Java8 Stream
目次
生成処理
streamインスタンス
配列から
Stream<String> stream1 = Stream.of(“A”, “B”, “C”, “D”, “E”);
Stream<String> stream2 = Arrays.stream(new String[] {“A”, “B”, “C”, “D”, “E”});
Listから
List<String> list = Arrays.asList(“A”,”B”,”C”,”D”,”E”);
Stream<String> stream3 = list.stream();
Mapから
Stream<Entry<String, Double>> map = exp.entrySet().stream();
数値範囲から
IntStream stream = IntStream.range(1, 5);
stream.forEach( System.out::print );
1 2 3 4
IntStream stream = IntStream.rangeClosed(1, 5);
stream.forEach( System.out::print );
1 2 3 4 5
直接中間操作、終端操作
通常はsteramインスタンス変数を作成せず、この方法を用いる
Arrays.asList(“A”,”B”,”C”,”D”,”E”).stream().forEach( ～ )
中間操作
map/flatMap
map
各要素に特定の処理を行う
Function型メソッドを介する
List<String> list = Arrays.asList(“A”,”B”,”C”,”D”,”E”);
List<String> newList = new ArrayList<>();
for (String s : list) {
s = s + “+”;
newList.add(s);
}
[A+, B+, C+, D+, E+]
List<String> newList = new ArrayList<>();
list.stream()
.map(s -> s + “+”)
.forEach(s -> newList.add(s));
[A+, B+, C+, D+, E+]
flatMap
public class Language {
private String name;
private int exp;
public Language(String name, int exp) {
this.name = name;
this.exp = exp;
}
}
public class Group{
private List<Language> students = new ArrayList<>();
public List<Language> getStudents() {
return this.students;
}
public void addStudent(Language students) {
this.students.add(students);
}
}
List<Group> groupList = new ArrayList<>();
Group group1 = new Group();
group1.addStudent(new Language(“Java”, 3));
group1.addStudent(new Language(“VB.NET”, 4));
group1.addStudent(new Language(“C#”, 2));
group1.addStudent(new Language(“VB6”, 2));
Group group2 = new Group();
group2.addStudent(new Language(“php”, 2));
group2.addStudent(new Language(“python”, 1));
groupList.add(group1);
groupList.add(group2);
List<List<Language>> map = groupList.stream().map(s -> s.getStudents())
.collect(Collectors.toList());
List<List<Language>>
└[0]
└Language
└name：Java
└exp：3
└Language
└ ～
└Language
└ ～
└Language
└ ～
└[1]
└Language
└name：php
└exp：2
└Language
└ ～
List<Language> flatMap = groupList.stream().flatMap(s -> s.getStudents().stream())
.collect(Collectors.toList());
List<Language>
└Language
└name：Java
└exp：3
└Language
└ ～
└Language
└ ～
└Language
└ ～
└Language
└name：php
└exp：2
└Language
└ ～
mapToInt
OptionalInt min = Arrays.asList(“Java”, “PHP”, “C#”).stream()
.mapToInt(x -> x.length()).min();
min.getAsInt()：2
OptionalInt max = Arrays.asList(“Java”, “PHP”, “C#”).stream()
.mapToInt(x -> x.length()).max();
max.getAsInt()：4
filter
条件を満たす要素を選別
Predicate型メソッドを介する
※Predicateがtrueを返す
Integer[] list = {1, 2, 3, 4, 5, 6 };
Predicate<Integer> isThree = (i) -> { return i % 3 == 0; };
lists.stream()
.filter(isThree)
.forEach(System.out::println);
→ 3 6
まとめて書いた場合
Arrays.stream(list).filter((i) -> { return i % 3 == 0; }).forEach(System.out::println);
limit
List<String> list = Arrays.asList(“A”,”B”,”C”,”D”,”E”);
List<String> newList = new ArrayList<>();
list.stream()
.limit(3)
.forEach(s -> newList.add(s));
[A, B, C]
sorted
List<String> list = Arrays.asList(“E”,”D”,”C”,”B”,”A”);
List<String> newList = new ArrayList<>();
list.stream()
.sorted()
