Tags: Collectors , List , Map , Stream

Code snippet demonstrate converting List<T> to Map<K,V>.

Source code(User.java)

public class User {
   private Long id;
   private String name;
 
   public Long getId() {
    return id;
   }
   public void setId(Long id) {
    this.id = id;
   }
   public String getName() {
    return name;
   }
   public void setName(String name) {
    this.name = name;
   }
}

Source code(ExampleListToMap.java)

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import javaQuery.beans.User;

public class ExampleListToMap {

    public static void main(String[] args) {
        /* Creating List<String> */
        List<String> listStrings = new ArrayList<String>();
        listStrings.add("Vicky");
        listStrings.add("Thakor");

        /**
         * Convert List<String> to Map<String, String> using `stream` API
         * Explanation:
         * Collectors.toMap(String::toString, listStringName -> listStringName)
         * - String::toString => Calling `toString()` method of `String` class. 
         *                       (Note: Its compulsory to set `key` of Map from String class.)  
         * 
         * - listStringName -> listStringName => Value of Map.
         */
        Map<String, String> mapName = listStrings.stream()
                                                 .collect(Collectors.toMap(String::toString, listStringName -> listStringName));
        /* Print the Map */
        System.out.println("List<String> to Map<String, String>: " + mapName);
        System.out.println("+++++++++++++++++++++++++++++++++++++++++++");

        /* Creating User objects */
        User objUser = new User();
        objUser.setId(1L);
        objUser.setName("Vicky Thakor");

        User objUser2 = new User();
        objUser2.setId(2L);
        objUser2.setName("Chirag Thakor");

        /* Add users to List<User> */
        List<User> listUsers = new ArrayList<User>();
        listUsers.add(objUser);
        listUsers.add(objUser2);

        /**
         * Convert List<User> to Map<Long, User> using `stream` API.
         * Explanation:
         * Collectors.toMap(User::getId, user -> user)
         * - User::getId => Calling `getter` method of `User` class. 
         *                  (Note: Its compulsory to set `key` of Map from User class.)  
         * 
         * - user -> user => Value of Map.
         */
        Map<Long, User> mapUsers = listUsers.stream().collect(Collectors.toMap(User::getId, user -> user));
  
        /* Print the Map */
        System.out.println("List<User> to Map<Long, User>:");
        mapUsers.keySet()
                .forEach(key -> {
                            User getUser = mapUsers.get(key);
                            System.out.println(key + "=" + getUser.getName());
                });
    }
}

Output

List<String> to Map<String, String>: {Vicky=Vicky, Thakor=Thakor}
+++++++++++++++++++++++++++++++++++++++++++
List<User> to Map<Long, User>:
1=Vicky Thakor
2=Chirag Thakor

Tags: Collectors , List , Map , Stream