常用代码集

一、List操作

1.1 List集合去重

public static void main(String[] args) {
        List<String> memberIds = new ArrayList<>();
        memberIds.add("A");
        memberIds.add("B");
        memberIds.add("A");

        List<String> distinctList = memberIds.stream().distinct().collect(Collectors.toList());
        
        //[A, B]
        System.out.println(distinctList);
    }

1.2 List排序

List<Student> sortedCivilStudents = students.stream()
        .filter(student -> "土木工程".equals(student.getMajor())).sorted((s1, s2) -> s1.getAge() - s2.getAge())
        .limit(2)
        .collect(Collectors.toList());

1.3 只取List集合对象的某一个字段

List<String> names = students.stream()
             .filter(student -> "计算机科学".equals(student.getMajor()))
             .map(Student::getName).collect(Collectors.toList());

1.4 快速创建List

List<Double> list1=Arrays.asList(32.5,78.3,45.6);

二、时间

2.1 毫秒转小时

public static void main(String[] args) {
        Long lastTime = 1650091356556L - 1650011356556L;
        System.out.println(TimeUnit.MILLISECONDS.toHours(lastTime));
 }

2.2 LocalDate格式化

 LocalDate now = LocalDate.now();
 DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy/MM/dd");

 String JDK8Time = now.format(dateTimeFormatter);
 System.out.println("JDK8格式化后的时间数据=="+JDK8Time);

2.3 LocalDate转String

LocalDate localDate = LocalDate.now();

DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy-MM-dd");

String dateStr = localDate.format(fmt);

System.out.println("LocalDate转String：" + dateStr);

2.4 Date转LocalDate

Date date = new Date();

System.out.println("LocalDateTime()：" + (new Date()).toInstant().atZone(ZoneId.systemDefault()).toLocalDateTime());

2.5 时间戳转LocalDateTime

long timestamp = System.currentTimeMillis();

Instant instant1 = Instant.ofEpochMilli(timestamp);

System.out.println("时间戳 转 LocalDateTime：" + LocalDateTime.ofInstant(instant1, ZoneId.systemDefault()));