List<String> newList = Stream.concat(listOne.stream(), listTwo.stream()).toList();List<Integer> newList = Stream.of(list1, list2, list3, list4) .flatMap(Collection::stream) .collect(Collectors.toList());import java.util.ArrayList; import java.util.List; public class JoinLists { public static void main(String[] args) { List<String> list1 = new ArrayList<String>(); list1.add("a"); List<String> list2 = new ArrayList<String>(); list2.add("b"); List<String> joined = new ArrayList<String>(); joined.addAll(list1); joined.addAll(list2); System.out.println("list1: " + list1); System.out.println("list2: " + list2); System.out.println("joined: " + joined); } } //output list1: [a] list2: [b] joined: [a, b]