Combine multiple lists in Java

Question

If I want to make two lists into one in Java, I can use ListUtils.union(List list1,List list2). But what if I want to combine multiple lists?

This works

Answer 1

Java 8 has an easy way of doing it with the help of Stream API shown in the code below. We have basically created a stream with all the lists , and then as we need the individual contents of the lists, there is a need to flatten it with flatMap and finally collect the elements in a List.

Listlist1=Arrays.asList(1,2,3);
Listlist2=Arrays.asList(4,5,6);
Listlist3=Arrays.asList(7,8,9);
Listlist4=Arrays.asList(10,0,-1);
List newList = Stream.of(list1, list2, list3,list4)
                                      .flatMap(Collection::stream)
                                      .collect(Collectors.toList());       
 System.out.println(newList); // prints [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0, -1]