Intersection and union of ArrayLists in Java

Question

Are there any methods to do so? I was looking but couldn\'t find any.

Another question: I need these methods so I can filter files. Some are AND filter

Answer 1

The solution marked is not efficient. It has a O(n^2) time complexity. What we can do is to sort both lists, and the execute an intersection algorithm as the one below.

private  static ArrayList interesect(ArrayList f, ArrayList s) { 
    ArrayList res = new ArrayList();

    int i = 0, j = 0; 
    while (i != f.size() && j != s.size()) { 

        if (f.get(i) < s.get(j)) {
            i ++;
        } else if (f.get(i) > s.get(j)) { 
            j ++;
        } else { 
            res.add(f.get(i)); 
            i ++;  j ++;
        }
    }


    return res; 
}

This one has a complexity of O(n log n + n) which is in O(n log n). The union is done in a similar manner. Just make sure you make the suitable modifications on the if-elseif-else statements.

You can also use iterators if you want (I know they are more efficient in C++, I dont know if this is true in Java as well).