code: public double locked (@Nonnull final DoubleSupplier aSupplier)
    {
      ValueEnforcer.notNull (aSupplier, "Supplier");
  
      lock ();
      try
      {
        return aSupplier.getAsDouble ();
      }
      finally
      {
        unlock ();
      }
    }
docs: Execute the provided callable in a read lock.
  
  @param aSupplier
  Callable to be executed. May not be <code>null</code>.
  @return The return value of the callable. May be <code>null</code>.