Cartesian product of arbitrary sets in Java

Question

Do you know some neat Java libaries that allow you to make cartesian product of two (or more) sets?

For example: I have three sets. One with objects of class Person

Answer 1

Index-based solution

Working with the indices is an alternative that is fast and memory-efficient and can handle any number of sets. Implementing Iterable allows easy use in a for-each loop. See the #main method for a usage example.

public class CartesianProduct implements Iterable, Iterator {

    private final int[] _lengths;
    private final int[] _indices;
    private boolean _hasNext = true;

    public CartesianProduct(int[] lengths) {
        _lengths = lengths;
        _indices = new int[lengths.length];
    }

    public boolean hasNext() {
        return _hasNext;
    }

    public int[] next() {
        int[] result = Arrays.copyOf(_indices, _indices.length);
        for (int i = _indices.length - 1; i >= 0; i--) {
            if (_indices[i] == _lengths[i] - 1) {
                _indices[i] = 0;
                if (i == 0) {
                    _hasNext = false;
                }
            } else {
                _indices[i]++;
                break;
            }
        }
        return result;
    }

    public Iterator iterator() {
        return this;
    }

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

    /**
     * Usage example. Prints out
     * 
     * 
     * [0, 0, 0] a, NANOSECONDS, 1
     * [0, 0, 1] a, NANOSECONDS, 2
     * [0, 0, 2] a, NANOSECONDS, 3
     * [0, 0, 3] a, NANOSECONDS, 4
     * [0, 1, 0] a, MICROSECONDS, 1
     * [0, 1, 1] a, MICROSECONDS, 2
     * [0, 1, 2] a, MICROSECONDS, 3
     * [0, 1, 3] a, MICROSECONDS, 4
     * [0, 2, 0] a, MILLISECONDS, 1
     * [0, 2, 1] a, MILLISECONDS, 2
     * [0, 2, 2] a, MILLISECONDS, 3
     * [0, 2, 3] a, MILLISECONDS, 4
     * [0, 3, 0] a, SECONDS, 1
     * [0, 3, 1] a, SECONDS, 2
     * [0, 3, 2] a, SECONDS, 3
     * [0, 3, 3] a, SECONDS, 4
     * [0, 4, 0] a, MINUTES, 1
     * [0, 4, 1] a, MINUTES, 2
     * ...
     * 
     */
    public static void main(String[] args) {
        String[] list1 = { "a", "b", "c", };
        TimeUnit[] list2 = TimeUnit.values();
        int[] list3 = new int[] { 1, 2, 3, 4 };

        int[] lengths = new int[] { list1.length, list2.length, list3.length };
        for (int[] indices : new CartesianProduct(lengths)) {
            System.out.println(Arrays.toString(indices) //
                    + " " + list1[indices[0]] //
                    + ", " + list2[indices[1]] //
                    + ", " + list3[indices[2]]);
        }
    }
}