code: @NonNull
      static <T> T checkNotNull(final T reference) {
          if (reference == null) {
              throw new NullPointerException();
          }
          return reference;
      }
docs: Ensures that an object reference passed as a parameter to the calling
  method is not null.
  
  @param reference an object reference
  @return the non-null reference that was validated
  @throws NullPointerException if {@code reference} is null