前後要素を引き連れながらIterableをイテレートする.java

import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

import lombok.AllArgsConstructor;
import lombok.Data;

import com.google.common.collect.Lists;

@AllArgsConstructor(staticName = "of")
public class LinkedLoop<T> implements Iterable<LinkedLoop.Link<T>> {

	@Data
	@AllArgsConstructor
	public static class Link<T> {
		private final T previous;
		private final T next;
		private final T value;

		public Link() {
			this(null, null, null);
		}

		public boolean isFirst() {
			return previous == null;
		}

		public boolean isLast() {
			return next == null;
		}

		public Link<T> push(T next) {
			Link<T> link = new Link<T>(value, next, this.next);
			return link;
		}
	}

	public static class LinkedLoopIterator<T> implements Iterator<Link<T>> {
		private final Iterator<T> src;
		private Link<T> current;
		private boolean last;

		protected LinkedLoopIterator(Iterator<T> src) {
			this.src = src;
			current = new Link<>();
			if (src.hasNext()) {
				current = current.push(src.next());
			}
		}

		@Override
		public boolean hasNext() {
			return !last;
		}

		@Override
		public Link<T> next() {
			if (!last) {
				last = !src.hasNext();
				T next = null;
				if (!last) {
					next = src.next();
				}
				current = current.push(next);
				return current;
			} else {
				throw new NoSuchElementException();
			}
		}

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

	}

	public static void main(String[] args) {
		List<String> list = Lists.newArrayList("hoge", "fuga", "foo", "bar");
		for (Link<String> link : LinkedLoop.of(list)) {
			Object[] a = {
					link.getPrevious(), link.getValue(), link.getNext(), link.isFirst(), link.isLast(),
			};
			System.out.println(String.format("previous: %s, value: %s, next: %s. isFirst: %b, isLast: %b", a));
		}
	}

	private final Iterable<T> src;

	@Override
	public Iterator<Link<T>> iterator() {
		return new LinkedLoopIterator(src.iterator());
	}

}