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Java 流收集器

2018-03-12 23:07 更新

Java流 - Java流收集器

要在Stream中對(duì)數(shù)據(jù)進(jìn)行分組，我們可以使用 collect()方法的Stream<T>接口。

collect()方法重載了兩個(gè)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()方法的第一個(gè)版本有三個(gè)參數(shù):

供應(yīng)商提供一個(gè)可變?nèi)萜鱽?lái)存儲(chǔ)結(jié)果。
累加器，將結(jié)果累積到可變?nèi)萜髦小?/li>
組合器，當(dāng)并行使用時(shí)組合部分結(jié)果。

我們使用以下步驟在ArrayList <String>中收集員工姓名。

首先，創(chuàng)建一個(gè)供應(yīng)商，它使用以下語(yǔ)句之一返回一個(gè) ArrayList< String> 以創(chuàng)建供應(yīng)商：

Supplier<ArrayList<String>> supplier = () -> new ArrayList<>();

要么

Supplier<ArrayList<String>> supplier = ArrayList::new;

第二，創(chuàng)建一個(gè)累加器，它接收兩個(gè)參數(shù)，它們是從供應(yīng)商返回的容器（在這種情況下為ArrayList <String>）。和流的元素。

累加器將名稱添加到列表中。

BiConsumer<ArrayList<String>, String>  accumulator = (list, name)  ->  list.add(name);
BiConsumer<ArrayList<String>, String>  accumulator = ArrayList::add;

最后，組合器將結(jié)果組合成一個(gè) ArrayList<String> 。

組合器僅用于并行流。

以下代碼顯示如何使用collect()方法來(lái)收集列表中所有員工的姓名。

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;
  }
}

上面的代碼生成以下結(jié)果。

收集器接口

第二個(gè)版本的collect()將Collector接口的一個(gè)實(shí)例作為參數(shù)。

Streams API提供了一個(gè)名為Collectors的實(shí)用程序類，為常用的收集器提供了開箱即用的實(shí)現(xiàn)。

三種最常用的方法之一收集器類是toList()，toSet()和toCollection()。

toList()方法返回收集列表中數(shù)據(jù)的收集器。
toSet()方法返回在集合中收集數(shù)據(jù)的收集器。
toCollecton()返回一個(gè)供應(yīng)商，返回一個(gè)集合用于收集數(shù)據(jù)。

以下代碼收集列表<String>中的所有員工姓名。

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;
  }
}

上面的代碼生成以下結(jié)果。

例2

以下代碼收集Set<String>中的所有名稱，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

我們可以使用toCollection()方法在排序集合中收集名稱，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;
  }
}

上面的代碼生成以下結(jié)果。