Question

What is the best way to find and mark duplicate objects in a Collection? Let us say we have a List persons and our duplicate strategy is based on exact match of first name and last name.

1. Identify all duplicates
2. Mark each duplicate person indicating it is a duplicate
3. For each duplicate person, identify the object it is the duplicate of

Is there a simple way of doing this with guava?

Answer 1

You don’t need Guava to do this:

List<Person> people = ...
Map<Name, Person> peopleByName = new HashMap<>();
for (Person person : people) {
  // Name is a simple value class with equality based on its fields
  Name name = new Name(person.getFirstName(), person.getLastName());
  Person firstPersonWithName = peopleByName.get(name);
  if (firstPersonWithName == null) {
    peopleByName.put(name, person);
  } else {
    // or whatever you do to mark a duplicate
    person.setDuplicateOf(firstPersonWithName);
  }
}

That said, you could use a Guava Table instead of a Map and avoid needing to create the Name… use first name as the row keys and last name as column keys, say.

Another choice would be to use Multimaps.index to index all the people in your list by name. Then for every list of people mapped to a particular name, the first person would be the first person with that name from your list and the others would be duplicates.