[COLLECTIONS-880] Add MultiMapUtils.invert(Map, MultiValuedMap) overload

src/main/java/org/apache/commons/collections4/MultiMapUtils.java

@@ -179,6 +179,26 @@ M invert(MultiValuedMap<? extends V, ? extends K> input, M output) {
         return output;
     }
 
+    /**
+     * A utility method to invert the mappings from an input Map
+     * and add them to an output MultiValuedMap. Use this method to have complete
+     * control of the output MultiValuedMap or when merging several inverse mappings.
+     *
+     * @param input take key-to-value mappings from here
+     * @param output add value-to-key mappings here
+     * @param <K> the output MultiValuedMap key type and the input Map value type
+     * @param <V> the output MultiValuedMap value type and the input Map key type
+     * @param <M> the output MultiValuedMap with key and value types reversed compared with input
+     * @return the updated output MultiValuedMap
+     */
+    public static <K, V, M extends MultiValuedMap<K, V>>
+    M invert(Map<? extends V, ? extends K> input, M output) {
+        for (Map.Entry<? extends V, ? extends K> e : input.entrySet()) {
+            output.put(e.getValue(), e.getKey());
+        }
+        return output;
+    }
+
     /**
      * Null-safe check if the specified {@code MultiValuedMap} is empty.
      * <p>

src/test/java/org/apache/commons/collections4/MultiMapUtilsTest.java

@@ -24,8 +24,10 @@
 
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
 import org.apache.commons.collections4.multimap.ArrayListValuedHashMap;
@@ -119,7 +121,7 @@ void testGetValuesAsSet() {
     }
 
     @Test
-    void testInvert() {
+    void testInvertMultiValuedMap() {
         final HashSetValuedHashMap<String, String> usages = new HashSetValuedHashMap<>();
 
         final LinkedHashSetValuedLinkedHashMap<String, String> deps = new LinkedHashSetValuedLinkedHashMap<>();
@@ -142,6 +144,30 @@ void testInvert() {
         assertEquals("[commons-collections, commons-configuration2]", codecUsagesAll.toString());
     }
 
+    @Test
+    void testInvertMap() {
+        final HashSetValuedHashMap<String, String> usages = new HashSetValuedHashMap<>();
+
+        final Map<String, String> deps = new HashMap<>();
+        deps.put("commons-configuration1", "commons-logging");
+        deps.put("commons-configuration2", "commons-lang3");
+        deps.put("commons-configuration3", "commons-text");
+        deps.put("commons-beanutils1", "commons-collections");
+        deps.put("commons-beanutils2", "commons-logging");
+        MultiMapUtils.invert(deps, usages);
+        final Set<String> loggingUsagesCompile = usages.get("commons-logging");
+        assertEquals("[commons-beanutils2, commons-configuration1]", loggingUsagesCompile.toString());
+        final Set<String> codecUsagesCompile = usages.get("commons-codec");
+        assertEquals("[]", codecUsagesCompile.toString());
+
+        final Map<String, String> optionalDeps = new HashMap<>();
+        optionalDeps.put("commons-configuration2", "commons-codec");
+        optionalDeps.put("commons-collections", "commons-codec");
+        MultiMapUtils.invert(optionalDeps, usages);
+        final Set<String> codecUsagesAll = usages.get("commons-codec");
+        assertEquals("[commons-collections, commons-configuration2]", codecUsagesAll.toString());
+    }
+
     @Test
     void testIsEmptyWithEmptyMap() {
         assertTrue(MultiMapUtils.isEmpty(new ArrayListValuedHashMap<>()));