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Group by field name in java
public class Person{
    String name;
    String surname;
    ....
}

List<Person> allPeople; // your list of all people
Map<String, List<Person>> map = new HashMap<String, List<Person>>();
for (Person person : allPeople) {
   String key = person.getName();
   if (map.get(key) == null) {
      map.put(key, new ArrayList<Person>());
   }
   map.get(key).add(person);
}

List<Person> davids = map.get("David");

void addPerson(Person p, Map<String, List<Person>> map){
  ArrayList<Person> lst = map.get(p.name);
  if(lst == null){
     lst = new ArrayList<Person>();
  }
  lst.add(p);
  map.put(p.name, lst);
}
...
for(Person p:personsCollection>){
   addPerson(p, map);
}

class Person (val name: String, val surname: String ="Smith")
val li = List (new Person ("David"), new Person ("Joe"), new Person ("Sue"), new Person ("David", "Miller"))
li.groupBy (_.name)