去重

对列表进行保留顺序的去重

List<Integer> list = List.of(1, 2, 4, 3, 1);
LinkedHashSet<Integer> set = new LinkedHashSet<>(list);
ArrayList<Integer> integers = new ArrayList<>(set);
// 1, 2, 4, 3

删除

按照单逻辑进行删除

List<Integer> integers = new ArrayList<>(List.of(1, 2, 3, 5));
integers.removeIf(x -> x == 1);

按照多种匹配进行删除

List<Integer> integers = new ArrayList<>(List.of(1, 2, 3, 5));
integers.removeAll(List.of(1, 2));

替换

空指针转空列表

ans = null;
ans = Objects.requireNonNullElse(ans, new ArrayList<>());

替换整个列表里面的null值

List<Integer> integers = new ArrayList<>(List.of(1, 2, 3, 5));
integers.add(null);
integers.replaceAll(integer -> integer == null ? 0 : integer);

List<Integer> integers = new ArrayList<>(List.of(1, 2, 3, 5));
integers.add(null);
boolean b = Collections.replaceAll(integers, null, 0);

翻转

List<Integer> integers = new ArrayList<>(List.of(1, 2, 3, 5));
integers.add(null);
Collections.reverse(integers);

排序(空指针安全的排序)

integers.sort(Comparator.nullsFirst(Comparator.naturalOrder()));

二维数组

展平数组

List<List<Integer>> lists = new ArrayList<>();
lists.add(Collections.nCopies(12, 5));
lists.add(Collections.nCopies(12, 6));List<Integer> ans = new ArrayList<>();
lists.forEach(ans::addAll);

行转列

public class ListUtils {public static List<List<String>>  rowNormaliser(List<List<String>> list) {TreeMap<Integer, List<String>> treeMap = new TreeMap<>();for (List<String> stringList : list) {for (int i = 0; i < stringList.size(); i++) {treeMap.computeIfAbsent(i, k -> new ArrayList<>()).add(stringList.get(i));}}return new ArrayList<>(treeMap.values());}
}

合并

ans = new ArrayList<>(List.of(1, 2, 3, 4));
StringJoiner stringJoiner = new StringJoiner(",", "[", "]");
ans.forEach(x -> stringJoiner.add(x.toString()));

其他

乱序洗牌（打乱顺序）

List<List<Integer>> lists = new ArrayList<>();
lists.add(Collections.nCopies(12, 5));
lists.add(Collections.nCopies(12, 6));
List<Integer> ans = new ArrayList<>();
for (List<Integer> list : lists) {ans.addAll(list);
}
Collections.shuffle(ans);