Merge two lists using collect

Question

I have 2 lists.

List<String> names = Arrays.asList("A","B","C","D");
List<String> wife = Arrays.asList("E","F","G");

I have used reduce() to merge it to a single list.

Stream.of(names, wife)
                .reduce(new ArrayList<String>(), (list, val) -> {
                    list.addAll(val);
                    return list;
                }, 
               (list1, list2) -> {list1.addAll(list2); return list1; }));

How do I perform the same operation using

collect 

method. I don't want to use functions from Collectors.java class. Is it possible to do it?

I tried this

List.of(names, wife).stream()
                .collect(new ArrayList<List<String>>(), (l1, l2) -> l1.addAll(l2));

However, I see that the l2 is of type List but not l1. What is l1? I want to learn how the collect method works. Please help!

Answer 1

Per the Javadoc for Stream#collect, you can pass ArrayList::new, ArrayList::add, ArrayList::addAll to the method to create and populate a collection such as ArrayList.

You could specify another concrete collection implementation besides ArrayList, what ever suits your needs.

To join our two lists together, I use Stream.concat.

List < String > first = List.of( "A" , "B" , "C" , "D" );
List < String > second = List.of( "E" , "F" , "G" );

List < String > result =
        Stream
                .concat( first.stream( ) , second.stream( ) )
                .collect( 
                        ArrayList :: new , 
                        ArrayList :: add , 
                        ArrayList :: addAll 
                );

But if you can accept any unmodifiable List, then it is simpler to call toList.

List < String > result =
        Stream
                .concat( first.stream( ) , second.stream( ) )
                .toList( );

For more on joining streams, see Merging Streams in Java by Baeldung.com.

Answer 2

If I understand your goal, you can simply use:

List<String> result = Stream.concat(names.stream(), wife.stream())
        .toList();

Or:

List<String> result = Stream.of(names, wife)
        .flatMap(List::stream)
        .toList();

Or:

List<String> result = new ArrayList<>(names);
result.addAll(wife);