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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.iterator;

import javax.annotation.Nullable;
import java.util.HashSet;
import java.util.NoSuchElementException;
import java.util.Set;

public class DistinctIterator extends UnaryReferenceIterator {
	private Set seen = new HashSet<>();

	@Nullable
	private T next;
	private boolean hasNext;

	@Override
	public boolean hasNext() {
		if (hasNext)
			return true;

		while (!hasNext && iterator.hasNext()) {
			T next = iterator.next();
			if (seen.add(next)) {
				hasNext = true;
				this.next = next;
			}
		}

		return hasNext;
	}

	@Override
	@Nullable
	public T next() {
		if (!hasNext())
			throw new NoSuchElementException();

		T result = next;
		hasNext = false;
		next = null;
		return result;
	}
}