7903328: Introduce a new method 'clear' in interface 'Multiset'

jmh-core/src/main/java/org/openjdk/jmh/profile/GCProfiler.java

@@ -309,7 +309,7 @@ public static synchronized void startChurnProfile() {
                 throw new IllegalStateException("Churn profile already started");
             }
             started = true;
-            churn = new HashMultiset<>();
+            churn.clear();
             try {
                 for (GarbageCollectorMXBean bean : ManagementFactory.getGarbageCollectorMXBeans()) {
                     ((NotificationEmitter) bean).addNotificationListener(listener, null, null);

jmh-core/src/main/java/org/openjdk/jmh/util/DelegatingMultiset.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, 2015, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -105,4 +105,10 @@ public int hashCode() {
         result = 31 * result + (int) (size ^ (size >>> 32));
         return result;
     }
+
+    @Override
+    public void clear() {
+        size = 0;
+        map.clear();
+    }
 }

jmh-core/src/main/java/org/openjdk/jmh/util/Multiset.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -82,4 +82,9 @@ public interface Multiset<T> {
      * @return the collections of keys
      */
     Collection<T> keys();
+
+    /**
+     * Remove all the elements.
+     */
+    void clear();
 }

jmh-core/src/test/java/org/openjdk/jmh/util/MultisetTest.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package org.openjdk.jmh.util;
+
+import static org.junit.Assert.*;
+import org.junit.Test;
+
+public class MultisetTest {
+    @Test
+    public void testClear() {
+        Multiset<String> set = new HashMultiset<>();
+        set.add("one");
+        set.add("two", 2);
+        assertFalse(set.isEmpty());
+        assertEquals(3, set.size());
+        assertEquals(2, set.keys().size());
+        assertEquals(1, set.count("one"));
+        assertEquals(2, set.count("two"));
+
+        // clear the set
+        set.clear();
+        assertTrue(set.isEmpty());
+        assertEquals(0, set.size());
+        assertEquals(0, set.keys().size());
+        assertEquals(0, set.count("one"));
+        assertEquals(0, set.count("two"));
+
+        // reuse the set
+        set.add("one");
+        set.add("two", 2);
+        assertFalse(set.isEmpty());
+        assertEquals(3, set.size());
+        assertEquals(2, set.keys().size());
+        assertEquals(1, set.count("one"));
+        assertEquals(2, set.count("two"));
+    }
+}