Even Integer Iterator

Problem : Given an iterator of integers, can you give me back another iterator which iterates over the given iterator of integers such that the new iterator gives only even integers.

There a couple of different ways to solve this problem.

* The first option is to create a new list of integers and add only the even integers from the given iterator and then return an iterator by calling list.iterator(). This has a time complexity of O (n) but also has a space complexity of O (n) in the worst case, which is not desirable.

Here is the code for it -

public static Iterator<Integer> giveEvenIntegersIterator(Iterator<Integer> iterator)
{
    List<Integer> evenIntegers = new ArrayList<Integer>();

    if (iterator != null)
    {
        while(iterator.hasNext())
        {
            Integer number = iterator.next();

            if (number != null && number % 2 == 0)
            {
                evenIntegers.add(number);
            }
        }
    }
    
    return evenIntegers.iterator();
}

* The second solution which is more efficient is to implement a new EvenIntegersIterator which takes in the given iterator as a parameter in the constructor and delegates the hasNext and next calls to the original iterator with logic to skip odd integers. This solution has a time complexity of O (n) in the worst case, but space complexity of O(1) as no extra memory is needed. Here is the code -

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

/**
 * This iterator implementation takes in an iterator of integers.
 *
 * This implementation will iterate over even integers only.
 */
public class EvenNumbersIterator implements Iterator
{
    private Iterator<Integer> iterator;
    
    private boolean hasNextCalled;

    private Integer next;
    private boolean hasNextValue;

    /**
     * Constructor which takes in an Iterator of Integers.
     * @param iterator - Iterator of integers.
     * @throws RuntimeException - Throws runtime exception if the iterator being passed is null.
     */
    public EvenNumbersIterator(Iterator<Integer> iterator)
    {
        if (iterator == null)
        {
            throw new RuntimeException("Iterator cannot be null.");
        }

        this.iterator = iterator;
    }

    /**
     * This returns if the iterator has the next element.
     * @return boolean
     */
    public boolean hasNext()
    {
        boolean hasNext = false;

        if (hasNextCalled)
        {
            hasNext = hasNextValue;
        }
        else
        {
            hasNextCalled = true;

            while (hasNext = iterator.hasNext())
            {
                Integer number = iterator.next();

                if (number != null && number % 2 == 0)
                {
                    next = number;
                    break;
                }
            }

            hasNextValue = hasNext;
        }

        return hasNext;
    }

    /**
     * Returns the next element of the iterator. Throws NoSuchElementException if there is no more elements.
     * @return Object
     */
    public Object next()
    {
        Integer returnValue = null;

        if (hasNextCalled)
        {
            hasNextCalled = false;
            returnValue = next;

            if (returnValue == null)
            {
                throw new NoSuchElementException();
            }
        }
        else
        {
            while(iterator.hasNext())
            {
                returnValue = iterator.next();

                if (returnValue != null && returnValue % 2 == 0)
                {
                    break;   
                }
            }

            if (returnValue == null)
            {
                throw new NoSuchElementException();
            }
        }

        return returnValue;
    }

    public void remove()
    {
        
    }
}