Identify duplicates in a List

Question

I have a List of type Integer eg:

[1, 1, 2, 3, 3, 3]

I would like a method to return all the duplicates eg:

[1, 3]

What is the best way to do this?

Answer 1

The method add of Set returns a boolean whether a value already exists (true if it does not exist, false if it already exists, see Set documentation).

So just iterate through all the values:

public Set<Integer> findDuplicates(List<Integer> listContainingDuplicates)
{ 
  final Set<Integer> setToReturn = new HashSet<>(); 
  final Set<Integer> set1 = new HashSet<>();

  for (Integer yourInt : listContainingDuplicates)
  {
   if (!set1.add(yourInt))
   {
    setToReturn.add(yourInt);
   }
  }
  return setToReturn;
}