Java Streams - Collectors toMap(Function keyMapper, Function valueMapper, BinaryOperator mergeFunction) example








Collectors toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction) returns a Collector that accumulates elements into a Map whose keys and values are from the mapping functions.

Syntax

toMap has the following syntax.

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

Example

The following example shows how to use toMap.

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;
/*w ww . jav a 2  s. co m*/
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;
  }
}

The code above generates the following result.