Java8 stream groupingBy example 2018-06-12 04:24
When we want to group the list data, we have to write some tedious code in the old version of the JDK. For example we want to group list of people by age. The code is like this before JDK8.
private static void oldGroupingBy(List<User> users) {
Map<Integer, List<User>> ageUsersMap = new HashMap<>();
for (User user : users) {
List<User> groupOfUser = ageUsersMap.get(user.getAge());
if (groupOfUser != null) {
groupOfUser.add(user);
continue;
}
groupOfUser = new ArrayList<>();
groupOfUser.add(user);
ageUsersMap.put(user.getAge(), groupOfUser);
}
System.out.println("age users map:" + ageUsersMap);
}
Use stream and Collectors.groupingBy can make it easy.
private static void newGroupingBy(List<User> users) {
Map<Integer, List<User>> ageUsersMap = users.stream().collect(Collectors.groupingBy(User::getAge));
System.out.println("age users map:" + ageUsersMap);
Map<Integer, Long> ageCountMap = users.stream().collect(Collectors.groupingBy(User::getAge, Collectors.counting()));
System.out.println("age count map:" + ageCountMap);
}
The complete code is as follows
public class StreamGroupingByClient {
public static void main(String[] args) {
List<User> users = new ArrayList<>();
users.add(new User("henry", 30));
users.add(new User("justin", 30));
users.add(new User("mathew", 27));
oldGroupingBy(users);
newGroupingBy(users);
}
private static void newGroupingBy(List<User> users) {
Map<Integer, List<User>> ageUsersMap = users.stream().collect(Collectors.groupingBy(User::getAge));
System.out.println("age users map:" + ageUsersMap);
Map<Integer, Long> ageCountMap = users.stream().collect(Collectors.groupingBy(User::getAge, Collectors.counting()));
System.out.println("age count map:" + ageCountMap);
}
private static void oldGroupingBy(List<User> users) {
Map<Integer, List<User>> ageUsersMap = new HashMap<>();
for (User user : users) {
List<User> groupOfUser = ageUsersMap.get(user.getAge());
if (groupOfUser != null) {
groupOfUser.add(user);
continue;
}
groupOfUser = new ArrayList<>();
groupOfUser.add(user);
ageUsersMap.put(user.getAge(), groupOfUser);
}
System.out.println("age users map:" + ageUsersMap);
}
}
output
age users map:{27=[User{name='mathew', age=27}], 30=[User{name='henry', age=30}, User{name='justin', age=30}]}
age users map:{27=[User{name='mathew', age=27}], 30=[User{name='henry', age=30}, User{name='justin', age=30}]}
age count map:{27=1, 30=2}