Java8 Collectors.mapping example 2019-04-11 11:00
There are many practical methods in the Collectors class. mapping method is one of them. In this page I will show you how to use mapping method in Collectors. Let's say you have a list of users(List). Let's say you want to group these users by age. The code will be like this.
Map<Integer, List<User>> userMap = users.stream().collect(Collectors.groupingBy(User::getAge));
If you want to get a collection of names that users group by age, then you need to use the mapping method. We can map User to user's name and collect them.
Map<Integer, List<String>> userAgeNameMap = users.stream().collect(Collectors.groupingBy(User::getAge, Collectors.mapping(User::getName, Collectors.toList())));
The complete code and output are as follows.
public class CollectorsMappingClient {
public static void main(String[] args) {
User user1 = new User("henry", 30);
User user2 = new User("justin", 29);
User user3 = new User("mathew", 25);
User user4 = new User("charles", 30);
List<User> users = Arrays.asList(user1, user2, user3, user4);
Map<Integer, List<User>> userMap = users.stream().collect(Collectors.groupingBy(User::getAge));
System.out.println(userMap);
Map<Integer, List<String>> userAgeNameMap = users.stream().collect(Collectors.groupingBy(User::getAge, Collectors.mapping(User::getName, Collectors.toList())));
System.out.println(userAgeNameMap);
}
}
public class User {
public User(String name, int age) {
this.name = name;
this.age = age;
}
private String name;
private int age;
//getter and setter
@Override
public String toString() {
return "User{" +
"name='" + name + '\'' +
", age=" + age +
'}';
}
}
output.
{25=[User{name='mathew', age=25}], 29=[User{name='justin', age=29}], 30=[User{name='henry', age=30}, User{name='charles', age=30}]}
{25=[mathew], 29=[justin], 30=[henry, charles]}