Add a main test class to prints out the difference time working with

MutableInt AtomicInteger.

src/test/java/org/apache/commons/lang3/mutable/PrintAtomicVsMutable.java

@@ -0,0 +1,44 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.commons.lang3.mutable;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.apache.commons.lang3.time.DurationUtils;
+
+/**
+ * Prints out the difference time working with {@link MutableInt} and {@link AtomicInteger}.
+ */
+public class PrintAtomicVsMutable {
+
+    public static void main(String[] args) {
+        MutableInt mInt = new MutableInt();
+        final int max = 100_000_000;
+        System.out.println("MutableInt " + DurationUtils.of(() -> {
+            for (int i = 0; i < max; i++) {
+                mInt.incrementAndGet();
+            }
+        }));
+        AtomicInteger aInt = new AtomicInteger();
+        System.out.println("AtomicInteger " + DurationUtils.of(() -> {
+            for (int i = 0; i < max; i++) {
+                aInt.incrementAndGet();
+            }
+        }));
+    }
+}