The following method collect() method takes an instance of the Collector interface as an argument and collects the data.
<R,A> R collect(Collector<? super T,A,R> collector)
The Collector interface is the java.util.stream package and it is declared as follows.
Only abstract methods are shown.
public interface Collector<T,A,R> { Supplier<A> supplier(); BiConsumer<A,T> accumulator(); BinaryOperator<A> combiner(); Function<A,R> finisher(); Set<Collector.Characteristics> characteristics(); }
The Collector interface takes three type parameters called T, A, and R, where
The finisher is used to transform the intermediate type A to result type R.
Collectors class provides out-of-box implementations for commonly used collectors.
Three of the most commonly used methods of the Collectors class are toList(), toSet(), and toCollection().
Method | Description |
---|---|
toList() | returns a Collector that collects the data in a List |
toSet() | returns a Collector that collects data in a Set |
toCollecton() | takes a Supplier that returns a Collection to be used to collect data. |
The following code collects all names of people in a List<String>:
List<String> names = Person.persons() .stream() .map(Person::getName) .collect(Collectors.toList()); System.out.println(names);
The following code collects all names in a Set<String>. Note that a Set keeps only unique elements.
Set<String> uniqueNames = Person.persons() .stream() .map(Person::getName) .collect(Collectors.toSet()); System.out.println(uniqueNames);
You can collect names in a sorted set using the toCollection() method as follows:
SortedSet<String> uniqueSortedNames= Person.persons()
.stream()
.map(Person::getName)
.collect(Collectors.toCollection(TreeSet::new));
System.out.println(uniqueSortedNames);
You can sort the list of names using the sorted operation.
sorted() method of the Stream interface produces another stream containing the same elements on a sorted order.
The following code shows how to collect sorted names in a list:
List<String> sortedName = Person.persons() .stream() .map(Person::getName) .sorted() .collect(Collectors.toList()); System.out.println(sortedName);
Counting the number of people in the streams:
long count = Person.persons() .stream() .collect(Collectors.counting()); System.out.println("Person count: " + count);
import java.time.LocalDate; import java.time.Month; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class Main { public static void main(String[] args) { List<String> sortedNames = Person.persons() .stream() .map(Person::getName) .sorted() .collect(Collectors.toList()); System.out.println(sortedNames); }/*from ww w. j a va 2 s . c o m*/ } class Person { // An enum to represent the gender of a person public static enum Gender { MALE, FEMALE } private long id; private String name; private Gender gender; private LocalDate dob; private double income; public Person(long id, String name, Gender gender, LocalDate dob, double income) { this.id = id; this.name = name; this.gender = gender; this.dob = dob; this.income = income; } public long getId() { return id; } public void setId(long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Gender getGender() { return gender; } public boolean isMale() { return this.gender == Gender.MALE; } public boolean isFemale() { return this.gender == Gender.FEMALE; } public void setGender(Gender gender) { this.gender = gender; } public LocalDate getDob() { return dob; } public void setDob(LocalDate dob) { this.dob = dob; } public double getIncome() { return income; } public void setIncome(double income) { this.income = income; } public static List<Person> persons() { Person ken = new Person(1, "Java", Gender.MALE, LocalDate.of(1970, Month.MAY, 4), 6020.0); Person jeff = new Person(2, "Jack", Gender.MALE, LocalDate.of(1970, Month.JULY, 15), 7320.0); Person donna = new Person(3, "Javascript", Gender.FEMALE, LocalDate.of(1972, Month.JULY, 29), 8720.0); Person chris = new Person(4, "XML", Gender.MALE, LocalDate.of(1993, Month.DECEMBER, 16), 5800.0); Person laynie = new Person(5, "Json", Gender.FEMALE, LocalDate.of(2002, Month.DECEMBER, 13), 0.0); Person lee = new Person(6, "Database", Gender.MALE, LocalDate.of(2011, Month.MAY, 9), 2400.0); // Create a list of persons List<Person> persons = Arrays.asList(ken, jeff, donna, chris, laynie, lee); return persons; } @Override public String toString() { String str = String.format("(%s, %s, %s, %s, %.2f)", id, name, gender, dob, income); return str; } }