转换List<>或List<List<>>

List<String> stringList = Arrays.asList("1", "2", "3");
List<Integer> integerList = new ArrayList<>();

for (String str : stringList) {
    integerList.add(Integer.parseInt(str));
}

System.out.println(integerList);

List<List<String>> stringListList = Arrays.asList(
        Arrays.asList("1", "2", "3"),
        Arrays.asList("4", "5", "6"),
        Arrays.asList("7", "8", "9")
);
List<List<Integer>> integerListList = new ArrayList<>();

for (List<String> innerList : stringListList) {
    List<Integer> innerIntegerList = new ArrayList<>();
    for (String str : innerList) {
        innerIntegerList.add(Integer.parseInt(str));
    }
    integerListList.add(innerIntegerList);
}

System.out.println(integerListList);