Convert Stream to Map using collect(toMap())

Stream collect(toMap())

toMap() method returns a Collector that accumulates elements into a Map.

Convert stream to Map – collect(toMap())

Stream’s collect(toMap()) gathers a stream elements into a key-value collection.

Here is an example:

public class Employee {

  private int id;

  private String name;

  public Employee(int id, String name) {
    this.id = id;
    this.name = name;
  }

  public String getName() {
    return name;
  }

  public int getId() {
    return id;
  }

  public String toString() { 
      return "Name: '" + getName() + "', Id: '" + getId() + "'";
  } 
}

package com.topjavatutorial.java8examples;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

import com.topjavatutorial.Employee;

public class StreamOperations {

  public static void main(String[] args) {
    Employee emp1 = new Employee(1,"Ajay");
    Employee emp2 = new Employee(2,"Bob");
    Employee emp3 = new Employee(3,"Chetan");
    
    List<Employee> empList = new ArrayList<Employee>();
    empList.add(emp1);
    empList.add(emp2);
    empList.add(emp3);
    
    Map<Integer,String> collectedMap = empList.stream()
        .collect(Collectors.toMap(Employee::getId,Employee::getName));
    collectedMap.forEach((k,v) -> System.out.println("Key : " + k + " , Value : " + v));
  }

}

Output:

Key : 1 , Value : Ajay
Key : 2 , Value : Bob
Key : 3 , Value : Chetan

 
When we want to get a mapping of Employee Ids to the employee objects, we could use an utility method Function.identity() :

package com.topjavatutorial.java8examples;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

import com.topjavatutorial.Employee;

public class StreamOperations {

  public static void main(String[] args) {
    
    Employee emp1 = new Employee(1,"Ajay");
    Employee emp2 = new Employee(2,"Bob");
    Employee emp3 = new Employee(3,"Chetan");
    
    List<Employee> empList = new ArrayList<Employee>();
    empList.add(emp1);
    empList.add(emp2);
    empList.add(emp3);
    
    Map<Integer,Employee> collectedMap = empList.stream()
        .collect(Collectors.toMap(Employee::getId,Function.identity()));
    collectedMap.forEach((k,v) -> System.out.println("Key [ " + k + "] , Value : [" + v + "]"));
  }

}

Output :

Key [ 1] , Value : [Name: ‘Ajay’, Id: ‘1’]
Key [ 2] , Value : [Name: ‘Bob’, Id: ‘2’]
Key [ 3] , Value : [Name: ‘Chetan’, Id: ‘3’]

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