Get all possible sums with indexes from array

PossibleSums.java

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

public class PossibleSums {

    public static List<Sum> getSumsList(final int[] numbersArray) {
        final List<Sum> returnSumsList = new ArrayList<>();
        final int numberOfSums = (int) Math.pow(2, numbersArray.length);
        for (int i = 1; i < numberOfSums; i++) {
            final Sum sum = new Sum();
            final boolean[] mask = getMask(i);
            for (int j = 0; j < mask.length; j++) {
                if (mask[j]) {
                    sum.addToIndexes(j);
                    sum.addToSum(numbersArray[j]);
                }
            }
            returnSumsList.add(sum);
        }
        return returnSumsList;
    }

    private static boolean[] getMask(final int digit) {
        final String reversedBinaryString = new StringBuilder(Integer.toBinaryString(digit)).reverse().toString();
        final boolean[] returnMask = new boolean[reversedBinaryString.length()];
        for (int i = 0; i < reversedBinaryString.length(); i++)
            returnMask[i] = reversedBinaryString.charAt(i) - '0' == 1;
        return returnMask;
    }

    public static void printSumsList(final List<Sum> sums) {
        System.out.println(new StringBuilder()
                .append("Sums: ")
                .append(sums.toString())
                .append("\nItem count: ")
                .append(sums.size())
        );
    }

    public static class Sum implements Comparable<Sum> {
        private final List<Integer> indexes = new ArrayList<>();
        private int sum;

        private Sum() {
        }

        private int getSum() {
            return sum;
        }

        private void addToSum(final int value) {
            sum += value;
        }

        private void addToIndexes(final int index) {
            indexes.add(index);
        }

        @Override
        public String toString() {
            return String.valueOf(sum) + indexes;
        }

        @Override
        public int compareTo(final Sum o) {
            return this.sum - o.getSum();
        }
    }
}