OAK-11226 : added new api in CollectionUtils to create concurrent has…

…h set

oak-commons/src/main/java/org/apache/jackrabbit/oak/commons/collections/CollectionUtils.java

@@ -31,6 +31,7 @@
 import java.util.Objects;
 import java.util.Set;
 import java.util.TreeSet;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 import java.util.stream.Stream;
@@ -221,6 +222,20 @@ public static <K> Set<K> newHashSet(final int capacity) {
         return new HashSet<>(ensureCapacity(capacity));
     }
 
+    /**
+     * Creates a new, empty {@link Set} which is backed by {@link ConcurrentHashMap} to allow concurrent access.
+     * Returning Set doesn't allow null keys and values.
+     *
+     * @return a new, empty {@link Set} which is backed by {@link ConcurrentHashMap}.
+     *
+     * @see CollectionUtils#newHashMap(int)
+     * @see CollectionUtils#newLinkedHashSet(int)
+     */
+    @NotNull
+    public static <K> Set<K> newConcurrentHashSet() {
+        return ConcurrentHashMap.newKeySet();
+    }
+
     /**
      * Creates a new, empty LinkedHashSet with expected capacity.
      * <p>

oak-commons/src/test/java/org/apache/jackrabbit/oak/commons/collections/CollectionUtilsTest.java

@@ -119,6 +119,17 @@ public void iteratorToSet() {
         Assert.assertEquals(s, CollectionUtils.toSet(iterable.iterator()));
     }
 
+    @Test
+    public void concurrentHashSet() {
+        // create a set of non-null values
+        final Set<String> s = data.stream().filter(Objects::nonNull).collect(Collectors.toSet());
+
+        Set<String> concurrentHashSet = CollectionUtils.newConcurrentHashSet();
+        concurrentHashSet.addAll(s);
+
+        Assert.assertEquals(s, concurrentHashSet);
+    }
+
     @Test
     public void toArrayDequeWithNonEmptyIterable() {
         List<String> list = Arrays.asList("one", "two", "three");