JDK8 部分代码测试

淩亂°似流年 2022-08-24 11:56 49阅读 0赞

JDK8 部分代码测试

*  ## [Lambda 表达式 -][Lambda _ -] ##

(p1, p2) -> p1.firstName

（）放参数，可不带类型

\->  特定

\{ \} 方法块 或者一行代码；

`(` `int` `x,` `int` `y) -> x + y   ` `//两整形参数，返回它们的和`

`() ->` `42`                   `//无参数，直接返回 42`

`x ->` `100` `;                 ` `//可推断出参数 x 的类型`

`(String s) -> { System.out.println(s); }` `//传入一个字符，只执行一个操作，无返回值`

*  “::” 域操作符

JDK8 又重新借用了 C++ 的那个 “::” 域操作符，全称为作用域解析操作符。

*  测试代码

package com.test.jdk;
    
    import java.time.Clock;
    import java.time.DayOfWeek;
    import java.time.Instant;
    import java.time.LocalDate;
    import java.time.LocalDateTime;
    import java.time.LocalTime;
    import java.time.Month;
    import java.time.ZoneId;
    import java.time.format.DateTimeFormatter;
    import java.time.format.FormatStyle;
    import java.time.temporal.ChronoField;
    import java.time.temporal.ChronoUnit;
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.Comparator;
    import java.util.Date;
    import java.util.LinkedList;
    import java.util.List;
    import java.util.Locale;
    import java.util.UUID;
    import java.util.concurrent.TimeUnit;
    import java.util.function.Consumer;
    import java.util.function.Function;
    import java.util.function.Supplier;
    
    public class JDK8Test {
    
    	public static void main(String[] args) {
    
    		 Comparator<Person> comparator = (p1, p2) -> p1.firstName
    		 .compareTo(p2.firstName);
    		
    		 Person p1 = new Person("John", "Doe");
    		 Person p2 = new Person("Alice", "Wonderland");
    		
    		 System.out.println(comparator.compare(p1, p2)); // > 0
    		
    		 System.out.println(comparator.reversed().compare(p1, p2)); // < 0
    		
    		 // Function 接口有一个参数并且返回一个结果，
    		 // 并附带了一些可以和其他函数组合的默认方法（compose, andThen）：
    		 Function<String, Integer> toInteger = Integer::valueOf;
    		 Function<String, String> backToString = toInteger
    		 .andThen(String::valueOf);
    		
    		 backToString.apply("123"); // "123"
    		
    		 System.out.println(backToString);
    		
    		 // Supplier 接口返回一个任意范型的值，和Function接口不同的是该接口没有任何参数
    		 Supplier<Object> personSupplier = Object::new;
    		 personSupplier.get(); // new Object
    		
    		 System.out.println(personSupplier.get());
    		
    		 new Thread(() -> {
    		 for (int i = 0; i < 9; i++) {
    		 System.out.println(String.format(
    		 "Message #%d from inside the thread!", i));
    		 }
    		 }).start();
    		
    		 new Thread(() -> {
    		 System.out.println(String.format(
    		 "Message #%d from inside the thread!", 1024));
    		 }).start();
    		
    		 List inputList = new LinkedList<>();
    		 List upper = new LinkedList<>();
    		 //有问题u
    //		 inputList.stream().filter(x -> (x != null &&
    //		 x.matches("[A-Z0-9]*"))).into(upper);
    //		
    //		 int sumX = inputList.stream()
    //		 .filter(x -> (x != null && x.matches("[A-Z0-9]*")))
    //		 .map(String::length).reduce(0, Integer::sum);
    //		 //从集合中取出每个元素然后打印
    //		 List list = new ArrayList<>();
    //		 list.add("123aa");
    //		 list.add("124aa");
    //		 list.add("125aa");
    //		 list.forEach(x -> System.out.println(x));
    //		 Arrays.asList("Alice", "Bob", "Charlie",
    //		 "Dave").forEach(System.out::println);
    //		 //names.filter 有问题 啊，，怎么回事啊~！~
    //		 List<String> names = Arrays.asList("Alice", "Bob", "Charlie",
    //		 "Dave");
    //		 List<String> filteredNames = names.filter((e) -> e.length() >= 4)
    //		 .into(new ArrayList<String>());
    //		
    //		 names.filter(e -> e.length() >= 4)
    //		 .forEach(x -> System.out.println(x));
    		 //并行Streams
    		 int max = 1000000;
    		 List<String> values = new ArrayList<>(max);
    		 for (int i = 0; i < max; i++) {
    		 UUID uuid = UUID.randomUUID();
    		 values.add(uuid.toString());
    		 }
    		 //串行排序：
    		 long t0 = System.nanoTime();
    		
    		 long count = values.stream().sorted().count();
    		 System.out.println(count);
    		
    		 long t1 = System.nanoTime();
    		
    		 long millis = TimeUnit.NANOSECONDS.toMillis(t1 - t0);
    		 System.out.println(String.format("sequential sort took: %d ms",
    		 millis));
    		
