Question

I’m writing some code that needs to process an arbitrary number of lists of doubles. However, although I can declare function parameters of type List<List<Double>> I’m having trouble creating actual instances since I need to create instances of a concrete class such as ArrayList

I’ve tried

List<? extends List<Double>> inputs = new ArrayList<List<Double>>();
inputs.add(new ArrayList<Double>());

and

List<? extends List<? extends Double>> inputs = new ArrayList<List<Double>>();
inputs.add(new ArrayList<Double>());

but in both cases I get a compile error on the call to add() saying that the method is not applicable for arguments of type ArrayList<Double>

This works

List<List<Double>> inputs = new ArrayList<List<Double>>();
inputs.add((List<Double>) new ArrayList<Double>());

but it’s kind of ugly to have to use casts in this way. Is there a better approach?

Answer 1

You can omit the cast since this one is perfectly valid. Each ArrayList is a List.

List<List<Double>> inputs = new ArrayList<List<Double>>();
inputs.add(new ArrayList<Double>());