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PeekableIterator.java

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

public class PeekableIterator<T> implements Iterator<T> {
    //
    private final Iterator<T> it;
    private T nextElem;
    private boolean hasNext = false;

    public PeekableIterator(Iterable<T> iterable) {
        this(iterable.iterator());
    }
    public PeekableIterator(Iterator<T> it) {
        this.it = it;

        if (it.hasNext()) {
            nextElem = it.next();
            hasNext = true;
        }
    }

    //
    public T peek() throws NoSuchElementException {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        return nextElem;
    }

    //
    @Override
    public boolean hasNext() {
        return hasNext;
    }
    @Override
    public T next() throws NoSuchElementException {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }

        final T ret = nextElem;
        if (it.hasNext()) {
            nextElem = it.next();
        } else {
            hasNext = false;
        }
        return ret;
    }
}