Lambda表達式與Stream流（終）

時間 2019-11-16

原文原文鏈接

package com.java.design.java8;

import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.time.*;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoField;
import java.util.*;
import java.util.concurrent.CompletableFuture;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

/**
 * @author  陳楊
 * 
 */
 
@SpringBootTest
@RunWith(SpringRunner.class)
public class LambdaInfo {

1、Lambda表達式與Stream流

/*
    A lambda expression can be understood as a concise representation of an anonymous function
        that can be passed around: it doesn’t have a name,
        but it has a list of parameters, a body, a return type,
        and also possibly a list of exceptions that can be thrown.
            That’s one big definition;

    let’s break it down:
    Anonymous:
        We say anonymous because it doesn’t have an explicit name like a method would normally have: less to write and think about!
    Function:
        We say function because a lambda isn’t associated with a particular class like a method is.
        But like a method, a lambda has a list of parameters, a body, a return type,
        and a possible list of exceptions that can be thrown.
    Passed around:
        A lambda expression can be passed as argument to a method or stored in a variable.
    Concise:
        You don’t need to write a lot of boilerplate like you do for anonymous classes.

*/

/*
    Stream :  A sequence of elements from a source that supports data processing operations.
        Sequence of elements
        Source
        Pipelining
        Internal iteration
        Traversable only once
        Collections: external interation using an interator behind the scenes

*/

2、初始化測試數據

private List<Integer> list;

@Before
public void init() {

    list = IntStream.rangeClosed(1, 100).boxed().collect(Collectors.toList());

    list.sort(Collections.reverseOrder());
}

3、各類API

1.allMatchhtml

@Test

public void testLambdaInfo() {


    System.out.println(">---------------------Match方法----------------------<");
       //  1、Match方法
        //  Returns whether all elements of this stream match the provided predicate.
        Optional.of(list.stream().mapToInt(Integer::intValue).allMatch(i -> i > 0))
                .ifPresent(System.out::println);

        //  Returns whether any elements of this stream match the provided predicate.
        Optional.of(list.stream().mapToInt(Integer::intValue).anyMatch(i -> i > 0))
                .ifPresent(System.out::println);

        //  Returns whether no elements of this stream match the provided predicate..
        Optional.of(list.stream().mapToInt(Integer::intValue).noneMatch(i -> i > 0))
                .ifPresent(System.out::println);

二、findjava

System.out.println(">--------------------Find方法-----------------------<");

//  2、Find方法
//  Returns an  Optional describing the first element of this stream,
//  or an empty Optional if the stream is empty.
//  If the stream has no encounter order, then any element may be returned.
list.stream().mapToInt(Integer::intValue).filter(i -> i > 10).findFirst()
        .ifPresent(System.out::println);

//  Returns an Optional describing some element of the stream, or an empty Optional if the stream is empty.
list.stream().mapToInt(Integer::intValue).filter(i -> i > 10).findAny()
        .ifPresent(System.out::println);

三、reducespring

System.out.println(">---------------------Reduce方法----------------------<");

//  3、Reduce方法
//  Performs a reduction on the elements of this stream, using the provided identity value
//      and an associative accumulation function, and returns the reduced value.

//  求和
System.out.println(list.stream().reduce(0, Integer::sum));
list.stream().mapToInt(Integer::intValue).reduce(Integer::sum)
        .ifPresent(System.out::println);


//  求最大值
System.out.println(list.stream().reduce(0, Integer::max));
list.stream().mapToInt(Integer::intValue).reduce(Integer::max)
        .ifPresent(System.out::println);


//  求最小值
System.out.println(list.stream().reduce(0, Integer::min));
list.stream().mapToInt(Integer::intValue).reduce(Integer::min)
        .ifPresent(System.out::println);
               System.out.println(">-------------------------------------------<");

    }

四、CompletableFuture APIexpress

@Test
public void testCompletableFuture() {

    //  4、CompletableFuture API
    /*
     * Returns a new CompletableFuture that is asynchronously completed by a task
     * running in the given executor with the value obtained by calling the given Supplier.
     */
    CompletableFuture.supplyAsync(list.stream().mapToInt(Integer::intValue)::sum, System.out::println);


    Optional.of(CompletableFuture.supplyAsync(list.stream().mapToInt(Integer::intValue)::sum)
            .complete(55)).ifPresent(System.out::println);

    //  thenAccept 無返回值  Consumer<? super T> action
    CompletableFuture.supplyAsync(list.stream().mapToInt(Integer::intValue)::sum)
            .thenAccept(System.out::println);

