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Program to Convert List to Map in Java

Last Updated : 11 Jul, 2025

The

is a child interface of Collection. It is an ordered collection of objects in which duplicate values can be stored. Since List preserves the insertion order, it allows positional access and insertion of elements. List Interface is implemented by

and

classes. The

interface represents a mapping between a key and a value. The Map interface is not a subtype of the

Collection

interface. Therefore it behaves a bit different from the rest of the collection types.

Example:

Input: List : [1="1", 2="2", 3="3"]
Output: Map : {1=1, 2=2, 3=3}

Input: List : [1="Geeks", 2="for", 3="Geeks"]
Output: Map : {1=Geeks, 2=for, 3=Geeks}

Below are various ways to convert List to Map in Java. For this, it is assumed that each element of the List has an identifier which will be used as a key in the resulting Map.

Using by object of list: Approach:

Get the List to be converted into Map
Create an empty Map
Iterate through the items in the list and add each of them into the Map.
Return the formed Map

Java


   // Java program for list convert in map
// with the help of Object method

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.HashMap;

// create a list
class Student {

    // id will act as Key
    private Integer id;

    // name will act as value
    private String name;

    // create curstuctor for reference
    public Student(Integer id, String name)
    {

        // assign the value of id and name
        this.id = id;
        this.name = name;
    }

    // return private variable id
    public Integer getId()
    {
        return id;
    }

    // return private variable name
    public String getName()
    {
        return name;
    }
}

// main class and method
public class GFG {

    // main Driver
    public static void main(String[] args)
    {

        // create a list
        List<Student>
            lt = new ArrayList<Student>();

        // add the member of list
        lt.add(new Student(1, "Geeks"));
        lt.add(new Student(2, "For"));
        lt.add(new Student(3, "Geeks"));

        // create map with the help of
        // Object (stu) method
        // create object of Map class
        Map<Integer, String> map = new HashMap<>();

        // put every value list to Map
        for (Student stu : lt) {
            map.put(stu.getId(), stu.getName());
        }

        // print map
        System.out.println("Map  : " + map);
    }
}

Output:

Map  : {1=Geeks, 2=For, 3=Geeks}

Using Collectors.toMap() method: This method includes creation of a list of the student objects, and uses Collectors.toMap() to convert it into a Map. Approach:

Get the List to be converted into Map
Convert the List into stream using List.stream() method
Create map with the help of Collectors.toMap() method
Collect the formed Map using stream.collect() method
Return the formed Map

Java


   // Java program for list convert  in map
// with the help of Collectors.toMap() method

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

// create a list
class Student {

    // id will act as Key
    private Integer id;

    // name will act as value
    private String name;

    // create curstuctor for reference
    public Student(Integer id, String name)
    {

        // assign the value of id and name
        this.id = id;
        this.name = name;
    }

    // return private variable id
    public Integer getId()
    {
        return id;
    }

    // return private variable name
    public String getName()
    {
        return name;
    }
}

// main class and method
public class GFG {

    // main Driver
    public static void main(String[] args)
    {

        // create a list
        List<Student> lt = new ArrayList<>();

        // add the member of list
        lt.add(new Student(1, "Geeks"));
        lt.add(new Student(2, "For"));
        lt.add(new Student(3, "Geeks"));

        // create map with the help of
        // Collectors.toMap() method
        LinkedHashMap<Integer, String>
            map = lt.stream()
                      .collect(
                          Collectors
                              .toMap(
                                  Student::getId,
                                  Student::getName,
                                  (x, y)
                                      -> x + ", " + y,
                                  LinkedHashMap::new));

        // print map
        map.forEach(
            (x, y) -> System.out.println(x + "=" + y));
    }
}

Output:

1=Geeks
2=For
3=Geeks

Creating MultiMap using Collectors.groupingBy(): Approach:

Get the List to be converted into Map
Convert the List into stream using List.stream() method
Create map with the help of Collectors.groupingBy() method
Collect the formed Map using stream.collect() method
Return the formed Map

Java


   // Java program for list convert  in map
// with the help of Collectors.groupingBy() method

import java.util.*;
import java.util.stream.Collectors;

// create a list
class Student {

    // id will act as Key
    private Integer id;

    // name will act as value
    private String name;

    // create curstuctor for reference
    public Student(Integer id, String name)
    {

        // assign the value of id and name
        this.id = id;
        this.name = name;
    }

    // return private variable id
    public Integer getId()
    {
        return id;
    }

    // return private variable name
    public String getName()
    {
        return name;
    }
}

// main class and method
public class GFG {

    // main Driver
    public static void main(String[] args)
    {

        // create a list
        List<Student> lt = new ArrayList<Student>();

        // add the member of list
        lt.add(new Student(1, "Geeks"));
        lt.add(new Student(1, "For"));
        lt.add(new Student(2, "Geeks"));
        lt.add(new Student(2, "GeeksForGeeks"));

        // create map with the help of
        // Object (stu) method
        // create object of Multi Map class

        // create multimap and store the value of list
        Map<Integer, List<String> >
            multimap = lt
                           .stream()
                           .collect(
                               Collectors
                                   .groupingBy(
                                       Student::getId,
                                       Collectors
                                           .mapping(
                                               Student::getName,
                                               Collectors
                                                   .toList())));

        // print the multiMap
        System.out.println("MultiMap = " + multimap);
    }
}

Output:

MultiMap = {1=[Geeks, For], 2=[Geeks, GeeksForGeeks]}

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