    		 //并行排序：
    		 t0 = System.nanoTime();
    		 count = values.parallelStream().sorted().count();
    		 System.out.println(count);
    		 t1 = System.nanoTime();
    		 millis = TimeUnit.NANOSECONDS.toMillis(t1 - t0);
    		 System.out.println(String.format("parallel sort took: %d ms",
    		 millis));
    		 //DATE API
    		// Clock 时钟
    		Clock clock = Clock.systemDefaultZone();
    		 millis = clock.millis();
    
    		Instant instant = clock.instant();
    		Date legacyDate = Date.from(instant); // legacy java.util.Date
    		System.out.println(legacyDate);
    		
    		// Timezones 时区
    		System.out.println(ZoneId.getAvailableZoneIds());
    		// prints all available timezone ids
    
    		ZoneId zone1 = ZoneId.of("Europe/Berlin");
    		ZoneId zone2 = ZoneId.of("Brazil/East");
    		System.out.println(zone1.getRules());// ZoneRules[currentStandardOffset=+01:00]
    		System.out.println(zone2.getRules());// ZoneRules[currentStandardOffset=-03:00]
    		//LocalTime 本地时间
    		LocalTime now1 = LocalTime.now(zone1);
    		LocalTime now2 = LocalTime.now(zone2);
    
    		System.out.println(now1.isBefore(now2));  // false
    
    		long hoursBetween = ChronoUnit.HOURS.between(now1, now2);
    		long minutesBetween = ChronoUnit.MINUTES.between(now1, now2);
    
    		System.out.println(hoursBetween);       
    		System.out.println(minutesBetween);     
    		LocalTime late = LocalTime.of(23, 59, 59);
    		System.out.println(late);       // 23:59:59
    
    		DateTimeFormatter germanFormatter =
    		    DateTimeFormatter
    		        .ofLocalizedTime(FormatStyle.SHORT)
    		        .withLocale(Locale.GERMAN);
    
    		LocalTime leetTime = LocalTime.parse("13:37", germanFormatter);
    		System.out.println(leetTime);   // 13:37
    		//LocalDate 本地日期
    		LocalDate today = LocalDate.now();
    		LocalDate tomorrow = today.plus(1, ChronoUnit.DAYS);
    		LocalDate yesterday = tomorrow.minusDays(2);
    
    		LocalDate independenceDay = LocalDate.of(2014, Month.JULY, 4);
    		DayOfWeek dayOfWeek = independenceDay.getDayOfWeek();
    		System.out.println(dayOfWeek); // FRIDAY
    		//从字符串解析一个LocalDate类型和解析LocalTime一样简单：
    		 germanFormatter =
    			    DateTimeFormatter
    			        .ofLocalizedDate(FormatStyle.MEDIUM)
    			        .withLocale(Locale.GERMAN);
    
    		LocalDate xmas = LocalDate.parse("24.12.2014", germanFormatter);
    		System.out.println(xmas);   // 2014-12-24
    		//LocalDateTime 本地日期时间
    		LocalDateTime sylvester = LocalDateTime.of(2014, Month.DECEMBER, 31, 23, 59, 59);
    
    		 dayOfWeek = sylvester.getDayOfWeek();
    		System.out.println(dayOfWeek);      // WEDNESDAY
    
    		Month month = sylvester.getMonth();
    		System.out.println(month);          // DECEMBER
    
    		long minuteOfDay = sylvester.getLong(ChronoField.MINUTE_OF_DAY);
    		System.out.println(minuteOfDay);    // 1439
    		//只要附加上时区信息，就可以将其转换为一个时间点Instant对象，
    		//Instant时间点对象可以很容易的转换为老式的java.util.Date。
    		 instant = sylvester
    		        .atZone(ZoneId.systemDefault())
    		        .toInstant();
    		 legacyDate = Date.from(instant);
    		System.out.println(legacyDate);     // Wed Dec 31 23:59:59 CET 2014
    		
    		DateTimeFormatter formatter =
    			    DateTimeFormatter
    			        .ofPattern("MMM dd, yyyy - HH:mm");
    
    		LocalDateTime parsed = LocalDateTime.parse("Nov 03, 2014 - 07:13", formatter);
    		String string = formatter.format(parsed);
    		System.out.println(string);     // Nov 03, 2014 - 07:13
    		
    	}
    
    }

[Lambda _ -]: http://unmi.cc/jdk8-lambda-lexical-scoping-variable-capture/