組合收集器

1.代碼

package org.java8.collector;

import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.toList;

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Stream;

import org.java8.vo.Album;
import org.java8.vo.Artist;
import org.java8.vo.Track;

public class MultiCollector {

    /**逆向統(tǒng)計，根據(jù)值來統(tǒng)計key或者整個對象**/
    public static void main(String[] args) {
        Artist tinygao2 = new Artist("tinygao2", "zh");
        Artist tinygao1 = new Artist("tinygao1", "zh");
        
        Album album1 = new Album(
                                "album1", 
                                Arrays.asList(new Track("tinygao", 1)), 
                                Arrays.asList(
                                        tinygao2,
                                        tinygao1
                                ));
        
        Album album2= new Album(
                                "album2", 
                                Arrays.asList(new Track("tinygao", 1)), 
                                Arrays.asList(
                                        tinygao2,
                                        tinygao1
                                ));
        
        Album album3  = new Album(
                                "album3", 
                                Arrays.asList(new Track("tinygao", 1)), 
                                Arrays.asList(
                                        tinygao1
                                ));
        Stream<Album> albums1 = Stream.of(album1,album2,album3);
        Stream<Album> albums2 = Stream.of(album1,album2,album3);
        numberOfAlbums(albums1).forEach((x, y)->System.out.println(x+"->"+y));
        /**使用完流之后就會關(guān)閉， 再使用就會報錯stream has already been operated upon or closed**/
        nameOfAlbums(albums2).forEach((x, y)->System.out.println(x+"->"+y));
        
    }
    
    public static Map<Artist, Long> numberOfAlbums(Stream<Album> albums) {
        /**先獲取Artist function，然后在做reduce操作**/
        /** Map<Artist,List<Album>> 為下面的返回值
            albums.collect(groupingBy(album -> album.getMainMusician()));
        **/
        return albums.collect(groupingBy(album -> album.getMainMusician(),
                counting()));
    }
    
    public static Map<Artist, List<String>> nameOfAlbums(Stream<Album> albums) {
        /**Album::getName 在mapping中是對Map<Artist,List<Album>> 后者做mapping**/
        return albums.collect(groupingBy(Album::getMainMusician,
        mapping(Album::getName, toList())));
    }
}

2.輸出

tinygao1->1
tinygao2->2
tinygao1->[album3]
tinygao2->[album1, album2]