    //  thenApply  有返回值  Function<? super T,? extends U> fn
    CompletableFuture.supplyAsync(() -> list.stream().mapToInt(Integer::intValue))
            .thenApply(IntStream::sum).thenAccept(System.out::println);

    //  對元素及異常進行處理  BiFunction<? super T, Throwable, ? extends U> fn
    CompletableFuture.supplyAsync(() -> list.stream().mapToInt(Integer::intValue))
            .handle((i, throwable) -> "handle：\t" + i.sum()).thenAccept(System.out::println);

    //  whenCompleteAsync 完成時執行 BiConsumer<? super T, ? super Throwable> action
    CompletableFuture.supplyAsync(list.stream().mapToInt(Integer::intValue)::sum)
            .whenCompleteAsync((value, throwable) -> System.out.println("whenCompleteAsync：\t" + value));

    //   組合CompletableFuture 將前一個結果做爲後一個輸入參數 （參照 組合設計模式）
    CompletableFuture.supplyAsync(() -> list.stream().mapToInt(Integer::intValue))
            .thenCompose(i -> CompletableFuture.supplyAsync(i::sum)).thenAccept(System.out::println);

    //   合併CompletableFuture
    CompletableFuture.supplyAsync(list.stream().mapToInt(Integer::intValue)::sum)
            .thenCombine(CompletableFuture.supplyAsync(() -> list.stream()
                    .mapToDouble(Double::valueOf).sum()), Double::sum).thenAccept(System.out::println);

    //   合併CompletableFuture
    CompletableFuture.supplyAsync(list.stream().mapToInt(Integer::intValue)::sum)
            .thenAcceptBoth(CompletableFuture.supplyAsync(list.stream()
                            .mapToDouble(Double::valueOf)::sum),
                    (r1, r2) -> System.out.println("thenAcceptBoth：\t" + r1 + "\t" + r2));

    //  2個CompletableFuture運行完畢後運行Runnable
    CompletableFuture.supplyAsync(() -> {
        System.out.println(Thread.currentThread().getName() + "\tis running");
        return list.stream().mapToInt(Integer::intValue).sum();
    })
            .runAfterBoth(
                    CompletableFuture.supplyAsync(() -> {
                        System.out.println(Thread.currentThread().getName() + "\tis running");
                        return list.stream().mapToDouble(Double::valueOf).sum();
                    }),
                    () -> System.out.println("The 2 method have done"));


    //  2個CompletableFuture 有一個運行完就執行Runnable
    CompletableFuture.supplyAsync(() -> {
        System.out.println(Thread.currentThread().getName() + "\tis running");
        return list.stream().mapToInt(Integer::intValue).sum();
    })
            .runAfterEither(
                    CompletableFuture.supplyAsync(() -> {
                        System.out.println(Thread.currentThread().getName() + "\tis running");
                        return list.stream().mapToDouble(Double::valueOf).sum();
                    }),
                    () -> System.out.println("The 2 method have done"));


    //  2個CompletableFuture 有一個運行完就執行Function<? super T, U> fn
    CompletableFuture.supplyAsync(
            list.stream().mapToInt(Integer::intValue).max()::getAsInt)
            .applyToEither(
                    CompletableFuture.supplyAsync(list.stream().mapToInt(Integer::intValue).min()::getAsInt)
                    , v -> v * 10)
            .thenAccept(System.out::println);

    //  2個CompletableFuture 有一個運行完就執行Consumer<? super T> action
    CompletableFuture.supplyAsync(
            list.stream().mapToInt(Integer::intValue).max()::getAsInt)
            .acceptEither(
                    CompletableFuture.supplyAsync(list.stream().mapToInt(Integer::intValue).min()::getAsInt)
                    , System.out::println);

    //  將集合中每個元素都映射成爲CompletableFuture<Integer>對象
    List<CompletableFuture<Integer>> collect =
            list.stream().map(i -> CompletableFuture.supplyAsync(i::intValue))
                    .collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
    //  集合轉數組
    CompletableFuture[] completableFutures = collect.toArray(CompletableFuture[]::new);

    //  有一個task執行完畢
    CompletableFuture.anyOf(completableFutures)
            .thenRun(() -> System.out.println("有一個task執行完畢--->first done"));
    //  有且僅有全部task執行完畢
    CompletableFuture.allOf(completableFutures)
            .thenRun(() -> System.out.println("有且僅有全部task執行完畢--->done"));

}

五、Java.time API設計模式

@Test
    public void testLocalDateTime() {

        //  5、Java.time API
        LocalDate localDate = LocalDate.of(2019, 12, 1);

