Java - Unir dos listas en Java

public class MezclarArreglos {

    public static void ordenamientoDescendente(int[] v) {

        for (int i = v.length - 1; i > 0; i--) {

            for (int j = 0; j < i; j++) {

                if (j + 1 <= i && v[j] < v[j + 1]) {

                    int aux = v[j];

                    v[j] = v[j + 1];

                    v[j + 1] = aux;

                }

            }

        }

    }

    public static int[] mezclar(int[] a, int[] b) {

        int[] x = new int[a.length + b.length];

        for (int idx = 0; idx < x.length; idx++) {

            if (idx < a.length) {

                x[idx] = a[idx];

            } else {

                x[idx] = b[idx - a.length];

            }

        }

        ordenamientoDescendente(x);

        return x;

    }

    public static void main(String[] args) {

        int[] a = {1, 3, 4, 5, 6, 10};

        int[] b = {2, 7, 8, 9, 11, 13};

        int[] x = mezclar(a, b);

        for (int idx = 0; idx < x.length; idx++) {

            System.out.println(idx + ": " + x[idx]);

        }

    }

}