PeekIterator.java

import java.util.*;

/**
 * Google:
 * Implement peek() for java iterator()
 *
 * Example [1,2,3,4,5]
 * peek() = 1, peek() = 1, next() = 1, peek() = 2, next() = 2, peek() = 3
 * hasNext() = true,
 *
 * Tags: Data Structures
 */
class PeekIterator<T> implements Iterator<T> {

    private final Iterator<T> iterator;

    /**
     * Store next item
     */
    private T nextItem;

    public PeekIterator(Iterator<T> iterator) {
        this.iterator = iterator;
    }

    @Override
    public boolean hasNext() {
        if (nextItem != null) return true; // check next item
        if (iterator.hasNext()) nextItem = iterator.next(); // set next item
        return nextItem != null;
    }

    @Override
    public T next() {
        if (!hasNext()) throw new NullPointerException();
        T temp = nextItem;
        nextItem = null; // reset next
        return temp;
    }

    public T peek() {
        if (!hasNext()) throw new NullPointerException();
        return nextItem;
    }

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