.forEach(s -> newList.add(s));
[E, D, C, B, A]
終端操作
forEach
Stream
List<String> language = Arrays.asList(“Java”, “C#”, “PHP”, “Python”);
language.stream().forEach( s -> {
System.out.println(s);
});
Java C# PHP Python
Map
Map<String, Integer> exp = new HashMap<>();
exp.put(“Java”, 3);
exp.put(“C#”, 2);
exp.put(“PHP”, 3);
exp.put(“Python”, 1);
exp.forEach((key, value) -> {
System.out.println(key + “:” + value);
});
C#:2 Java:2 PHP:1 Python:1
collect
Collectors.toList
Listへ変換
List<String> list = Arrays.asList(“E”,”D”,”C”,”B”,”A”);
List<String> newList =
list.stream()
.sorted()
.collect(Collectors.toList());
[E, D, C, B, A]
Collectors.joining
文字列として結合
String s =
list.stream()
.sorted()
.collect(Collectors.joining());
ABCDE
Collectors.groupingBy
集約
List<Language> languages = new ArrayList<>();
languages.add(new Language(“Java”, 3));
languages.add(new Language(“C#”, 2));
languages.add(new Language(“php”, 2));
languages.add(new Language(“python”, 1));
戻り値はMap型
Map<Integer, List<Language>> map = languages.stream().collect(Collectors.groupingBy(i -> i.getExp()));
Map<Integer, List<Language>>
└[0]
└key：1
└value
└Language
└name：Java
└exp：3
└[1]
└key：2
└value
└Language
└name：PHP
└exp：2
└Language
└ ～
PHPとC#が集約される
└[2]
└key：3
└value
└Language
└ ～
集約したListから値を取得
List<Language> list = map.get(2);
List<Language>
Language
└name：PHP
└exp：2
Language
└name：C#
└exp：2
Collectors.counting
List<String> language = Arrays.asList(“Java”, “C#”, “PHP”, “C#”, “Python”, “Java”);
language内の要素で集約、同値の要素数を取得
Map<String, Long> counter = language.stream().collect(Collectors.groupingBy(s -> s, Collectors.counting()));
counter.forEach((key, value) -> {
System.out.println(key + “:” + value);
});
C#:2 Java:2 PHP:1 Python:1
Match
anyMatch
List<String> list = Arrays.asList(“A”, “B”, “C”, “D”, “E”);
Boolean isMatch = list.stream().anyMatch(s -> s.equals(“C”));
isMatch:true
Boolean isMatch = list.stream().anyMatch(s -> s.equals(“Z”));
isMatch:false
allMatch
List<String> list = Arrays.asList(“A”, “B”, “C”, “D”, “E”);
Boolean isMatch = list.stream().allMatch(s -> s.length() == 1 );
isMatch:true
Boolean isMatch = list.stream().allMatch(s -> s.length() == 2 );
isMatch:false
noneMatch
List<String> list = Arrays.asList(“A”, “B”, “C”, “D”, “E”);
Boolean isExist = list.stream().noneMatch(s -> s.equals(“Z”));
isExist:true
Boolean isExist = list.stream().noneMatch(s -> s.equals(“C”));
isExist:false
抽出
findFirst
Optional<Integer> list = Arrays.asList(1, 2, 3, 5, 6).stream().findFirst();
list.get()：1
findAny
Optional<Integer> list = Arrays.asList(1, 2, 3, 5, 6).stream().findAny();
list.get()：1
min
Optional<Integer> list = Arrays.asList(1, 2, 3, 5, 6).stream().min(Comparator.naturalOrder());
list.get()：1
max
Optional<Integer> list = Arrays.asList(1, 2, 3, 5, 6).stream().max(Comparator.naturalOrder());
list.get()：6
集計
count
long ret = Arrays.asList(1,2,3,5,7).stream().mapToInt(x -> x).count();
ret：5
sum
int ret = Arrays.asList(1, 2, 3, 5, 6).stream().mapToInt(x -> x).sum();
ret：17
average
OptionalDouble ret = Arrays.asList(1, 2, 3, 5, 6).stream().mapToInt(x -> x).average();
ret.getAsDouble()：3.4
