Question

I have an ArrayList(String) which contains a list of formatted dates like this:

element 1: "2012-5-1"
element 2: "2012-8-10"
element 3: "2012-12-5"
element 4: "2013-12-21"
element 5: "2013-12-13"
element 6: "2014-5-8"

What is the most efficient/framework way to create another list or normal primitive array that contains the unique year entries? For example my new list would contain:

element 1: "2012"
element 2: "2013"
element 3: "2014"

Answer 1

Try this

ArrayList<String> yearsOnlylist = new ArrayList<String> ();
for(String s : elements) {
    String yearExtracted = s.substring(0,4);
    yearsOnlylist.add(yearExtracted);
}

Where elements is the name of your list of date in the extended form.

Using as destination list

 LinkedList<String> yearsOnlylist = new LinkedList<String> ();

instead of an ArrayList could sightly improve the conversion efficiency (because the add is O(1) in LinkedList) but access a specific position in a second time, has a lower efficiency (O(n) vs O(1)).