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A lightweight alternative to Java 8 sequential Stream

There is a newer version: 2.3.0

/*
 * Copyright 2016 Daniel Skogquist Åborg
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.d2ab.iterator;

import org.d2ab.collection.Maps;

import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.function.Function;

public class KeyFlatteningEntryIterator
		extends DelegatingReferenceIterator, Map.Entry> {
	private final Function, ? extends Iterable> mapper;

	private Iterator keyIterator = Iterators.empty();
	private Map.Entry entry;

	public KeyFlatteningEntryIterator(Iterator> iterator,
	                                  Function, ? extends Iterable> mapper) {
		super(iterator);
		this.mapper = mapper;
	}

	@Override
	public boolean hasNext() {
		while (!keyIterator.hasNext() && iterator.hasNext()) {
			entry = iterator.next();
			keyIterator = mapper.apply(entry).iterator();
		}
		return keyIterator.hasNext();
	}

	@Override
	public Map.Entry next() {
		if (!hasNext())
			throw new NoSuchElementException();

		return Maps.entry(keyIterator.next(), entry.getValue());
	}

	@Override
	public void remove() {
		throw new UnsupportedOperationException();
	}
}