Java 流收集映射

Java流 - Java流收集映射

我们可以从流中收集数据到映射。

在三个版本中重载的toMap()方法Collectors类返回一个收集器以在Map中收集数据。

toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M>  mapSupplier)

toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)

第一个版本有两个参数。这两个参数都是一个函数。

第一个参数将流元素映射到映射中的键。

第二个参数将流元素映射到映射中的值。

如果重复键，则抛出IllegalStateException。

以下代码在Map<long，String>中收集员工的数据，其中的关键字是员工的ID，值是员工的姓名。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {
  public static void main(String[] args) {
    Map<Long,String>  idToNameMap  = Employee.persons()
        .stream()
        .collect(Collectors.toMap(Employee::getId,  Employee::getName));
     System.out.println(idToNameMap);
  }
}

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 long getId() {
    return id;
  }
  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

下面列出的第二种形式的toMap具有额外的合并函数。

toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)

合并函数传递重复键的旧值和新值。 该函数应合并两个值，并返回将用于键的新值。

以下代码显示如何连接所有男性和女性的名称。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {
  public static void main(String[] args) {
    Map<Employee.Gender,String> genderToNamesMap  = 
    Employee.persons()
            .stream()
            .collect(Collectors.toMap(Employee::getGender,
                                      Employee::getName,
                                      (oldValue, newValue)  ->  String.join(", ", oldValue,  newValue))); 
    System.out.println(genderToNamesMap);
  }
}

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 void setName(String name) {
    this.name = name;
  }

  public Gender getGender() {
    return gender;
  }
  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3

下面列出的第三个版本允许我们使用供应商提供一个Map对象。

toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M>  mapSupplier)

以下代码按性别总结人数。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {
  public static void main(String[] args) {
    Map<Employee.Gender, Long> countByGender  = Employee.persons()
        .stream()
        .collect(Collectors.toMap(Employee::getGender, p  ->  1L, (oldCount, newCount)  ->  newCount+oldCount));

    System.out.println(countByGender);
  }
}
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 Gender getGender() {
    return gender;
  }

  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4

以下代码在映射中按性别取得最高收入。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Main {
  public static void main(String[] args) {
    Map<Employee.Gender, Employee>  highestEarnerByGender = Employee.persons()
        .stream()
        .collect(Collectors.toMap(Employee::getGender, Function.identity(), 
                                  (oldPerson, newPerson)  -> newPerson.getIncome() > oldPerson.getIncome() ? newPerson : oldPerson));
    System.out.println(highestEarnerByGender);
  }
}

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 Gender getGender() {
    return gender;
  }
  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;
  }
}

上面的代码生成以下结果。