Convert a Nested Array of int into a List of Lists with Streams in Java 8

September 3, 2022

I have an array as:

int[][] bookings= {{6, 7, 20}, {2, 0, 30}, {7, 20, 15}};

Which I want to be:

ArrayList = {
    ArrayList<Integer>{6, 7, 20},
    ArrayList<Integer>{2, 0, 30},
    ArrayList<Integer>{7, 20, 15},
}

How can I achieve that?

I’ve tried to use:

Arrays.stream(bookings)
    .flatMapToInt(x -> Arrays.stream(x))
    .boxed()
    .collect(Collectors.toList());

But this return only a single Arraylist, like:

ArrayList list = {6, 7, 20, 2, 0, 30, 7, 20, 15};

>Solution :

You don’t need to flatten your data with flatMapToInt(), since you need a nested list as a result.

Instead, you need to transform each array into a list, which can be done via map() operation, you can create a nested stream and collect array contents into a list.

And then collect stream lists into a list.

List<List<Integer>> result = Arrays.stream(bookings) // Stream<int[]>
    .map(booking -> Arrays.stream(booking).boxed()   // Stream<List<Integer>>
        .collect(Collectors.toList()))
    .collect(Collectors.toList())