Advent of Code 2024 (Java)

I picked Java again because I really need to step up my game with the language I use for work. With this I force myself to actually learn some new APIs and methods and to also try them out for the first time.

Day 1

I always thought the Pair was a default class, but to no avail it's not available anymore in Java 23. It was in the JavaFX package before. Luckily it's easy to make a class on your own.

This was also the first time I have used Class.forName to dynamically get a class and to call a method on it. It was easier than I expected it to be.

Reminder for myself, don't forget Math.abs last time I forgot it was there and use the min and max functions in the Math class as substitute 🤦🏻

IntStream.rangeClosed and IntStream.range are also useful if you just want to iterate over something with an index.

Day 2

A wild day indeed, I had some difficulty getting up to speed as I was doing some thinking wrong. But just doing a sort after the initial sort() and sort(Collections.reverseOrder()) made me eliminate a check for going up or down.

I also learned again that Collection.remove(object) does not work with a list like this [1,2,3,3,4] as there are multiple 3 in the collection.

Day 3

Regex was king today, making everything easier. I must say I was pretty rusty with remembering how to program patterns. But a quick Google was

Pattern multiplier = Pattern.compile("mul\\(\\d{1,3},\\d{1,3}\\)|do\\(\\)|don't\\(\\)");
Matcher matcher = multiplier.matcher(input);

List<String> captureGroups = matcher.results().map(MatchResult::group).toList();

Day 4

I learned something valuable today, which I don't even know if other people in the Java community know where I work.

What do you think what the next code wil do?

List<Integer> numbers = new ArrayList<>(List.of(1, 3));

numbers.add(1, 2);

System.out.println(numbers); // will print [1,2,3]

I DIDN'T KNOW! I didn't know that there was an easier way of inserting items in an existing array/list. I normally did this by hand. I thought that it will override the current existing index of 1, which was a 3. So this makes programming with insertions a lot easier in the future.

Also instead of doing a lot of if statements and checking positions by hand I wanted to make it a bit smarter and use offsets, and it worked out great in my opinion.

Day 5

Once again I keep forgetting that not every Regex engine uses single escape characters \| this for example wouldn't work. You would need two \\ to make it work.

Day 6

Do you want to reverse a collection? Well you better not use Collections.reversOrder() because this will also sort your list, without you even wanting it! What I was actually look for was the Collections.reverse(list); method!

Day 7

I again was reminded that giving a List in a recursive function would update the list in the above stack as well. Which is confusing if you don't know what to look out for. And of course the new Switch that we can use in Java:

return switch (operator) {
  case '+' -> left + right;
  case '*' -> left * right;
  case '|' -> Long.parseLong(String.format("%s%s", left, right));
  default -> throw new IllegalStateException("Unexpected value: " + operator);
};

Day 8

Grouping by is something I almost never use at my job. So having to use it again was nice. Especially because it's not too hard to implement.

var groupedAntennas = antennas.stream().collect(Collectors.groupingBy(x -> x.left));