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Why is adding a subclass of type a to the collection is illegal?

given this piece of code

    //Creates a list of List numbers
    List<List<Number>> num = new ArrayList<List<Number>>();
    //Creates a list of List doubles
    List<List<Double>> doub = new ArrayList<List<Double>>();
    //List of doubles
    List<Double> d = new ArrayList<Double>();
    d.add(2.5);
    d.add(2.6);
    doub.add(d);

    num.add(d);//This code will not compile

Why won't this num.add (doub) be resolved? not a List<List<Number>>supertype List<List<Double>>?

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4 answers

Inheritance generics is slightly different from our understanding of inheritance. If you want to use subclass types, you need to define wildcards using either extends (or) super in generics.

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You are trying to add a list of lists to a list that accepts a list. (not after the edit I see)

Or maybe a little more confusing:

Your num list can only add List<Number>, you are trying to add a List<List<Double>>.

, , , .

num :

    List<List<? extends Number>> num = new ArrayList<List<? extends Number>>();

:

    num.add(d);

:

    num.add(doub);
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List<Double> List<Number>, List<>.

, List<List<Double>> List<List<Number>>, Java Generics. extends, .

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, doub.add(d) num.add(doub) add(). doub.add(d) , num.add(doub).

,

num.add(d)


num.add(doub)

?

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