1.DateTimeFormatter对当前日期进行格式化，并提取出年、月、日的字符串表示。

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DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyyMMdd");

// 获取当前日期并格式化为 "yyyyMMdd" 格式的字符串
String day = LocalDate.now().format(dtf); // 输出例如 "20240628"

// 提取出前六位字符作为月份，格式为 "yyyyMM"
String month = day.substring(0, 6); // 输出例如 "202406"

// 提取出前四位字符作为年份，格式为 "yyyy"
String year = day.substring(0, 4); // 输出例如 "2024"

2.通常用于将集合的数据根据某个字段进行分类累加 eg：如下根据废料集合的marker来进行计算重量并整合

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List<Map<String, Object>> mapList = new ArrayList<>();
List<String> markerList = new ArrayList<>();

for (IntoWarehouse intoWarehouse : wasteList) {
    if (markerList.contains(intoWarehouse.getMarker())) {
        for (Map<String, Object> map : mapList) {
            if (map.get("marker").equals(intoWarehouse.getMarker())) {
                BigDecimal weight = intoWarehouse.getWeight().add(new BigDecimal(map.get("weight").toString()));
                map.put("weight", weight);
            }
        }
    } else {
        markerList.add(intoWarehouse.getMarker());
        Map<String, Object> map = new HashMap<>();
        map.put("marker", intoWarehouse.getMarker());
        map.put("weight", intoWarehouse.getWeight());
        mapList.add(map);
    }
}