Find all the years in a list of dates

So, I have a list of dates that is something like this:

List<String> dates = ["2012-05-16", "2012-05-18", "2012-06-19", "2013-01-18", "2013-01-10", "2013-08-05", "2010-07-10"...]

The list goes on with like 100 dates and I want to retrieve all the years that exists on the list. (to get a result like this)

List<String> years = ["2012", "2013", "2010"...]

Solution

Use a substring to split the first 4 characters of each item out. It will leave you with the year

   for (String s : dates) {
         String year = s.subString(0,5);
         years.add(year);

      }

or you could use the .split() method

String year =  s.split("-")[0];

EDIT

Upon further clarification, the question was to get unique years in the array list.

This can be done by passing the arraylist into a Set which does not accept duplicate values and then passing the set back into the array

// add elements to al, including duplicates
Set<String> hs = new HashSet<>();
hs.addAll(years);
years.clear();
years.addAll(hs);