java常用技巧

list按字母排序

List<String> list = Arrays.asList("dfg", "dafff", "dfe", "bde", "dbffg");

方式一

list.sort((a, b) -> a.compareTo(b)); list.forEach(System.out::println);

方式二

list.stream().sorted().forEach(System.out::println);

以下方法返回list

list = list.stream().sorted().collect(Collectors.toList());

list通过“,”隔开成字符串

Optional<String> reduced = list.stream().reduce((s1, s2) -> s1 + "#" + s2); reduced.ifPresent(System.out::println); String key = reduced.get(); System.out.println(key);

计算时间

long t0 = System.nanoTime(); long count = list.stream().sorted().count(); long t1 = System.nanoTime(); long millis = TimeUnit.NANOSECONDS.toMillis(t1 - t0); System.out.println(String.format("sequential sort took: %d ms", millis));

遍历Map

Map<Integer, String> map = new HashMap<>(); for (int i = 0; i < 10; i++) { map.put(i, "val" + i); } map.forEach((id, val) -> System.out.println(id+","+val));

时间转化

LocalDateTime localDateTime = LocalDateTime.now(); DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd hh:mm:ss"); String format = localDateTime.format(formatter); DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd hh:mm:ss"); LocalDate localDate = LocalDate.parse("2020-06-30 21:51:43",formatter);

stream-filter 过滤

lsit.stream().filter((s) -> s.startsWith("b")).forEach(System.out::println);

stream-Map 映射转大写

list.stream().map(String::toUpperCase).forEach(System.out::println);