How can I use a custom `Comparator` to define a specific sort order in Java?

General Discussions
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1

I want to sort a list of cars by color, but not in alphabetical order. Instead, I need a java custom comparator that ensures colors appear in a predefined sequence (e.g., Red first, then Blue, etc.). How can I implement this while keeping sorting efficient?

2

From my experience, the cleanest and most efficient way to define a specific order is by using a Map to assign priorities. It’s a classic move when working with a java custom comparator.

Here’s how I usually do it:

Map<String, Integer> colorOrder = Map.of("Red", 1, "Blue", 2, "Green", 3);
cars.sort(Comparator.comparingInt(car -> colorOrder.getOrDefault(car.color, Integer.MAX_VALUE)));

:white_check_mark: Why I prefer this?

You get constant-time lookups (O(1)), your logic stays clean, and it scales well with more items.

3

Right on, @macy-davis! I’ve used that Map-based approach a lot too. But sometimes, when I want something simpler or more human-readable—especially in smaller use cases—I go for a List with indexOf.

It still works within the realm of a java custom comparator, but it trades off a bit of performance for clarity:

List<String> colorOrder = List.of("Red", "Blue", "Green");
cars.sort(Comparator.comparingInt(car -> colorOrder.indexOf(car.color)));

:white_check_mark: Why?

Super easy to update the order later, and the intent is visually obvious, which helps when sharing code across teams.

4

Jumping in here—both those approaches are solid! But if you’re aiming for flexibility without pulling in additional data structures like a separate Map or List, you can directly define the logic inside the compare() method. It’s still part of building a java custom comparator, just more embedded:

cars.sort((c1, c2) -> {
    List<String> order = List.of("Red", "Blue", "Green");
    return Integer.compare(order.indexOf(c1.color), order.indexOf(c2.color));
});

:white_check_mark: Why? You avoid maintaining an external reference and keep everything tightly scoped—great for one-off custom sort logic inside modules.