Convert list of objects to /from map in java 8 lambda stream (example)

Leave a Comment / By /
  • Given the list of objects, convert list of objects to map.
  • We will use lambda streams to convert list objects to map & vice versa.
  • We will create Person class & we will convert list of person objects to/from map.

1. Convert list of objects to/from map in java8 lambda stream

  1. Convert list of objects to Map <Key,Value>
    • We will convert List<Person> to Map<String,Person>
  2. Convert Map<Key,Value> to list of objects.
    1. Convert Map<String,Person> to List<String>
      • Convert map of key value pairs to List of Keys
    2. Convert Map<String,Person> to List<Person>
      • Convert map of key value pairs to List of Values

2. Program – convert list of objects to/from map in java8 (lambda streams)

2.1.) Person Class:

  • Person class containing required data members is as follows.
public class Person {
    private String firstName;
    private String lastName;
    private String uniqueId;
 
    public Person(String firstName, String lastName,
            String uniqueId) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.uniqueId = uniqueId;
    }
 
    public String toString() {
        return "[" + firstName + " " + lastName +
               " " + uniqueId +"]";
    }  
 
    public String getUniqueId() {
        return uniqueId;
    }
 
    public void setUniqueId(String uniqueId) {
        this.uniqueId = uniqueId;
    }  
}

2.2. ListMapConversion Class:

  • ListMapConversion class is responsible for following operations
    1. Convert List<Person> to Map<String,Person>
    2. Convert Map<String,Person> to List<String> (List of keys).
    3. Convert Map<String,Person> to List<Person> (List of values).
package org.learn.ConvertListToMap;
 
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
 
public class ListMapConversion
{
    public static void main( String[] args )
    {
        List <Person> personList = new ArrayList<Person>();
         
        personList.add(new Person("Mike", "harvey", "100268"));
        personList.add(new Person("Nick", "young", "32654"));
        personList.add(new Person("Jack", "slater", "123642"));
         
        System.out.println("1. Convert List of objects to map :");
        Map<String, Person> mapIdToPerson = convertListToMap(personList);
         
        System.out.println("\n2. Convert Map to list of objects :");
        convertMapToList(mapIdToPerson);
    }
     
    private static Map<String, Person> convertListToMap(List <Person> personList) {
        Map<String, Person>mapIdToPerson = null;
        mapIdToPerson = personList.stream()
                                        .collect(
                                            Collectors
                                            .toMap(
                                                    Person::getUniqueId,
                                                    person->person)
                                                );
         
        System.out.println(mapIdToPerson);
        return mapIdToPerson;
    }
     
    private static void convertMapToList(Map<String, Person>mapIdToPerson) {
         
        System.out.println("2.1. List of keys :");
        List<String>uniqueIdList = new ArrayList<>(mapIdToPerson.keySet());
        System.out.println(uniqueIdList);
         
        System.out.println("\n2.2. List of person objects :");
        List<Person>personList = new ArrayList<>(mapIdToPerson.values());
        System.out.println(personList);    
    }
}

3. Output – convert list of objects to/from map in java (lambda streams)

1. Convert List of objects to map :
{32654=[Nick young 32654], 123642=[Jack slater 123642], 100268=[Mike harvey 100268]}
 
2. Convert Map to list of objects :
2.1. List of keys :
[32654, 123642, 100268]
 
2.2. List of person objects :
[[Nick young 32654], [Jack slater 123642], [Mike harvey 100268]]

You may also like:

Join or combine two / multiple Strings using StringJoiner in java8 (example)

Convert one/ heterogeneous object into another – stream lambda (java8 /example)

Find/ search String object from arraylist collection of objects – stream lambda java 8 (example)

Find object from arraylist of objects using lambda stream java 8 (example)

Java 8 Functional interface explained with examples (Comparator)

groupBy, mapping & statistics features in Java8 lambda stream (examples)

Primitive streams example in java8 (IntStream, LongStream, DoubleStream)