src/java.base/share/classes/java/util/ArrayList.java

@@ -174,10 +174,15 @@
      *
      * @param c the collection whose elements are to be placed into this list
      * @throws NullPointerException if the specified collection is null
      */
     public ArrayList(Collection<? extends E> c) {
+        if (c instanceof ArrayList) {
+            elementData = new Object[((ArrayList)c).elementData.length];
+            System.arraycopy(((ArrayList)c).elementData, 0, elementData, 0, ((ArrayList)c).elementData.length);
+            size = ((ArrayList)c).size;
+        } else {
         elementData = c.toArray();
         if ((size = elementData.length) != 0) {
             // defend against c.toArray (incorrectly) not returning Object[]
             // (see e.g. https://bugs.openjdk.java.net/browse/JDK-6260652)
             if (elementData.getClass() != Object[].class)

@@ -185,10 +190,11 @@
         } else {
             // replace with empty array.
             this.elementData = EMPTY_ELEMENTDATA;
         }
     }
+    }
 
     /**
      * Trims the capacity of this {@code ArrayList} instance to be the
      * list's current size.  An application can use this operation to minimize
      * the storage of an {@code ArrayList} instance.