Convert list of complex objects to Map using Java 8 - java-8

Question

I have an array of subjects
List<String> subjects = Arrays.asList(“physics”, “maths”);
I wanted to create a dummy list of users for each of these subjects and add them to a map with key as subject and value as List
Something like
Map<String,List<User>> userMap = new HashMap<>();
for(String subject: subjects){
List<User> users = new ArrayList<User>();
for(int i=0;i<10;i++){
User user = new User(“first name”+i+subject,”last name”+i+subject);
users.add(user);
}
userMap.put(subject,users);
}
I wanted to try this with the Java 8. Just tried something below, but doesn’t look like the right way.
subjects.stream().map((subjectName)->{
List<User> userList = new ArrayList<User>();
for(int i=0;i<10;i++){
User user = new User(“first name”+i+subject,”last name”+i+subject);
userList.add(user);
}
})

Answer 1

subjects.stream()
.map(subjectName -> {
List<User> users = .... create the users;
return new SimpleEntry<>(subjectName, users);
})
.collect(Collectors.toMap(Entry::getKey, Entry::getValue))
This would be one way if you really wanted to do it with java-8 and streams. One improvement would be to have a method that takes a String subjectName and create that Entry for example:
private static Entry<String, List<User>> createEntry(String subjectName) {
List<User> users = .... create the user;
return new SimpleEntry<>(subjectName, users);
}
And use it with:
subjects.stream()
.map(YourClass::createEntry)
.collect(Collectors.toMap(Entry::getKey, Entry::getValue))
Just notice that your loop is the cleanest way to do it IMO

Answer 2

One way to do it with java 8:
Map<String,List<User>> userMap = new HashMap<>();
subjects.forEach(s -> {
for (int i = 0; i < 10; i++)
userMap.computeIfAbsent(s, k -> new ArrayList<>())
.add(new User("first name" + i + subject, "last name" + i + subject));
});

Answer 3

Let's do this one step at a time. First, the inner loop for creating 10 users can be written with streams as:
import static java.util.stream.Collectors.toList;
import static java.util.stream.Collectors.toMap;
List<User> userList = IntStream.range(0, 10)
.mapToObj(i -> new User("first name" + i + subject, "last name" + i + subject)
.collect(toList());
And the outer loop can be written as
subjects.stream()
.collect(toMap(
subject -> subject, // key for the map is the subject
subject -> ... // whatever should be the value of the map
));
And now we can put it together:
Map<String, List<User>> userMap = subjects.stream()
.collect(toMap(
subject -> subject,
subject -> IntStream.range(0, 10)
.mapToObj(i -> new User("first name" + i + subject, "last name" + i + subject))
.collect(toList())
));