The Microsoft Graph Java SDK example for iterating through items looks like this:

PageIterator<ListItem, ListItemCollectionResponse> pageIterator = new PageIterator.Builder<ListItem, ListItemCollectionResponse>()
    .client(graphClient)
    // The first page of the collection is passed to the collectionPage method
    .collectionPage(response)
    // CollectionPageFactory is called to create a new collection page from the nextLink
    .collectionPageFactory(ListItemCollectionResponse::createFromDiscriminatorValue)
    // ProcessPageItemCallback is called for each item in the collection
    .processPageItemCallback(li -> {
        String driveItemId = li.getDriveItem().getId();
        InputStream is = graphClient.drives().byDriveId(siteDriveId)
                .items()
                .byDriveItemId(driveItemId)
                .content()
                .get();
        try {
            byte[] bytes = IOUtils.toByteArray(is);
            LOGGER.info("List item downloaded - id={}, driveItemId={}, numBytes={}", li.getId(), driveItemId, bytes.length);

            try {
                InputStream isFromFetcher = microsoftGraphFetcher.fetch(siteDriveId + "," + driveItemId,  new Metadata());
                byte[] bytesFromFetcher = IOUtils.toByteArray(isFromFetcher);
                Assertions.assertEquals(bytesFromFetcher, bytes);
            } catch (TikaException e) {
                Assertions.fail(e);
            }

        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        return true;
    }).build();
    pageIterator.iterate();

Can someone please describe what the collectionPageFactory method’s purpose is? ListItemCollectionResponse::createFromDiscriminatorValue – what is a discriminator?