java8 practical note, bit by bit

1, java8 time use, really good, highly recommended, the calculation of time, for example, want to know how many days before the current time, the last day of each month, such as addition and subtraction operations on time and very convenient. But the use of the process (so far), if the backend receives eg time with LocalDate / LocaDateTime: private LocalDate contractEndTime may experience some incompatible, it can be used, for example, can not easily copy or compatibility issues with other versions ) .

// 说明：均为示例片段
**java8的时间格式化**
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
DateTimeFormatter olddtf = DateTimeFormatter.ofPattern("yyyy-MM");
LocalDateTime dts = LocalDateTime.parse(“2020-03-04 23:59:59”, dtf);

*获取当前时间*
LocalDate ld = LocalDate.now();

*当前时间减去1天(昨天)*
ld = ld.minusDays(1);

*格式化时间变成String*
String endDate = ld.format(DateTimeFormatter.ofPattern("yyyy-MM-dd"));

*上个月第一天*
String lastMonthOfFirstDay = ld.minusMonths(1).with(TemporalAdjusters.firstDayOfMonth())
					.format(DateTimeFormatter.ofPattern("yyyy-MM-dd"))
					

2, java8 for collection on the intersection, union, difference, and the like very convenient and practical to weight.

// 只是示例片段
*取差集（lastMonthProjectList比thisMonthProjectList大的差集，谁在前面表示谁减），Set同理*
List<String> xList = lastMonthProjectList.stream().filter(i -> !thisMonthProjectList.contains(i))
					.collect(Collectors.toList());
					
*取交集*
List<String> rList = bl.stream().filter(i -> xList.contains(i)).collect(Collectors.toList());
			list.addAll(rList);
			
*通过distinct去重*
List<String> list = list.stream().distinct().collect(Collectors.toList());

*对集合进行map（重新组装）操作*
List<Node> ll = list.stream().map(nf -> {
                	String nodeId = nf.getNodeId();
                	Org org = orgRestful.getOrgById(nodeId);
                	Node node = org2Node(org);
                return node;
         }).collect(toList());
                 
*通过filter对集合进行过滤操作（很常规操作）*
List<CcpVirtualTreeOperateLogs> ll = ll.stream().filter(log -> log.getParentFullId().contains(parentId)).collect(toList());

*java8 的函数（Function）运用---java8的4大函数之一*
Node node = this.fetchNode(log.getNodeId(), f -> org2Node(this.orgRestful.getOrgById(f)));
private Node fetchNode(String id, Function<String, Node> function) {
    return function.apply(id);
}

*组装集合数据到map中*
Map<String, String> resourceMap = resourceList.stream()
				.collect(Collectors.toMap(Resource::getResourceId, Resource::getName));
*根据集合对象某字段进行去重，然后获取新的该集合*
List<NcCfgImportExcelForm> distinctList = list.stream()
		.collect(Collectors.collectingAndThen(
			Collectors.toCollection(
					() -> new TreeSet<>(Comparator.comparing(NcCfgImportExcelForm::getAccountId))),
						ArrayList::new));
						
*通过groupingBy进行分组去重*
int s = list.stream().collect((Collectors.groupingBy(NcCfgImportExcelForm::getProjectId))).size();
if (s != 1) {
	throw new BaseException("每次导入只能是一个项目，即项目id应该全部相同");
}