Create java collection with n clones of an object

Question

In Java, is there a one-line way to create a collection that is initialized with n clones of an object?

I'd like the equivalent of this:

• foo = vector<vector<int> >(10); c++, creates 10 different empty vectors
• [ [] for i in range(10) ] Python, an array of 10 distinct empty arrays
• Array.new(10) { [] } Ruby, same as Python

In Java, I've only found

new ArrayList<ArrayList<Integer> >(Collections.nCopies(10, new ArrayList<Integer>()))

However, this is not equivalent to the other examples, because the lists alias.

Is there a way to create an array of distinct object clones, without using a for loop, and preferably without resorting to external libraries?

Answer 1

If you're using Java 8 you could use its streams:

Stream.generate(ArrayList<Integer>::new)
    .limit(10).collect(Collectors.toList());

The Stream.generate() method takes a Supplier that knows how to produce a value and generates an infinite stream of those values (each value is obtained by calling the supplier again, so they are all different, unlike Collections.nCopies()). Placing a limit() on the stream and then collecting the results to a list thus yields a list of distinct entries.

Note that starting in Java 16 Stream has a toList() method, so this can become a little cleaner:

Stream.generate(ArrayList<Integer>::new).limit(10).toList();