Question

Possible Duplicate:
Java Compare Two Lists

If I have two ordered sequences of numbers (I have a lot of flexibility here, and could put my data in a list, set, array, etc.), what’s the most efficient way to go about extracting the matches? For example, if I have:

[1, 2, 4, 6, 9] and [2, 3, 4], I would like to return [2, 4].

There are obviously lots of ways to go about this; I’m curious what the most efficient way is.

Answer 1

If you store the two sequences into Sets you can use Set.retainAll() which:

Retains only the elements in this set that are contained in the specified collection (optional operation). In other words, removes from this set all of its elements that are not contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the intersection of the two sets.

For example:

Set<Integer> s1 = new TreeSet<Integer>();
s1.add(1);
s1.add(2);
s1.add(6);

Set<Integer> s2 = new TreeSet<Integer>();
s2.add(2);
s2.add(6);

s2.retainAll(s1);

for (Integer i: s2)
{
    System.out.println(i);
}

Output:

2
6