yotamoo yotamoo - 10 months ago 66
Java Question

Intersection and union of ArrayLists in Java

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

filters and some are
filters (like in set theory), so I need to filter according to all files and the unite/intersects ArrayLists that holds those files.

Should I use a different data structure to hold the files? Is there anything else that would offer a better runtime?

Answer Source

Here's a plain implementation without using any third-party library. Main advantage over retainAll, removeAll and addAll is that these methods don't modify the original lists input to the methods.

public class Test {

    public static void main(String... args) throws Exception {

        List<String> list1 = new ArrayList<String>(Arrays.asList("A", "B", "C"));
        List<String> list2 = new ArrayList<String>(Arrays.asList("B", "C", "D", "E", "F"));

        System.out.println(new Test().intersection(list1, list2));
        System.out.println(new Test().union(list1, list2));

    public <T> List<T> union(List<T> list1, List<T> list2) {
        Set<T> set = new HashSet<T>();


        return new ArrayList<T>(set);

    public <T> List<T> intersection(List<T> list1, List<T> list2) {
        List<T> list = new ArrayList<T>();

        for (T t : list1) {
            if(list2.contains(t)) {

        return list;