java 8新特性2 Stream、新的时间API

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  lambda 表达式简写。

public class Test {
	public static void main(String[] args) {
		// 消费型
		Consumer<Integer> con = s->System.out.println(s);
		
		//简化
		Consumer<Integer> con2 = System.out::println;
		con2.accept(666);
		
		// 比较
		Comparator<Integer> comparator = new Comparator<Integer>() {

			@Override
			public int compare(Integer o1, Integer o2) {
				return Integer.compare(o1, o2);
			}
		};

		//简化 ::静态方法
		Comparator<Integer> comp = Integer::compare;

		// 函数型
		Function<String, String> function = s->s.toUpperCase();
		
		//简化 ::实例方法
		Function<String, String> function = String::toUpperCase;
		
		// 供给型
		Supplier<String> supplier = ()->new String();
			
		//简化 ::new
		Supplier<String> supplier = String::new;
	}
}

  Stream，类似集合，只不过集合存数据，流存的是操作，处理中间过程。
  创建Stream对象。集合对象调用 stream 和parallelStream 方法；Arrays 接口调用 stream 方法；Stream 接口调用 of、iterate、generate 方法；IntStream 流的创建，IntStream 接口的 of、range、rangeClose 方法。

//
public class Person {
	private String name;
	private int    age;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	public Person(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}
	public Person() {
		super();
		// TODO Auto-generated constructor stub
	}
	@Override
	public String toString() {
		return "Person [name=" + name + ", age=" + age + "]";
	}
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Person other = (Person) obj;
		if (age != other.age)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}
}

public class StreamTest {
	public static void main(String[] args) {
		List<Person> list = new ArrayList<Person>();
		list.add(new Person("张三", 66));
		list.add(new Person("李四", 25));
		list.add(new Person("王五", 25));
		list.add(new Person("赵六", 25));
		
		//forEach :循环打印 1
		list.stream().forEach(System.out::println);

		//多线程打印
		list.parallelStream().forEach(System.out::println);
		
		String[] as = {"zs","ls"};
		//循环打印 2
		Arrays.stream(as).forEach(System.out::println);
		
		// 	循环打印 3
		Stream.of("sunwokong","zhubajie").forEach(System.out::println);
		
		//迭代3次，打印 5 7 9
		Stream.iterate(3, x->x+2).limit(3).forEach(System.out::println);
		
		Stream.generate(()->new Random().nextInt(5)).limit(3).forEach(System.out::println);
	
		//3, 6, 8
		IntStream.of(3,6,8).forEach(System.out::println);
		
		//3, 4
		IntStream.range(3, 5).forEach(System.out::println);
		
		//3, 4, 5
		IntStream.rangeClosed(3, 8).forEach(System.out::println);
	}
}

  中间过程操作，filter、limit、skip，dintinct(重写 hashCode 和 equals)，sorted，map(映射)，parallel(多线程)。

public class StreamTest2 {
	public static void main(String[] args) {
		
		List<Person> list = new ArrayList<Person>();
		list.add(new Person("张三", 66));
		list.add(new Person("李四", 25));
		list.add(new Person("王五", 25));
		list.add(new Person("赵六", 25));
		list.add(new Person("赵六", 35));
		
		//过滤：filter
		list.stream().filter(p->p.getAge()>30).forEach(System.out::println);
		
		//限制集合个数：limit
		list.stream().limit(3).forEach(System.out::println);
		
		//跳过个数
		list.stream().skip(1).forEach(System.out::println);
		
		//去重：distinct:重写hashCode和equals
		list.stream().distinct().forEach(System.out::println);
		
		//排序
		list.stream().sorted((o1,o2)->o1.getAge()-o2.getAge()).forEach(System.out::println);
		
		//匹配映射规则：map
		list.stream().map(p->p.getName()).forEach(System.out::println);
		
		//多线程执行：parallel
		list.stream().parallel().forEach(System.out::println);
	}
}

  最终结果 forEach、min、max、count、reduce (汇总)、collect (收集-转 List和 Set)。

public class StreamTest3 {
	public static void main(String[] args) {
		
		//获取最小的对象:min
		System.out.println(list.stream().min((o1,o2)->o1.getAge()-o2.getAge()));  
		
		//获取最大的对象:max
		System.out.println(list.stream().max((o1,o2)->o1.getAge()-o2.getAge())); 
		
		//个数
		System.out.println(list.stream().count());  
		
		//reduce：规约，汇总--转成存整数的流，再汇总
		Optional<Integer> o3 = list.stream().map(p->p.getAge()).reduce((o1,o2)->o1+o2);
		// 141
		System.out.println(o3.get());
		
		//collect:收集
		List<String> list2 = list.stream().map(p->p.getName()).collect(Collectors.toList());
		System.out.println(list2);
	}
}

  新的时间 API。
  之前的时间 API 存在着线程不安全，在多线程中用新时间API取代。

public class DateTest {
	public static void main(String[] args) throws InterruptedException, ExecutionException {
		ExecutorService es = Executors.newFixedThreadPool(10);
		
		//SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
		
		DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyyMMdd");
		
		Callable<LocalDate> callable = new Callable<LocalDate>() {
			@Override
			public LocalDate call() throws Exception { //10个线程
				Thread.sleep(9);
				
				//return sdf.parse("20200703");
				
				return LocalDate.parse("20200703", dtf);  //此处解决线程安全
			}
		};
		List<Future<LocalDate>> list = new ArrayList<>();
		for(int i=0;i<10;i++) {
			Future<LocalDate> future = es.submit(callable);
			list.add(future);
		}
		
		for(Future<LocalDate> future : list) {
			System.out.println(future.get());  //遍历打印
		}
		es.shutdown();
	}
}

  本地化日期时间，LocalDate、LocalTime、LocalDateTime。时间戳，Instant。时区，ZoneId。

public class DateTest2 {
	public static void main(String[] args) throws InterruptedException, ExecutionException {
		// 日期
		LocalDate localDate = LocalDate.now();
		System.out.println(localDate);
		
		// 时间
		LocalTime localTime = LocalTime.now();
		System.out.println(localTime);
		
		// 日期 + 时间
		LocalDateTime localDateTime = LocalDateTime.now();
		System.out.println(localDateTime);
		
		System.out.println(localDateTime.getYear());
		
		// 时间戳
		Instant instant = Instant.now();
		System.out.println(instant);
		
		//系统默认时区
		ZoneId zoneId = ZoneId.systemDefault();  
		System.out.println(zoneId);
	}
}

  日期间的格式转换，Date、Instant、LocalDateTime转换。格式化日期类，DateTimeFormat。

public class DateTest3 {
	public static void main(String[] args) {
		// Date->Instant->LocalDateTime
		Date date = new Date();
		Instant instant = date.toInstant();
		System.out.println(instant);
		LocalDateTime ldt = instant.atZone(ZoneId.systemDefault()).toLocalDateTime();
		System.out.println(ldt);
		
		// LocalDateTime->Instant->Date
		Instant instant2 = ldt.atZone(ZoneId.systemDefault()).toInstant();
		System.out.println(instant2);
		Date date2 = Date.from(instant2);
		System.out.println(date2);
		
		DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
		String strDate = dateTimeFormatter.format(LocalDateTime.now());
		System.out.println(strDate);
	}
}

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本文地址：https://blog.csdn.net/liu1shi/article/details/110825665