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Java流 – Java流收集器
要在Stream中对数据进行分组，我们可以使用 collect()方法的Stream<T>接口。
collect()方法重载了两个版本:
<R> R collect(Supplier<R> supplier, BiConsumer<R,? super T> accumulator, BiConsumer<R,R> combiner) <R,A> R collect(Collector<? super T,A,R> collector)
collect()方法的第一个版本有三个参数:
- 供应商提供一个可变容器来存储结果。
- 累加器，将结果累积到可变容器中。
- 组合器，当并行使用时组合部分结果。
我们使用以下步骤在ArrayList <String>中收集员工姓名。
首先，创建一个供应商，它使用以下语句之一返回一个 ArrayList< String> 以创建供应商：
Supplier<ArrayList<String>> supplier = () -> new ArrayList<>();
要么
Supplier<ArrayList<String>> supplier = ArrayList::new;
第二，创建一个累加器，它接收两个参数，它们是从供应商返回的容器（在这种情况下为ArrayList <String>）。 和流的元素。
累加器将名称添加到列表中。
BiConsumer<ArrayList<String>, String> accumulator = (list, name) -> list.add(name); BiConsumer<ArrayList<String>, String> accumulator = ArrayList::add;
最后，组合器将结果组合成一个 ArrayList<String> 。
组合器仅用于并行流。
以下代码显示如何使用collect()方法来收集列表中所有员工的姓名。
import java.time.LocalDate;
import java.time.Month;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Main {
public static void main(String[] args) {
List<String> names = Employee.persons()
.stream()
.map(Employee::getName)
.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
System.out.println(names);
}
}
class Employee {
public static enum Gender {
MALE, FEMALE
}
private long id;
private String name;
private Gender gender;
private LocalDate dob;
private double income;
public Employee(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 String getName() {
return name;
}
public static List<Employee> persons() {
Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
Month.JANUARY, 1), 2343.0);
Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
Month.JULY, 21), 7100.0);
Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
Month.MAY, 29), 5455.0);
Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
Month.OCTOBER, 16), 1800.0);
Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
Month.DECEMBER, 13), 1234.0);
Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
Month.JUNE, 9), 3211.0);
List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);
return persons;
}
}
上面的代码生成以下结果。
收集器接口
第二个版本的collect()将Collector接口的一个实例作为参数。
Streams API提供了一个名为Collectors的实用程序类，为常用的收集器提供了开箱即用的实现。
三种最常用的方法之一收集器类是toList()，toSet()和toCollection()。
- toList()方法返回收集列表中数据的收集器。
- toSet()方法返回在集合中收集数据的收集器。
- toCollecton()返回一个供应商，返回一个集合用于收集数据。
以下代码收集列表<String>中的所有员工姓名。
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> names = Employee.persons()
.stream()
.map(Employee::getName)
.collect(Collectors.toList());
System.out.println(names);
}
}
class Employee {
public static enum Gender {
MALE, FEMALE
}
private long id;
private String name;
private Gender gender;
private LocalDate dob;
private double income;
public Employee(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 String getName() {
return name;
}
public static List<Employee> persons() {
Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
Month.JANUARY, 1), 2343.0);
Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
Month.JULY, 21), 7100.0);
Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
Month.MAY, 29), 5455.0);
Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
Month.OCTOBER, 16), 1800.0);
Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
Month.DECEMBER, 13), 1234.0);
Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
Month.JUNE, 9), 3211.0);
List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);
return persons;
}
}
上面的代码生成以下结果。
例2
以下代码收集Set<String>中的所有名称，它只保留唯一的元素。
Set<String> uniqueNames = Person.persons()
.stream()
.map(Person::getName)
.collect(Collectors.toSet());
System.out.println(uniqueNames);
例3
我们可以使用toCollection()方法在排序集合中收集名称，如下所示:
import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.SortedSet;
import java.util.TreeSet;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
SortedSet<String> uniqueSortedNames= Employee.persons()
.stream()
.map(Employee::getName)
.collect(Collectors.toCollection(TreeSet::new));
System.out.println(uniqueSortedNames);
System.out.println(uniqueSortedNames);
}
}
class Employee {
public static enum Gender {
MALE, FEMALE
}
private long id;
private String name;
private Gender gender;
private LocalDate dob;
private double income;
public Employee(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 String getName() {
return name;
}
public static List<Employee> persons() {
Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
Month.JANUARY, 1), 2343.0);
Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
Month.JULY, 21), 7100.0);
Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
Month.MAY, 29), 5455.0);
Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
Month.OCTOBER, 16), 1800.0);
Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
Month.DECEMBER, 13), 1234.0);
Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
Month.JUNE, 9), 3211.0);
List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);
return persons;
}
}
上面的代码生成以下结果。
