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Is it possible to have a collection of shared collections?

I am writing code that should handle an arbitrary number of doubling lists. However, although I can declare functional parameters like List<List<Double>> , I cannot create the actual instances, since I need to create instances of a specific class, such as ArrayList

I 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 compilation error when add() called, saying that the method is not applicable for arguments like ArrayList<Double>

It works

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

but it’s somehow ugly to use throws in this way. Is there a better approach?

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3 answers

You can omit the cast, as this is absolutely true. Each ArrayList is a list.

 List<List<Double>> inputs = new ArrayList<List<Double>>(); inputs.add(new ArrayList<Double>());
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You do not understand what a wildcard means. <? extends List> <? extends List> does not mean "everything that extends the list", it means "Some specific thing that extends the list, but we don’t know what it is." So it is illegal to add an ArrayList to this because we don’t know if the ArrayList is specific to that in this collection.

Just old:

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

works great

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Two very different situations when defining a type of type must be declared:

1) in an open interface. use the correct abstract type.

  public List<List<Double>> querySomething();

2) as a implementation detail. use concrete type

  ArrayList<ArrayList<Double>> list = new ArrayList<ArrayList<Double>>();

I assume that you are in the second case.

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