Streams – groupingBy() operation

The groupingBy() function belongs to the Collectors class from java.util.stream package. It groups the elements based on a specified property, into a Stream. It is equivalent to the groupBy() operation in SQL.

Syntax

 public static <T, K> Collector<T, ?, Map<K, List<T>>>
    groupingBy(Function<? super T, ? extends K> classifier)

Java Stream groupingBy() operation – examples

Example 1:

A program that will take a list of strings and group them by the length attribute:

class Test {

  public static void main(String[] args) {
    List<String> cities = new ArrayList<>(Arrays.asList("Paris", "Bern", "London", "Tokyo", "Boston"));

    Map<Integer, List<String>> resultMap = cities.stream()
            .collect(Collectors.groupingBy(String::length));

    System.out.println(resultMap);
  }
}

Output: {4=[Bern], 5=[Paris, Tokyo], 6=[London, Boston]}

Example 2:

Let’s write a program that will take a list of Users and group them by the membership type:

class Test {

  public static void main(String[] args) {
    Map<String, List<User>> resultMap = getAllUsers().stream()
            .collect(Collectors.groupingBy(User::getMembershipType));

    System.out.println(resultMap);
  }

  private static List<User> getAllUsers() {
    List<User> users = new ArrayList<>();
    users.add(new User("John", "john123", "premium", "5th Avenue"));
    users.add(new User("Megan", "meganusr", "gold", "New Light Street"));
    users.add(new User("Steve", "steve1234", "regular", "New Street Avenue"));
    users.add(new User("Melissa", "mell1", "premium", "Ser Kingston Street"));
    users.add(new User("Joan", "joan1234", "gold", "New Light Street"));

    return users;
  }
}

class User {
  private String name;
  private String username;
  private String membershipType;
  private String address;

  public User(String name, String username, String membershipType, String address) {
    this.name = name;
    this.username = username;
    this.membershipType = membershipType;
    this.address = address;
  }

  public String getMembershipType() {
    return membershipType;
  }

  @Override
  public String toString() {
    return "name: " + name + " ," + "username: " + username;
  }
}

Output:

{ 
gold=[name: Megan ,username: meganusr, name: Joan ,username: joan1234], 
premium=[name: John ,username: john123, name: Melissa ,username: mell1], 
regular=[name: Steve ,username: steve1234] 
}

I hope this tutorial was helpful to you. To learn more, check out other Java Functional Programming tutorials.