Provide an iterator over the contents of two lists simultaneously?

Anyway, the problem is that I can’t really return just a single object from next(), and I also can’t have a Iterator take more than one type. So, any thoughts?

Obviously you are going to need a light-weight “pair” class. This is roughly analogous to the Map.Entry inner class.

Here’s a rough cut at a generic solution:

public class ParallelIterator <T1, T2> implements Iterator<Pair<T1, T2>> {

    public class Pair<TT1, TT2> {
        private final TT1 v1;
        private final TT2 v2;
        private Pair(TT1 v1, TT2 v2) { this.v1 = v1; this.v2 = v2; }

    private final Iterator<T1> it1;
    private final Iterator<T2> it2;

    public ParallelIterator(Iterator<T1> it1, Iterator<T2> it2) { 
        this.it1 = it1; this.it2 = it2;

    public boolean hasNext() { return it1.hasNext() && it2.hasNext(); }

    public Pair<T1, T2> next() {
        return new Pair<T1, T2>(,;



Note: this doesn’t explicitly deal with cases where the lists have different lengths. What will happen is that extra elements at the end of the longer list will be silently ignored.

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