Shell Sort

package iteration;

public interface Iterable {
    public Iterator iterator();
}

package dataStructures;

import iteration.Iterable;

public interface List extends Iterable {
    public void insert(int index, Object value)
            throws IndexOutOfBoundsException;
    public void add(Object value);
    public Object delete(int index) throws IndexOutOfBoundsException;
    public boolean delete(Object value);
    public void clear();
    public Object set(int index, Object value)
            throws IndexOutOfBoundsException;
    public Object get(int index) throws IndexOutOfBoundsException;
    public int indexOf(Object value);
    public boolean contains(Object value);
    public int size();
    public boolean isEmpty();
}

package sorting;

import dataStructures.List;

public interface ListSorter {
    public List sort(List list);
}

package sorting;

import dataStructures.List;

public class ShellsortListSorter implements ListSorter {
    private final Comparator _comparator;
    private final int[] _increments = {121, 40, 13, 4, 1};

    public ShellsortListSorter(Comparator comparator) {
        assert comparator != null : "comparator cannot be null";
        _comparator = comparator;
    }

    public List sort(List list) {
        assert list != null : "list cannot be null";
        for (int i = 0; i < _increments.length; i++) {
            int increment = _increments[i];
            hSort(list, increment);
        }
        return list;
    }

    private void hSort(List list, int increment) {
        if (list.size() < (increment * 2)) {
            return;
        }
        for (int i = 0; i < increment; ++i) {
            sortSublist(list, i, increment);
        }
    }

    private void sortSublist(List list, int startIndex, int increment) {
        for (int i = startIndex + increment; i < list.size(); i += increment) {
            Object value = list.get(i);
            int j;
            for (j = i; j > startIndex; j -= increment) {
                Object previousValue = list.get(j - increment);
                if (_comparator.compare(value, previousValue) >= 0) {
                    break;
                }
                list.set(j, previousValue);
            }
            list.set(j, value);
        }
    }
}