code: @Nullable
    public static Class <?> getPrimitiveWrapperClass (@Nullable final Class <?> aClass)
    {
      if (isPrimitiveWrapperType (aClass))
        return aClass;
      return PRIMITIVE_TO_WRAPPER.get (aClass);
    }
docs: Get the primitive wrapper class of the passed primitive class.
  
  @param aClass
  The primitive class. May be <code>null</code>.
  @return <code>null</code> if the passed class is not a primitive class.