Convert List to Map in Java

We often need to convert list to map in Java. In this lesson, you will learn how to do that with Java 8.

Let’s say we have a list of User objects. 

User.class

class User {

  private String name;
  private String city;
  private String state;
  
 // constructors, getters, setters and toString() method
}

Now, we have a requirement to collect users based on a field. We can do that by converting the provided list into a map.

Convert List to Map with Java 8

In this example, we use the Java Streams API and the Collectors class to collect all users based on a “city”.

class Test {

  public static void main(String[] args) throws IOException {

    Map<String, List<User>> usersMap = getUsers().stream().collect(
           Collectors.groupingBy(User::getCity, HashMap::new, Collectors.toList()));

    System.out.println(usersMap.get("London"));
  }

  private static List getUsers() {
    List users = new ArrayList<>();
    users.add(new User("Tom", "London", "UK"));
    users.add(new User("Jim", "Paris", "France"));
    users.add(new User("Megan", "London", "UK"));
    users.add(new User("Melissa", "Birmingham", "UK"));
    users.add(new User("Steve", "Paris", "France"));

    return users;
  }
}