モジュール java.base
パッケージ java.util.stream

クラスCollectors

java.lang.Object
java.util.stream.Collectors
public final class Collectors extends Object
要素をコレクションに蓄積したり、さまざまな条件に従って要素を要約するなど、有用な各種リダクション操作を実装したCollector実装。

定義済のコレクタを使って一般的な可変リダクション・タスクを実行する例を、次に示します。 


 // Accumulate names into a List
 List<String> list = people.stream()
   .map(Person::getName)
   .collect(Collectors.toList());

 // Accumulate names into a TreeSet
 Set<String> set = people.stream()
   .map(Person::getName)
   .collect(Collectors.toCollection(TreeSet::new));

 // Convert elements to strings and concatenate them, separated by commas
 String joined = things.stream()
   .map(Object::toString)
   .collect(Collectors.joining(", "));

 // Compute sum of salaries of employee
 int total = employees.stream()
   .collect(Collectors.summingInt(Employee::getSalary));

 // Group employees by department
 Map<Department, List<Employee>> byDept = employees.stream()
   .collect(Collectors.groupingBy(Employee::getDepartment));

 // Compute sum of salaries by department
 Map<Department, Integer> totalByDept = employees.stream()
   .collect(Collectors.groupingBy(Employee::getDepartment,
                                  Collectors.summingInt(Employee::getSalary)));

 // Partition students into passing and failing
 Map<Boolean, List<Student>> passingFailing = students.stream()
   .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));

導入されたバージョン:
1.8