Java Stream Guide

Assume we have the following classes,

@Data
public class User {
  private long id;
  private String name;
  private int age;

  private List<Address> address;
}


@Data
public class Address {
  private String postCode;
  private String street;
  private String building;
}

1. Conver a List of User into a Map with key id

   Java

   List<User> users = getUsers();
   Map<Long, User> userIdMap = users.stream().collect(Collectors.toMap(User::getId, Function.identity))
   

   Function.identity is a predefined method that always return the input parameter itself.

2. Convert a property of User into a list

   Java

   List<User> users = getUsers();
   // get all user names as a list
   List<String> userNames = users.stream().map(User::getName).toList();
   

3. Flat map a list property of User into a list

   Java

   List<User> users = getUsers();
   List<Address> addresses = users.stream().map(User::getAddress).flatMap(List::stream).toList();
   

   We also can user flatMap to merge a list of list objects.

4. Filter by property

   Java

   List<User> users = getUsers();
   List<Address> addresses = users.stream().filter(u -> u.getAge() >= 18).toList();
   

5. Group By Age

   Java

   // group by age and collect user
   List<User> users = getUsers();
   Map<Integer, List<User>> ageUsers = users.stream().collect(Collectors.groupingBy(User::getAge));
   
   // group by age and collect name
   
   Map<Integer, Set<String>> ageNames = users.stream().collect(Collectors.groupingBy(User::getAge, TreeMap::new, Collectors.mapping(User::getName, Collectors.toSet())));