いつもお世話になっております。 CollectorsクラスのgroupingByメソッドの仕様について、質問をさせていただきたいです。上記のメソッドを使用した際に、グルーピングが行われますが、その結果を出力した際の出力順序には、決まりはないのでしょうか。 groupByの対象となるListの中身 - new Musician("Jimi Hendrix", Musician.Category.ROCK) - new Musician("Eric Dolphy", Musician.Category.JAZZ) - new Musician("J.S.Bach", Musician.Category.CLASSICAL) - new Musician("Charles Mingus", Musician.Category.JAZZ) [Collectorsクラス：groupingBy Javadoc](https://docs.oracle.com/javase/jp/8/docs/api/java/util/stream/Collectors.html#groupingBy-java.util.function.Function-java.util.stream.Collector-) リストの中身の順番を変えた場合の出力結果をいくつか記載致します。 <出力結果> {JAZZ=[Eric Dolphy, Charles Mingus], ROCK=[Jimi Hendrix], CLASSICAL=[J.S.Bach]} ```Java import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class Test { public static void main(String[] args) { List musicList = Arrays.asList( 　　　　　　　　new Musician("Jimi Hendrix", Musician.Category.ROCK), new Musician("Eric Dolphy", Musician.Category.JAZZ), new Musician("J.S.Bach", Musician.Category.CLASSICAL), new Musician("Charles Mingus", Musician.Category.JAZZ)); Map> map = musicList.stream().collect(Collectors .groupingBy(Musician::getCategory, Collectors.mapping(Musician::getName, Collectors.toList()))); System.out.println(map); } } ``` ```Java public class Musician { public enum Category { ROCK, JAZZ, CLASSICAL } private String name; private Category category; public Musician(String name, Category category) { this.name = name; this.category = category; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Category getCategory() { return category; } public void setCategory(Category category) { this.category = category; } } ``` <出力結果> {CLASSICAL=[J.S.Bach], JAZZ=[Eric Dolphy, Charles Mingus], ROCK=[Jimi Hendrix]} ```Java import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class Test { public static void main(String[] args) { List musicList = Arrays.asList( new Musician("Eric Dolphy", Musician.Category.JAZZ), new Musician("J.S.Bach", Musician.Category.CLASSICAL), new Musician("Charles Mingus", Musician.Category.JAZZ), new Musician("Jimi Hendrix", Musician.Category.ROCK)); Map> map = musicList.stream().collect(Collectors .groupingBy(Musician::getCategory, Collectors.mapping(Musician::getName, Collectors.toList()))); System.out.println(map); } } ``` <出力結果> {JAZZ=[Charles Mingus, Eric Dolphy], CLASSICAL=[J.S.Bach], ROCK=[Jimi Hendrix]} ```Java import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class Test { public static void main(String[] args) { List musicList = Arrays.asList( new Musician("J.S.Bach", Musician.Category.CLASSICAL), new Musician("Charles Mingus", Musician.Category.JAZZ), new Musician("Jimi Hendrix", Musician.Category.ROCK), 　　　　 new Musician("Eric Dolphy", Musician.Category.JAZZ)); Map> map = musicList.stream().collect(Collectors .groupingBy(Musician::getCategory, Collectors.mapping(Musician::getName, Collectors.toList()))); System.out.println(map); } } ```

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1	- 【Java】CollectorsクラスのgroupingByメソッドの出力順序について
1	+ 【Java】CollectorsクラスのgroupingByメソッドを使用した際の出力順序について

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