        //  當前時間
        Optional.of(LocalDate.now()).ifPresent(System.out::println);

        //  年份
        Optional.of(localDate.getYear()).ifPresent(System.out::println);
        OptionalInt.of(localDate.get(ChronoField.YEAR)).ifPresent(System.out::println);

        //  月份 (Jan-->Dec)
        Optional.of(localDate.getMonth()).ifPresent(System.out::println);


        //  月份(1-->12)
        Optional.of(localDate.getMonthValue()).ifPresent(System.out::println);
        OptionalInt.of(localDate.get(ChronoField.MONTH_OF_YEAR)).ifPresent(System.out::println);


        //  年中的第幾天
        Optional.of(localDate.getDayOfYear()).ifPresent(System.out::println);
        OptionalInt.of(localDate.get(ChronoField.DAY_OF_YEAR)).ifPresent(System.out::println);


        //  月中的第幾天
        Optional.of(localDate.getDayOfMonth()).ifPresent(System.out::println);
        OptionalInt.of(localDate.get(ChronoField.DAY_OF_MONTH)).ifPresent(System.out::println);

        //  星期幾(Mon-->Sun)
        Optional.of(localDate.getDayOfWeek()).ifPresent(System.out::println);

        //  星期幾(1-->7)
        OptionalInt.of(localDate.get(ChronoField.DAY_OF_WEEK)).ifPresent(System.out::println);

        //  時代（公元前、後） CE BCE
        Optional.of(localDate.getEra()).ifPresent(System.out::println);

        //  時代（公元前、後） 1--->CE 0--->BCE
        Optional.of(localDate.getEra().getValue()).ifPresent(System.out::println);
        OptionalInt.of(localDate.get(ChronoField.ERA)).ifPresent(System.out::println);

        //  ISO年表
        Optional.of(localDate.getChronology().getId()).ifPresent(System.out::println);

        //  當前時間
        LocalTime time = LocalTime.now();

        //   時
        OptionalInt.of(time.getHour()).ifPresent(System.out::println);
        OptionalInt.of(time.get(ChronoField.HOUR_OF_DAY)).ifPresent(System.out::println);

        //   分
        OptionalInt.of(time.getMinute()).ifPresent(System.out::println);
        OptionalInt.of(time.get(ChronoField.MINUTE_OF_DAY)).ifPresent(System.out::println);

        //   秒
        OptionalInt.of(time.getSecond()).ifPresent(System.out::println);
        OptionalInt.of(time.get(ChronoField.SECOND_OF_DAY)).ifPresent(System.out::println);

        //   納秒
        OptionalInt.of(time.getNano()).ifPresent(System.out::println);
        OptionalLong.of(time.getLong(ChronoField.NANO_OF_SECOND)).ifPresent(System.out::println);

        //  中午時間
        Optional.of(LocalTime.NOON).ifPresent(System.out::println);

        //  午夜時間
        Optional.of(LocalTime.MIDNIGHT).ifPresent(System.out::println);

        //  自定義格式化時間
        DateTimeFormatter customDateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss E");
        LocalDateTime localDateTime = LocalDateTime.of(localDate, time);
        Optional.of(localDateTime.format(customDateTimeFormatter)).ifPresent(System.out::println);

        //   根據傳入的文本匹配自定義指定格式進行解析
        Optional.of(LocalDateTime.parse("2019-12-25 12:30:00 週三", customDateTimeFormatter))
                .ifPresent(System.out::println);

        //   時間點 Instant
        Instant start = Instant.now();
        try {
            Thread.sleep(10_000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        Instant end = Instant.now();

        //    Duration 時間段
        Duration duration = Duration.between(start, end);
        OptionalLong.of(duration.toNanos()).ifPresent(System.out::println);

        //   Period  時間段
        Period period = Period.between(LocalDate.now(), localDate);

        OptionalInt.of(period.getYears()).ifPresent(System.out::println);
        OptionalInt.of(period.getMonths()).ifPresent(System.out::println);
        OptionalInt.of(period.getDays()).ifPresent(System.out::println);

        //   The Difference Between Duration And Period
        //   Durations and periods differ in their treatment of daylight savings time when added to ZonedDateTime.
        //   A Duration will add an exact number of seconds, thus a duration of one day is always exactly 24 hours.
        //   By contrast, a Period will add a conceptual day, trying to maintain the local time.

    }


}

4、備註

一、Why do we need a new date and time library?

https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html

二、java.time API

https://docs.oracle.com/javase/8/docs/api/java/time/package-summary.html