How do I generate a Cartesian product in Java?

Question

I have a number of ArrayList with each ArrayList having objects and each one can have different length. I need to generate permutation like in the

Answer 1

With an Iterable+Iterator:

import java.util.*;

class CartesianIterator  implements Iterator > {

    private final List > lilio;    
    private int current = 0;
    private final long last;

    public CartesianIterator (final List > llo) {
        lilio = llo;
        long product = 1L;
        for (List  lio: lilio)
            product *= lio.size ();
        last = product;
    } 

    public boolean hasNext () {
        return current != last;
    }

    public List  next () {
        ++current;
        return get (current - 1, lilio);
    }

    public void remove () {
        ++current;
    }

    private List get (final int n, final List > lili) {
        switch (lili.size ())
        {
            case 0: return new ArrayList  (); // no break past return;
            default: {
                List  inner = lili.get (0);
                List  lo = new ArrayList  ();
                lo.add (inner.get (n % inner.size ()));
                lo.addAll (get (n / inner.size (), lili.subList (1, lili.size ())));
                return lo;
            }
        }
    }
}

class CartesianIterable  implements Iterable > {

    private List > lilio;  

    public CartesianIterable (List > llo) {
        lilio = llo;
    }

    public Iterator > iterator () {
        return new CartesianIterator  (lilio);
    }
}

You can use them in a simplified for-loop:

class CartesianIteratorTest {

    public static void main (String[] args) {
        List  la = Arrays.asList (new Character [] {'a', 'b', 'c'});
        List  lb = Arrays.asList (new Character [] {'d', 'e'});      
        List > llc = new ArrayList > ();
        llc.add (la);
        llc.add (lb);

        CartesianIterable  ci = new CartesianIterable  (llc);
        for (List lo: ci)
            show (lo);
    }

    public static void show (List  lo) {
        System.out.print ("(");
        for (Object o: lo)
            System.out.print (o);
        System.out.println (")");
    }
}