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A lightweight alternative to Java 8 sequential Stream
There is a newer version: 2.3.0
/*
 * Copyright 2015 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.iterable;

import org.d2ab.iterator.ChainingIterator;
import org.d2ab.util.Arrayz;

import javax.annotation.Nullable;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.function.Function;
import java.util.stream.Stream;

import static java.util.Objects.requireNonNull;

public class ChainingIterable implements Iterable {
	private final Collection> iterables = new ArrayList<>();

	public ChainingIterable() {
	}

	public ChainingIterable(Iterable iterable) {
		iterables.add(requireNonNull(iterable));
	}

	@SafeVarargs
	public ChainingIterable(Iterable... iterables) {
		Arrayz.forEach(e -> this.iterables.add(requireNonNull(e)), iterables);
	}

	public static  Iterable flatten(Iterable containers) {
		return new ChainingIterable().flatAppend(requireNonNull(containers));
	}

	public static  Iterable flatMap(Iterable iterable,
	                                         Function> mapper) {
		requireNonNull(mapper);
		requireNonNull(iterable);
		ChainingIterable result = new ChainingIterable<>();
		iterable.forEach(each -> result.append(mapper.apply(each)));
		return result;
	}

	public Iterable flatAppend(Iterable containers) {
		for (Object each : requireNonNull(containers))
			append(Iterables.from(each));
		return this;
	}

	public Iterable append(Iterable iterable) {
		iterables.add(requireNonNull(iterable));
		return this;
	}

	public Iterable append(Iterator iterator) {
		return append(Iterables.from(requireNonNull(iterator)));
	}

	@SuppressWarnings("unchecked")
	public Iterable append(T... objects) {
		return append(Iterables.from(requireNonNull(objects)));
	}

	public Iterable append(Stream stream) {
		return append(Iterables.from(requireNonNull(stream)));
	}

	@Override
	public Iterator iterator() {
		return new ChainingIterator<>(iterables);
	}

	@Override
	public int hashCode() {
		return iterables.hashCode();
	}

	@Override
	public boolean equals(@Nullable Object o) {
		if (this == o)
			return true;
		if ((o == null) || (getClass() != o.getClass()))
			return false;

		ChainingIterable that = (ChainingIterable) o;

		return iterables.equals(that.iterables);
	}

	@Override
	public String toString() {
		return "ChainingIterable" + iterables;
	}
}