Java 流組合

Java流 - Java流組合

reduce()操作組合流中的所有元素以產(chǎn)生單個(gè)值。

reduce操作采用兩個(gè)稱為種子(初始值)和累加器的參數(shù)。

累加器是一個(gè)函數(shù)。如果流是空的，種子是結(jié)果。

種子和一個(gè)元素被傳遞給累加器，它返回部分結(jié)果。然后將部分結(jié)果和下一個(gè)元素傳遞給累加器函數(shù)。

這重復(fù)，直到所有元素被傳遞到累加器。累加器返回的最后一個(gè)值是reduce操作的結(jié)果。

流相關(guān)接口包含兩個(gè)稱為reduce()和collect()的方法來執(zhí)行通用reduce操作。

諸如sum()，max()，min()，count()等方法在IntStream，LongStream和DoubleStream接口中定義。

count()方法適用于所有類型的流。

Stream<T> 接口包含一個(gè)reduce()方法來執(zhí)行reduce操作。該方法有三個(gè)重載版本:

T  reduce(T identity, BinaryOperator<T> accumulator)
<U> U reduce(U identity, BiFunction<U,? super  T,U> accumulator, BinaryOperator<U> combiner)
Optional<T> reduce(BinaryOperator<T> accumulator)

第一個(gè)版本的reduce()方法使用一個(gè)標(biāo)識和一個(gè)累加器作為參數(shù)，并將流reduce為同一類型的單個(gè)值。

import java.util.Arrays;
import java.util.List;

public class Main {
  public static void main(String[] args) {
    List<Integer> numbers  = Arrays.asList(1, 2, 3, 4, 5);
    int sum = numbers.stream()
    .reduce(0, Integer::sum); 
    System.out.println(sum);
  }
}

上面的代碼生成以下結(jié)果。

例2

計(jì)算所有員工的收入總和。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;

public class Main {
  public static void main(String[] args) {
    double sum = Employee.persons()
        .stream()
        .map(Employee::getIncome)
        .reduce(0.0, Double::sum);
    System.out.println(sum);
  }
}

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 double getIncome() {
    return income;
  }
  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3

第二個(gè)版本的reduce方法如下所示允許我們執(zhí)行一個(gè)map操作，隨后執(zhí)行reduce操作。

<U> U reduce(U identity, BiFunction<U,? super T,U> accumulator, BinaryOperator<U> combiner)

第三個(gè)參數(shù)用于組合部分結(jié)果當(dāng)并行執(zhí)行縮減操作時(shí)。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;

public class Main {
  public static void main(String[] args) {
    double sum = Employee.persons()
        .stream()
        .reduce(0.0, (partialSum, person) -> partialSum + person.getIncome(), Double::sum); 
    System.out.println(sum);

  }
}

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 double getIncome() {
    return income;
  }
  public void setIncome(double income) {
    this.income = income;
  }
  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。

Java流映射并行reduce

Java Streams API支持并行映射縮減操作。

當(dāng)使用以下reduce方法時(shí)，每個(gè)線程使用累加器累加部分結(jié)果。最后，組合器用于組合來自所有線程的部分結(jié)果以獲得結(jié)果。

<U> U reduce(U identity, BiFunction<U,? super  T,U> accumulator, BinaryOperator<U> combiner)

以下代碼顯示了如何順序并行reduce操作工作。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;

public class Main {
  public static void main(String[] args) {
    double sum = Employee
        .persons()
        .stream()
        .reduce(
            0.0,
            (Double partialSum, Employee p) -> {
              double accumulated = partialSum + p.getIncome();
              System.out.println(Thread.currentThread().getName()
                  + "  - Accumulator: partialSum  = " + partialSum
                  + ",  person = " + p + ", accumulated = " + accumulated);
              return accumulated;
            },
            (a, b) -> {
              double combined = a + b;
              System.out.println(Thread.currentThread().getName()
                  + "  - Combiner:  a  = " + a + ", b  = " + b
                  + ", combined  = " + combined);
              return combined;
            });
    System.out.println("--------------------------------------");
    System.out.println(sum);

    sum = Employee
        .persons()
        .parallelStream()
        .reduce(
            0.0,
            (Double partialSum, Employee p) -> {
              double accumulated = partialSum + p.getIncome();
              System.out.println(Thread.currentThread().getName()
                  + "  - Accumulator: partialSum  = " + partialSum
                  + ",  person = " + p + ", accumulated = " + accumulated);
              return accumulated;
            },
            (a, b) -> {
              double combined = a + b;
              System.out.println(Thread.currentThread().getName()
                  + "  - Combiner:  a  = " + a + ", b  = " + b
                  + ", combined  = " + combined);
              return combined;
            });
    System.out.println(sum);
  }
}
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 double getIncome() {
    return income;
  }
  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。

流reduce沒有默認(rèn)值

第三個(gè)版本的reduce()如下列方法用于執(zhí)行沒有默認(rèn)值的縮減操作。

reduce(BinaryOperator<T> accumulator)

如果流是空的，我們不能使用默認(rèn)值為0。

Optional<T>
用于包裝結(jié)果或不存在結(jié)果。

以下代碼顯示如何計(jì)算流中的整數(shù)的最大值:

import java.util.Optional;
import java.util.stream.Stream;

public class Main {
  public static void main(String[] args) {
    Optional<Integer> max = Stream.of(1, 2, 3, 4, 5).reduce(Integer::max);

    if (max.isPresent()) {
      System.out.println("max = " + max.get());
    } else {
      System.out.println("max is not  defined.");
    }
    
    max = Stream.<Integer> empty().reduce(Integer::max);
    if (max.isPresent()) {
      System.out.println("max = " + max.get());
    } else {
      System.out.println("max is not  defined.");
    }
    
  }
}

上面的代碼生成以下結(jié)果。

例5

以下代碼打印員工列表中最高收入者的詳細(xì)信息。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;

public class Main {
  public static void main(String[] args) {
    Optional<Employee> person = Employee
        .persons()
        .stream()
        .reduce((p1, p2) -> p1.getIncome() > p2.getIncome() ? p1 : p2);
    if (person.isPresent()) {
      System.out.println("Highest earner: " + person.get());
    } else {
      System.out.println("Could not  get   the   highest earner.");
    }
  }
}

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 double getIncome() {
    return income;
  }
  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;
  }

  @Override
  public String toString() {
    String str = String.format("(%s, %s,  %s,  %s,  %.2f)\n", id, name, gender,
        dob, income);
    return str;
  }
}

上面的代碼生成以下結(jié)果。