Java Generics Puzzler, extending a class and using wildcards

Question

I\'ve been beating my head against this one for awhile and thought that maybe some fresh eyes will see the issue; thanks for your time.

import java.util.*;

clas         


        

Answer 1

This happens because of the way capture conversion works:

There exists a capture conversion from a parameterized type G1,...,Tn> to a parameterized type G1,...,Sn>, where, for 1 ≤ i ≤ n :

• If Ti is a wildcard type argument of the form ? extends Bi, then Si is a fresh type variable [...].

Capture conversion is not applied recursively.

Note the end bit. So, what this means is that, given a type like this:

    Map>
//      │  │    └ no capture (not applied recursively)
//      │  └ T2 is not a wildcard
//      └ T1 is a wildcard

Only "outside" wildcards are captured. The Map key wildcard is captured, but the List element wildcard is not. This is why, for example, we can add to a List>, but not a List. The placement of the wildcard is what matters.

Carrying this over to TbinList, if we have an ArrayList>, the wildcard is in a place where it does not get captured, but if we have a TbinList, the wildcard is in a place where it gets captured.

As I alluded to in the comments, one very interesting test is this:

ArrayList> test3 = new TbinList<>();

We get this error:

error: incompatible types: cannot infer type arguments for TbinList<>
    ArrayList> test3 = new TbinList<>();
                                                        ^
    reason: no instance(s) of type variable(s) T exist so that
            TbinList conforms to ArrayList>

So there's no way to make it work as-is. One of the class declarations needs to be changed.

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Additionally, think about it this way.

Suppose we had:

class Derived1 extends Base {}
class Derived2 extends Base {}

And since a wildcard allows subtyping, we can do this:

TbinList test4 = new TbinList();

Should we be able to add a Tbin to test4? No, this would be heap pollution. We might end up with Derived2s floating around in a TbinList.