import java.util.*;

public class UnionIntersectionOfTwoListsExample {
	
	public static void main (String[] args) {
		
		List<String> list1 = new ArrayList<String>(Arrays.asList("a","b","c","d","e"));
		List<String> list2 = new ArrayList<String>(Arrays.asList("b","d","f","g"));
		
		//Find union of two lists
		System.out.println("Union of List1 and List2 :" + getUnionOfLists(list1, list2));
		
		//Find intersect of lists using Stream API (Java 8)
		System.out.println("Intersection of List1 & List2 Method 1: " + getIntersectOfLists1(list1, list2));	
				
		//Find intersect of lists using retainAll() method
		System.out.println("Intersection of List1 & List2 Method 2: " + getIntersectOfLists2(list1, list2));	
	}

	private static List<String> getUnionOfLists(List<String> list1, List<String> list2) {
		
		Set<String> set = new HashSet<>();
		set.addAll(list1);
		set.addAll(list2);
		
		return new ArrayList<>(set);
	}

	private static List<String> getIntersectOfLists1(List<String> list1, List<String> list2) {
		List<String> intersectElements = list1.stream().filter(list2 :: contains).collect(Collectors.toList());
		
		if(!intersectElements.isEmpty()) {
			return intersectElements;
		}else {
			return Collections.emptyList();
		}
	}

	private static List<String> getIntersectOfLists2(List<String> list1, List<String> list2) {
		list1.retainAll(list2);
		return list1;
	}

}