javalistunionintersection

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 AND filters and some are OR 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?


Solution

  • 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>();
    
            set.addAll(list1);
            set.addAll(list2);
    
            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)) {
                    list.add(t);
                }
            }
    
            return list;
        }
    }