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

*** 174,183 ****
--- 174,188 ----
       *
       * @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)
*** 185,194 ****
--- 190,200 ----
              } 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.