freshest freshest - 1 year ago 74
Java Question

Identify duplicates in a List

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 Source

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))
  return setToReturn;
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