CODEWITHSUNDEEP
javaarraylistduplicatesarray-merge
John Evans Solachuk
John Evans Solachuk

Reputation: 2085

How do I obtain the duplicated values of two ArrayLists?

Let's say I have 2 ArrayList of Points:

  1. (0,2)->(0,3)->(0,4)
  2. (0,2)->(0,3)->(0,6)

And I want to obtain a new list : (0,2)->(0,3)

How do I do that?

current solution

Using two foreach loops to compare the two lists, element by element. I think it's a very inefficient way. Are there any other ways?

Upvotes: 2

Views: 84

Answers (2)

Nitin Dandriyal
Nitin Dandriyal

Reputation: 1607

If large lists, add elements of one list to a HashSet and iterate other while keep adding elements to the new list which the HashSet contains

List<Point> list1 = new ArrayList<Point>(Arrays.asList(new Point[]{new Point(0,2), new Point(0,3), new Point(0,4)}));
List<Point> list2 = new ArrayList<Point>(Arrays.asList(new Point[]{new Point(0,2), new Point(0,3), new Point(0,6)}));
Set<Point> setList1 = new HashSet<Point>(list1);
List<Point> intersection  = list2.stream().filter( l -> setList1.contains(l)).collect(Collectors.toList());

Time complexity, Adding to Set = O(n), Iterating list = O(k) time hashset lookup O(1) ~ overall O(n)

Upvotes: 0

Konstantin Yovkov
Konstantin Yovkov

Reputation: 62864

You can use the List#retainAll(Collection<?> c) method, which:

Retains only the elements in this list that are contained in the specified collection (optional operation). In other words, removes from this list all of its elements that are not contained in the specified collection.

List<Point> first = ...
List<Point> second = ...
first.retainAll(second);

Upvotes: 6

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