Group by field name in Java

java dictionary grouping
London picture London · Mar 22, 2011 · Viewed 28.7k times · Source

I'm trying to group Java objects by their field, i.e Person.java

public class Person {
    String name;
    String surname;
    ....
}

So if I have n Person objects, what would be the easiest way to get all people name "David" into a map like Map<String, List<Person>> map;?

I found this on Google (but it doesn't compile), it seems to be the thing I'm looking for: http://www.anzaan.com/2010/06/grouping-objects-using-objects-property/

Answer

Kaleb Brasee picture Kaleb Brasee · Mar 22, 2011

There's probably a library that can do this more simply, but it's not too hard to do it manually:

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");

Related questions

How do I efficiently iterate over each entry in a Java Map?

If I have an object implementing the Map interface in Java and I wish to iterate over every pair contained within it, what is the most efficient way of going through the map? Will the ordering of elements depend on …

Read More
Sort a Map<Key, Value> by values

I am relatively new to Java, and often find that I need to sort a Map<Key, Value> on the values. Since the values are not unique, I find myself converting the keySet into an array, and sorting …

Read More
How to directly initialize a HashMap (in a literal way)?

Is there some way of initializing a Java HashMap like this?: Map<String,String> test = new HashMap<String, String>{"test":"test","test":"test"}; What would be the correct syntax? I have not found anything regarding this. …

Read More