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 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?

Answer 1

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;     } }