Question

In the Hidden Features of Java question, I was interested in the answer about instance initializers.

I was wondering how to modify this line :

List<Integer> numbers = new ArrayList<Integer>(){{ add(1); add(2); }};

in order to make it perform the same job with nested Arraylists:

ArrayList<ArrayList<Integer>> numbers = ...

is that possible?

Answer 1

List<Integer> numbers = new ArrayList<Integer>(){{ add(1); add(2); }};

I don’t really recommend this approach, as it creates an (anonymous) class for no good reason.

Use either:

List<Integer> numbers = Arrays.asList(1, 2);

or

List<Integer> numbers = new ArrayList<Integer>(Arrays.asList(1, 2));

For 2 levels, you can use:

List<List<Integer>> numbers = Arrays.asList(Arrays.asList(1, 2), Arrays.asList(2,3,4));

With a static import you could even reduce it to this, if you really want to:

List<List<Integer>> numbers = asList(asList(1, 2), asList(2,3,4));