一. 題目
Given the definition of the Country class:
| public class Country { | |
| public enum Continent { | |
| ASIA, EUROPE | |
| } | |
| String name; | |
| Continent region; | |
| public Country(String na, Continent reg) { | |
| name = na; | |
| region = reg; | |
| } | |
| public String getName() { | |
| return name; | |
| } | |
| public Continent getRegion() { | |
| return region; | |
| } | |
| } |
and the code fragment:
| List<Country> couList = Arrays.asList( | |
| new Country("Japan", Country.Continent.ASIA), | |
| new Country("Italy", Country.Continent.EUROPE), new Country("Germany", Country.Continent.EUROPE)); | |
| Map<Country.Continent, List<String>> regionNames = couList.stream(). | |
| collect(Collectors.groupingBy(Country::getRegion, | |
| Collectors.mapping(Country::getName, Collectors.toList()))); | |
| System.out.println(regionNames); |
What is the output?
A. {EUROPE = [Italy, Germany], ASIA = [Japan]}
B. {ASIA = [Japan], EUROPE = [Italy, Germany]}
C. {EUROPE = [Germany, Italy], ASIA = [Japan]}
D. {EUROPE = [Germany], EUROPE = [Italy], ASIA = [Japan]}
二. 題解
先把 3個 Country物件放入 couList，
couList轉成 stream流，再使用 stream物件的 collect方法，把 stream物件轉成 Collection物件，
使用 Collectors類別的 groupingBy方法，會回傳一個 Map，
https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html
第一個參數，使用 Country::getRegion，取得 region，依序是 Asia、Europe，
第二個參數，使用 Collectors.mapping方法，將 stream物件元素，Country物件呼叫 getName方法，回傳的字串會依據相同的 region，組合成一個 List物件，
第一個 List為 [Japan]，第二個 List為[Italy, Germany]。
https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html
輸出結果
答案是B. {ASIA = [Japan], EUROPE = [Italy, Germany]}
三. 參考
[OCPJP]串流與Lambda的精簡應用
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