Generate Combinations

import java.util.ArrayList;
import java.util.List;

public class Combinations {
    private final int n;
    private int x;
    private String[] disp;

    public static void main(String[] args) {
        System.out.println(new Combinations(8).generateAll());
        System.out.println(new Combinations(new String[] { "a", "b", "c" }).generateAll());
    }

    public Combinations(int n) {
        this.n = n;
        disp = new String[n];
        for (int i = 0; i < n; ++i) {
            disp[i] = Integer.toString(i + 1);
        }
    }

    public Combinations(String[] ss) {
        n = ss.length;
        disp = ss;
    }

    private int lowBit(int n) {
        return (1 << n) - 1;
    }

    private boolean next() {
        int smallest = x & -x;
        int ripple = x + smallest;
        int new_smallest = ripple & -ripple;
        int ones = ((new_smallest / smallest) >>> 1) - 1;
        x = ripple | ones;
        return (x & ~lowBit(n)) == 0;
    }

    public List<List<String>> generate(int k) {
        List<List<String>> ret = new ArrayList<>();
        x = lowBit(k);
        do {
            List<String> lis = new ArrayList<>();
            for (int s = x, i = 1; i <= n; s >>>= 1, ++i) {
                if ((s & 1) == 1) {
                    lis.add(disp[i - 1]);
                }
            }
            ret.add(lis);
        } while (next());
        return ret;
    }

    public List<List<List<String>>> generateAll() {
        List<List<List<String>>> ret = new ArrayList<>();
        for (int i = 1; i <= n; ++i) {
            ret.add(generate(i));
        }
        return ret;
    }
}