Comparator based on a configurable order

Question

Is there a way to write custom comparator, following this example:

There are at most 10 items coming in at a random order i.e.

first item:  item_one
second:      second_one
third:       third_one

I want result them to be sorted like : second_one, third_one, first_one. I'd like to pull this order from configuration file, sort of like template for sorting.

Am I using the wrong data structure, does anyone have experience with this?

Answer 1

Sure. Here is an "OrderedComparator" that compares elements according to a predefined order:

class OrderedComparator implements Comparator<String> {

    List<String> predefinedOrder;

    public OrderedComparator(String[] predefinedOrder) {
        this.predefinedOrder = Arrays.asList(predefinedOrder);
    }

    @Override
    public int compare(String o1, String o2) {
        return predefinedOrder.indexOf(o1) - predefinedOrder.indexOf(o2);
    }

}

And here is some test code. (I used a List instead of a Set since it 1) seem more natural when talking about the order of the elements and 2) better illustrate what happens with duplicate elements upon sorting using this comparator.)

class Test {

    public static void main(String[] args) {

        // Order (could be read from config file)
        String[] order = { "lorem", "ipsum", "dolor", "sit" };


        List<String> someList = new ArrayList<String>();

        // Insert elements in random order.
        someList.add("sit");
        someList.add("ipsum");
        someList.add("sit");
        someList.add("lorem");
        someList.add("dolor");
        someList.add("lorem");
        someList.add("ipsum");
        someList.add("lorem");


        System.out.println(someList);

        Collections.sort(someList, new OrderedComparator(order));

        System.out.println(someList);
    }

}

Output:

[sit, ipsum, sit, lorem, dolor, lorem, ipsum, lorem]
[lorem, lorem, lorem, ipsum, ipsum, dolor, sit, sit]