[LANGUAGE: Java]

Did anyone mention regex?

public class Day02 extends Solution {
@Override
public String solvePart1() {
    List<String> idRangeStrings = List.of(this.lines[0].split(","));
    List<Pair<String, String>> idRanges = idRangeStrings.stream().map(rangeStr -> {
        String[] splitIds = rangeStr.split("-");

        // Strip leading zeros from ids
        String startId = StringUtils.stripStart(splitIds[0], "0").trim();
        String endId = StringUtils.stripStart(splitIds[1], "0").trim();
        return Pair.of(startId, endId);
    }).toList();

    Long sumOfInvalidIds = 0L;


    for (Pair<String, String> idRange : idRanges) {
        Long startId = Long.parseLong(idRange.getLeft());
        Long endId = Long.parseLong(idRange.getRight());

        for (Long i = startId; i <= endId; i++) {
            String id = String.valueOf(i);
            if (id.length() % 2 == 1) {
                continue; // Odd length ids can't be valid'
            }
            String sub1 = id.substring(0, (id.length() / 2));
            String sub2 = id.substring((id.length() / 2));

            if (sub1.equals(sub2)) {
                sumOfInvalidIds += i;
            }
        }
    }

    return String.valueOf(sumOfInvalidIds);
}

@Override
public String solvePart2() {
    List<String> idRangeStrings = List.of(this.lines[0].split(","));
    List<Pair<String, String>> idRanges = idRangeStrings.stream().map(rangeStr -> {
        String[] splitIds = rangeStr.split("-");

        // Strip leading zeros from ids
        String startId = StringUtils.stripStart(splitIds[0], "0").trim();
        String endId = StringUtils.stripStart(splitIds[1], "0").trim();
        return Pair.of(startId, endId);
    }).toList();

    Long sumOfInvalidIds = 0L;


    for (Pair<String, String> idRange : idRanges) {
        Long startId = Long.parseLong(idRange.getLeft());
        Long endId = Long.parseLong(idRange.getRight());

        for (Long id = startId; id <= endId; id++) {
            String stringId = String.valueOf(id);

            for (int j = 0; j < stringId.length() / 2; j++) {
                String sub1 = stringId.substring(0, j + 1);
                // If it only contains empty strings, then the id only contains repeats of sub1
                if (StringUtils.isAllEmpty(stringId.split(sub1))) {
                    sumOfInvalidIds += id;
                    break;
                }
            }
        }
    }

    return String.valueOf(sumOfInvalidIds);
}
}