map原理

map原理

map內(nèi)部函數(shù)接口依賴

filter原理

filter原理

filter內(nèi)部函數(shù)接口依賴

flatMap原理

flatMap原理

reduce操作累加

reduce操作累加

package org.java8.stream;

import static java.util.Arrays.asList;
import static java.util.stream.Collectors.toList;

import java.util.Comparator;
import java.util.List;
import java.util.function.BinaryOperator;
import java.util.stream.Stream;

public class StreamMethodTest {

    public static void main(String[] args) {
        //初始化list
        List<String> list = asList("one", "two", "three", "four");
        Stream.of("one", "two", "three", "four").collect(toList());

/**map:**/
        /**map功能（將一種類型數(shù)據(jù)的值轉(zhuǎn)化成另一種類型）
        //比如將小寫轉(zhuǎn)成大寫
        //注意map的函數(shù)接口是Function壤圃，一進(jìn)一出變個(gè)身**/
        List<String> mapList = Stream.of("one", "two", "three", "four")
            .map(value->value.toUpperCase())
            .collect(toList());
        //forEach內(nèi)部函數(shù)接口是Consumer念链，只進(jìn)不出消化了
        mapList.forEach(x->System.out.println(x));

/**filter:**/   
        /**遍歷對(duì)數(shù)據(jù)進(jìn)行檢查用filter我磁，一進(jìn)出來(lái)成boolean。其實(shí)就是for里面的if語(yǔ)句
        filter內(nèi)部的函數(shù)接口是Predicate**/      
        List<String> filterList = Stream.of("one", "two", "three", "four")
                .filter(x->x.equalsIgnoreCase("one"))
                .collect(toList());
        filterList.forEach(x->System.out.println(x));

/**flatMap:**/
        //有時(shí)：對(duì)map中的值用新的stream來(lái)替換他
        List<Integer> together = Stream.of(asList(1, 2), asList(3, 4))
                .flatMap(numbers -> numbers.stream())
                .collect(toList());
        together.forEach(x->System.out.print(x));
        System.out.println();
        
/**reduce:**/
        /** reduce操作可以實(shí)現(xiàn)從一組值中生成一個(gè)值
            如何通過(guò)reduce操作對(duì)Stream中的數(shù)字求和浮定。以0作起點(diǎn)——一個(gè)空流 
            Stream的求和結(jié)果，每一步都將Stream中的元素累加至accumulator，遍歷至Stream中的
            最后一個(gè)元素時(shí)，accumulator的值就是所有元素的和燎含。**/
        /**BinaryOperator reduce包含初始值和BinaryOperator函數(shù)接口**/
        int count = Stream.of(1, 2, 3)
                .reduce(0, (acc, element) -> acc + element);
        System.out.println(count);
        
        /**展開(kāi)過(guò)程**/
        BinaryOperator<Integer> accumulator = (acc, element) -> acc + element;
        int anotherCount = accumulator.apply(
                    accumulator.apply(
                        accumulator.apply(0, 1),
                    2),
                3);
        System.out.println(anotherCount);
        
        /**等同功能**/
        int acc = 0;
        for (Integer element : asList(1, 2, 3)) {
            acc = acc + element;
        }
        System.out.println(acc);
        
/**max和min: 其實(shí)包含了reduce功能：reduce(BinaryOperator.minBy(comparator));**/
        List<Track> tracks = asList(new Track("Bakai", 524),
                                    new Track("Violets for Your Furs", 378),
                                    new Track("Time Was", 451));
        Track shortestTrack = tracks.stream()
                                .min(Comparator.comparing(track -> track.getLength()))
                                .get();
        System.out.println(shortestTrack);
    }
    
    static class Track {
        String name;
        int length;
        
        public Track(String name, int length) {
            this.name = name;
            this.length = length;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getLength() {
            return length;
        }

        public void setLength(int length) {
            this.length = length;
        }

        @Override
        public String toString() {
            StringBuilder builder = new StringBuilder();
            builder.append("Track [name=").append(name).append(", length=")
                    .append(length).append("]");
            return builder.toString();
        }
        
    }
}