Helper to iterate over pageable sources. Should reduce memory usage when querying large tables via Spring Data.

PageableCollection.java

======================================
Usage:

Fetcher<Source, MyEntity> f = new Fetcher<Source, MyEntity>(source) {
	@Override
	public List<MyEntity> fetch(Pageable pageRequest)
	{
		return source.findAll(pageRequest);
	}
};

PageableCollection<Source,MyEntity> pageableCollection = new PageableCollection<Source, MyEntity>(f);

for (MyEntity e : pageableCollection) {
	// ...
}

============================================================================

public class PageableCollection<T> implements Iterable<T>
{
	private static final int DEFAULT_PAGE_SIZE = 100;
	
	private Fetcher<?, T> fetcher;
	private int pageSize;

	public PageableCollection(Fetcher<?, T> f) {
		this(f, DEFAULT_PAGE_SIZE);
	}
	
	public PageableCollection(Fetcher<?, T> f, int pageSize)
	{
		this.fetcher = f;
		this.pageSize = pageSize;
	}

	@Override
	public Iterator<T> iterator()
	{
		return new PageableIterator<T>(fetcher, pageSize);
	}
}

/**
 * Initially makes sense only inside the PageableCollection class
 */
class PageableIterator<T> implements Iterator<T>
{
	private static final int FIRST_PAGE = 0;
	
	private List<T> currentData;
	private int cursor;
	private int pageSize;
	private Pageable page;
	private Fetcher<?, T> fetcher;
	
	public PageableIterator(Fetcher<?, T> f, int pageSize) {
		this.fetcher = f;
		this.pageSize = pageSize;
		page = new PageRequest(FIRST_PAGE, pageSize);
		currentData = new ArrayList<T>();
	}
	
	@Override
	public boolean hasNext()
	{
		if (hasDataLoaded()) {
			return true;
		}
		
		tryToFetchMoreData();
		return !currentData.isEmpty();
	}
	
	private void tryToFetchMoreData()
	{
		currentData = fetcher.fetch(page);
		page = new PageRequest(page.getPageNumber()+1, pageSize);
		cursor = 0;
	}
	
	private boolean hasDataLoaded()
	{
		return cursor < currentData.size();
	}
	
	@Override
	public T next()
	{
		return currentData.get(cursor++);
	}
	
	@Override
	public void remove()
	{
		throw new UnsupportedOperationException();
	}
}

public abstract class Fetcher<S, T>
{
  protected S source;
	
	public Fetcher(S s) {
		this.source = s;
	}
	
	public abstract List<T> fetch(Pageable pageRequest);
}

Thanks a lot,
I just refactored it to make usage more readable and general:

=============================================================
Usage:

    final Fetcher<Portfolio> fetcher = (pageable) ->
              portfolioRepository.getAllByAssetType(AssetType.PORTFOLIO, pageable);

    for (final Portfolio portfolio : fetcher.toPageableCollection())
    {
        System.out.println(portfolio);
    }

=============================================================

    @FunctionalInterface
    public interface Fetcher<T>
    {
        List<T> fetch(Pageable pageRequest);
    
        default PageableCollection<T> toPageableCollection()
        {
            return new PageableCollection<>(this);
        }
    }

=============================================================

    public class PageableCollection<T> implements Iterable<T>
    {
        private static final int DEFAULT_PAGE_SIZE = 1000;
    
        private final Fetcher<T> fetcher;
        private final int pageSize;
    
        public PageableCollection(final Fetcher<T> fetcher)
        {
            this(fetcher, DEFAULT_PAGE_SIZE);
        }
    
        public PageableCollection(final Fetcher<T> fetcher, final int pageSize)
        {
            this.fetcher = fetcher;
            this.pageSize = pageSize;
        }
    
        @Override
        public Iterator<T> iterator()
        {
            return new PageableIterator<>(fetcher, pageSize);
        }
    }

=============================================================

    @Slf4j
    class PageableIterator<T> implements Iterator<T>
    {
        private static final int FIRST_PAGE = 0;
    
        private final Fetcher<T> fetcher;
        private final int pageSize;
    
        private List<T> currentData;
        private int cursor;
        private Pageable page;
    
        public PageableIterator(final Fetcher<T> fetcher, final int pageSize)
        {
            this.fetcher = fetcher;
            this.pageSize = pageSize;
            page = PageRequest.of(FIRST_PAGE, pageSize);
            currentData = new ArrayList<>();
        }
    
        @Override
        public boolean hasNext()
        {
            if (hasDataLoaded())
            {
                return true;
            }
    
            tryToFetchMoreData();
            return !currentData.isEmpty();
        }
    
        private void tryToFetchMoreData()
        {
            log.debug("try to fetch more data on page {}", page);
            currentData = fetcher.fetch(page);
            page = PageRequest.of(page.getPageNumber() + 1, pageSize);
            cursor = 0;
        }
    
        private boolean hasDataLoaded()
        {
            return cursor < currentData.size();
        }
    
        @Override
        public T next()
        {
            return currentData.get(cursor++);
        }
    
        @Override
        public void remove()
        {
            throw new UnsupportedOperationException();
        }
    }