public static <R extends IRegionKey<R, L>, L extends IKey<R, L>> CachedRegionProvider<R, L> makeProvider(Path directory, int sectorSize,
            int maxSize) {
        // error here
        return new CachedRegionProvider<>(SimpleRegionProvider.createDefault(directory, sectorSize), maxSize);
    }


SimpleRegionProvider.createDefault:
    public static <R extends IRegionKey<R, L>, L extends IKey<R, L>> SimpleRegionProvider<R, L> createDefault(Path directory, int sectorSize) {
        return new SimpleRegionProvider<>(directory, (p, r) ->
                new Region.Builder()
                        .setPath(p)
                        .setEntriesPerRegion(r.getKeyCount())
                        .setSectorSize(sectorSize)
                        .build()
        );
    }

SimpleRegionProvider:

public class SimpleRegionProvider<R extends IRegionKey<R, L>, L extends IKey<R, L>> implements IRegionProvider<R, L> {

    // fields

    public SimpleRegionProvider(Path directory, RegionFactory<R, L> regionBuilder) {
          // init...
    }

    // methods
}

CachedRegionProvider:

public class CachedRegionProvider<R extends IRegionKey<R, L>, L extends IKey<R, L>> implements IRegionProvider<R, L> {

    // fields
    public CachedRegionProvider(IRegionProvider<R, L> sourceProvider, int maxCacheSize) {
        //init
    }