Intersection and union of ArrayLists in Java
intersection, java, list, union
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;
}
}
Problem
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?