1、计算时间间隔
Java8之前，我们想要确定一个方法的运行时间长度，可以这样：
long start = System.currentTimeMillis();
doSomething();
long end = System.currentTimeMillis();
System.out.println(end-start);
Java8中，可以这样
Instant start = Instant.now();
doSomething();
Instant end = Instant.now();
Duration time = Duration.between(start, end);
long seconds = time.getSeconds();//秒表示
long millis = time.toMillis();//毫秒表示
System.out.println(seconds);
System.out.println(millis);
可以轻松选择用纳秒、毫秒、秒、分钟、小时或者天来表示时间间隔的单位。
可以这样来比较第一个方法是不是比第二个执行得更快：
Instant start = Instant.now();
doSomething();
Instant end = Instant.now();
Duration time = Duration.between(start, end);
Instant start2 = Instant.now();
doSomething2();
Instant end2 = Instant.now();
Duration time2 = Duration.between(start2, end2);
boolean fast = time.minus(time2).isNegative();
System.out.println(fast);
2、本地日期和时间
#
LocalDate now = LocalDate.now();
LocalDate date = LocalDate.of(2005, 5, 10);
LocalDate date2 = LocalDate.of(2003, Month.FEBRUARY, 5);
System.out.println(now);
System.out.println(date);
System.out.println(date2);
LocalTime now2 = LocalTime.now();
LocalTime time = LocalTime.of(22, 50, 56);
System.out.println(now2);
System.out.println(time);
3、日期格式化
#
//按照内置的不同方式格式化
String format = DateTimeFormatter.ISO_LOCAL_DATE.format(LocalDate.now());
String format2 = DateTimeFormatter.ISO_LOCAL_TIME.format(LocalTime.now());
String format3 = DateTimeFormatter.ISO_DATE.format(LocalDateTime.now());
String format4 = DateTimeFormatter.ISO_INSTANT.format(Instant.now());
System.out.println(format);
System.out.println(format2);
System.out.println(format3);
System.out.println(format4);
//按照标准格式格式化
DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.FULL);
String format5 = formatter.format(LocalDateTime.now());
System.out.println(format5);
//按照指定方式格式化
DateTimeFormatter pattern = DateTimeFormatter.ofPattern("yyyy-MM-dd E HH:mm:ss");
String format6 = pattern.format(LocalDateTime.now());
System.out.println(format6);