiValuedMap(map, K1, K1_V2);
+        ArrayListInHashMap.addKeyToApacheMultiValuedMap(map, K2, K2_V1);
+        verifyMap(map.asMap());
+    }
+
+    @Test
+    void whenUsingSpringLinkedMultiValueMap_thenMapMatches() {
+        MultiValueMap<String, String> map = new LinkedMultiValueMap<>();
+        ArrayListInHashMap.addKeyToSpringLinkedMultiValueMap(map, K1, K1_V1);
+        ArrayListInHashMap.addKeyToSpringLinkedMultiValueMap(map, K1, K1_V2);
+        ArrayListInHashMap.addKeyToSpringLinkedMultiValueMap(map, K2, K2_V1);
+        verifyMap(map);
+    }
+
+    @Test
+    void whenUsingGuavaMultimap_thenMapMatches() {
+        Multimap<String, String> map = ArrayListMultimap.create();
+        ArrayListInHashMap.addKeyToGuavaMultimap(map, K1, K1_V1);
+        ArrayListInHashMap.addKeyToGuavaMultimap(map, K1, K1_V2);
+        ArrayListInHashMap.addKeyToGuavaMultimap(map, K2, K2_V1);
+        verifyMap(map.asMap());
+    }
+
+}