001/*- 002 * #%L 003 * HAPI FHIR - Core Library 004 * %% 005 * Copyright (C) 2014 - 2024 Smile CDR, Inc. 006 * %% 007 * Licensed under the Apache License, Version 2.0 (the "License"); 008 * you may not use this file except in compliance with the License. 009 * You may obtain a copy of the License at 010 * 011 * http://www.apache.org/licenses/LICENSE-2.0 012 * 013 * Unless required by applicable law or agreed to in writing, software 014 * distributed under the License is distributed on an "AS IS" BASIS, 015 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 016 * See the License for the specific language governing permissions and 017 * limitations under the License. 018 * #L% 019 */ 020package ca.uhn.fhir.model.api; 021 022import ca.uhn.fhir.i18n.Msg; 023 024import java.util.Iterator; 025import java.util.LinkedList; 026import java.util.NoSuchElementException; 027import java.util.function.Consumer; 028 029public class PagingIterator<T> implements Iterator<T> { 030 031 public interface PageFetcher<T> { 032 void fetchNextPage(int thePageIndex, int theBatchSize, Consumer<T> theConsumer); 033 } 034 035 static final int PAGE_SIZE = 100; 036 037 private int myPage; 038 039 private boolean myIsFinished; 040 041 private final LinkedList<T> myCurrentBatch = new LinkedList<>(); 042 043 private final PageFetcher<T> myFetcher; 044 045 public PagingIterator(PageFetcher<T> theFetcher) { 046 myFetcher = theFetcher; 047 } 048 049 @Override 050 public boolean hasNext() { 051 fetchNextBatch(); 052 053 return !myCurrentBatch.isEmpty(); 054 } 055 056 @Override 057 public T next() { 058 fetchNextBatch(); 059 060 if (myCurrentBatch.isEmpty()) { 061 throw new NoSuchElementException(Msg.code(2098) + " Nothing to fetch"); 062 } 063 064 return myCurrentBatch.remove(0); 065 } 066 067 private void fetchNextBatch() { 068 if (!myIsFinished && myCurrentBatch.isEmpty()) { 069 myFetcher.fetchNextPage(myPage, PAGE_SIZE, myCurrentBatch::add); 070 myPage++; 071 myIsFinished = myCurrentBatch.size() < PAGE_SIZE; 072 } 073 } 074}