---

前言

常用語法匯總

一、lambda

1. 排序

1.1 按照對象屬性排序：

List<AccidentEduSLResp> respList =list.stream().sorted(Comparator.comparing(AccidentEduSLResp::getOrgUnit)).collect(Collectors.toList());
district.sort(Comparator.comparing(AccidentEduSLResp::getOrgUnit));  //性能優

1.2 字符串List排序：

List<String> sortedDistrict = district.stream().sorted().collect(Collectors.toList());

1.3 數據庫排序jpa

dao.findAll(spec, Sort.by(Sort.Order.desc(ExpertConstants.UPDATE_TIME), Sort.Order.asc(ExpertConstants.EXAMINE_STATUS)));

2. 聚合

2.1 基本聚合（返回對象list）

Map<String, List<AccidentSupervisePO>> collect = cityList.stream().collect(Collectors.groupingBy(s -> s.getDistrictCode()));

2.2 多字段組合聚合（直接返回對象list數量）

Map<String, Long> collect = list.stream().collect(Collectors.groupingBy(o -> o.getDistrictCode() + "_" + o.getOrgUnit(), Collectors.counting()));

二、基礎語法

2.1 List

2.1.1 數組初始化賦值

Arrays.asList 不允許add remove操作 UnsupportedOperationException

public static final List CITY_ARR = Arrays.asList("230100","230200","230300","230400","230500","230600");

需要add等操作需要以下寫法支持

List<String> list = new ArrayList<>(Arrays.asList(arr));

2.1.2. 逗號分割字符串、list互轉

List 轉字符串：

Joiner.on(",").join(list)

字符串轉List:

//-> 引入guava-28.2-jre.jar//-> CommonConstants常量類定義
public static final Splitter SPLITTER_COMMA = Splitter.on(",").omitEmptyStrings().trimResults();
//需要判斷字符串是否為空
List<String> list = CommonConstants.SPLITTER_COMMA.splitToList(a.getDistrictCode());

2.1.3 去重

用hashset去重list 性能優

List<String> testListDistinctResult = new ArrayList<>(new HashSet(testList));

2.2. Json解析

2.2.1 Gson

Book b = new Book("書名1","簡介1");
//使用gson將對象轉為json字符串
String json = new Gson().toJson(b);
System.out.println(json);//使用gson將json字符轉轉為對象(第一個參數為json字符串，第二個參數為要轉為的類)
Book b2 = new Gson().fromJson("{\\"name\\":\\"書名1\\",\\"info\\":\\"簡介1\\"}",Book.class);

2.2.2 Fastjson

Book b = new Book("書名2","簡介2");
//使用fastjson將對象轉為json字符串
String json= JSON.toJSONString(b);
System.out.println(json);//使用fastjson將json字符轉轉為對象(第一個參數為json字符串，第二個參數為要轉為的類)
Book b2 = JSON.parseObject("{\\"name\\":\\"書名1\\",\\"info\\":\\"簡介1\\"}", Book.class);

2.3. LocalDateTime

2.3.1 String->LocalDateTime

//1.具有轉換功能的對象
DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
//2.要轉換的對象	
LocalDateTime time = LocalDateTime.now();//3.發動功能
String localTime = df.format(time);
System.out.println("LocalDateTime轉成String類型的時間："+localTime);//3.LocalDate發動，將字符串轉換成  df格式的LocalDateTime對象，的功能
LocalDateTime LocalTime = LocalDateTime.parse(strLocalTime,df)
System.out.println("String類型的時間轉成LocalDateTime："+LocalTime);

2.3.2 Date->LocalDateTime

//Date轉LocalDateTime 
Date date = new Date();
Instant instant = date.toInstant();
ZoneId zoneId = ZoneId.systemDefault();
LocalDateTime localDateTime = instant.atZone(zoneId).toLocalDateTime();
System.out.println("Date = " + date);
System.out.println("LocalDateTime = " + localDateTime);//LocalDateTime轉Date 
ZoneId zoneId = ZoneId.systemDefault();
LocalDateTime localDateTime = LocalDateTime.now();
ZonedDateTime zdt = localDateTime.atZone(zoneId);
Date date = Date.from(zdt.toInstant());
System.out.println("LocalDateTime = " + localDateTime);
System.out.println("Date = " + date);

2.4. Optional

2.4.1 map/flatMap

● map適用于基礎數據類型
● flatMap適用于對象類型
user不為空的時候取address address為空取city city為空異常

Optional.ofNullable(user).map(u-> u.getAddress()).map(a->a.getCity()).orElseThrow(()->new Exception("錯誤"));

2.4.2 ifPresent

user不為空時dosomething

Optional.ofNullable(user)
.ifPresent(u->{dosomething(u);
});

2.4.3 filter

如果user的name的是zhangsan的，則返回當前對象。否則返回構造的user對象。

Optional.ofNullable(user)
.filter(u->"zhangsan".equals(u.getName()))
.orElseGet(()-> {User user1 = new User();user1.setName("zhangsan");return user1;
});

2.5. EsayExcel

2.5.1 導出示例

response.setContentType("application/octet-stream");
response.setCharacterEncoding("utf-8");
// 這里URLEncoder.encode可以防止中文亂碼 當然和easyExcel沒有關系
String fileName = URLEncoder.encode("專家報銷匯總" + DateUtil.localDateToString(LocalDate.now()), "UTF-8");
response.setHeader("Content-Disposition", "attachment; filename=" + fileName + ".xlsx");
// 前端需要FileName頭否則會會有問題
response.setHeader("FileName", fileName + ".xlsx");
response.setHeader("Access-Control-Expose-Headers", "FileName");
WriteCellStyle contentWriteCellStyle = new WriteCellStyle();
contentWriteCellStyle.setHorizontalAlignment(HorizontalAlignment.CENTER);
WriteCellStyle headWriteCellStyle = new WriteCellStyle();
HorizontalCellStyleStrategy horizontalCellStyleStrategy =
new HorizontalCellStyleStrategy(headWriteCellStyle, contentWriteCellStyle);
EasyExcel//將數據映射到DownloadDTO實體類并響應到瀏覽器
.write(response.getOutputStream(), ExpertAndCostInfoOutDTO.class)
//07的excel版本,節省內存
.excelType(ExcelTypeEnum.XLSX)
//是否自動關閉輸入流
.autoCloseStream(Boolean.TRUE)
.registerWriteHandler(horizontalCellStyleStrategy)
.sheet("專家報銷匯總").doWrite(findExpertAndCostGatherList(dto).getExpertList());

2.5.2 相關注解

//不映射excel
@ExcelIgnore    //列寬 class上控制全部字段  屬性上控制該屬性字段
@ColumnWidth(40)  //2.1.4 表頭名稱及排序   
//@ExcelProperty(order = 2)  2.2.7 排序寫法 index被兼容
@ExcelProperty(value = "任務內容", index = 1)

三、 Linux命令

3.1 磁盤爆滿占用情況

df -h
du -h --max-depth=1

3.2 nacos單機啟動命令

sh startup.sh -m standalone

3.3 防火墻狀態查詢及操作

systemctl stop firewalld
systemctl status firewalld
systemctl disable firewalld

3.4 清理緩存（buff/cache）

清除前后使用 free -h 查看效果

free -h
sysctl -w vm.drop_caches=3
free -h