Does Collections.sort keep order on equal elements?

Question

I have a list of objects ordered by a date parameter and want to reorder them by category parameter, but keeping the date order within the category.

Is something like this enough, or do I have to implement a comparator that takes on account the date for objects of the same category?

// sort the list by category asc(, date asc )
Collections.sort((List<Object>)entries, new Comparator<Object>() {

    @Override public int compare(Object elementA, Object elementB) {
        return elementA.category.compareTo(elementB.category); // what happens when elementA.category.equals(elementB.category)?
    }

});

Answer 1

The code in your question will do what you need it to, since Collections.sort() does preserve the order of equal elements.

From the documentation:

This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.

In other words, if the entries are ordered by date before the sort(), they will stay ordered by date within each category after the sort().

If you don't want to rely on the original ordering, you can easily extend your comparator to first compare the categories and then break ties using the dates.