Created
August 8, 2018 15:10
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JavaのflatMapがちょっと気に食わない
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import java.util.ArrayList; | |
import java.util.Arrays; | |
import java.util.List; | |
import java.util.stream.Collectors; | |
public class FlatMapSample { | |
public static void main(String[] arg) { | |
List<Integer> listA = Arrays.asList(1, 3, 5); | |
List<Integer> listB = Arrays.asList(2, 4, 6, 8); | |
List<Integer> listC = Arrays.asList(7, 9, 10); | |
// リストのリスト | |
List<List<Integer>> listOfList = Arrays.asList(listA, listB, listC); | |
System.out.println("listOfList=" + listOfList); | |
// output: listOfList=[[1, 3, 5], [2, 4, 6, 8], [7, 9, 10]] | |
// 中の数字を取り出してリストを作れる | |
List<Integer> list = listOfList.stream() | |
.flatMap(List::stream) // .flatMap(l -> l.stream())と同じ | |
.collect(Collectors.toList()); | |
System.out.println("list=" + list); | |
// output: list=[1, 3, 5, 2, 4, 6, 8, 7, 9, 10] | |
// もちろん中間操作もできる | |
List<Integer> sortedList = listOfList.stream() | |
.flatMap(List::stream) | |
.sorted() | |
.collect(Collectors.toList()); | |
System.out.println("sortedList=" + sortedList); | |
// output: sortedList=[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] | |
// 参考: Streamを使わずに書くとこんなん | |
List<Integer> listFor = new ArrayList<>(); | |
for (List<Integer> l : listOfList) { | |
for (Integer integer : l) { | |
listFor.add(integer); | |
} | |
} | |
System.out.println("listFor=" + sortedList); | |
// output: listFor=[1, 3, 5, 2, 4, 6, 8, 7, 9, 10] | |
List<Integer> sortedListFor = new ArrayList<>(); | |
for (List<Integer> l : listOfList) { | |
for (Integer integer : l) { | |
sortedListFor.add(integer); | |
} | |
} | |
sortedList.sort(null); | |
System.out.println("sortedListFor=" + sortedListFor); | |
// output: sortedListFor=[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] | |
} | |
} |
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