Unable to case Set to Set

Question

I am getting error while casting Set < Supertype > to Set< Subtype >

I am getting following error :

Cannot cast from Set< AppUserRole > to Set< AppUserRoleEntity >

interface AppUserRole {
    void m1();
}

class AppUserRoleEntity implements AppUserRole {

    @Override
    public void m1() {

    }

}

interface AppUser {

    void m2(Set<AppUserRole> abc);
}

class AppUserEntity implements AppUser {

    Set<AppUserRoleEntity> role;

    @Override
    public void m2(Set<AppUserRole> abc) {
        this.role = (Set<AppUserRoleEntity>) abc;// Compilation fails here

    }

}

After reading PECS I have modified my code like below

    Set< ? super AppUserRoleEntity> role;

But not on getter method of same class I am getting warning

Type safety: Unchecked cast from Set< capture#2-of ? super AppUserRoleEntity > to Set

@Override
    public Set<AppUserRole> getAppUserRoles() {
        return (Set<AppUserRole>) appUserRoles;
    }

Can some one tell me how to fix this problem ? Please help

You need co/contravariance: https://stackoverflow.com/questions/2501023/demonstrate-covariance-and-contravariance-in-java and then it will work one way (either co or contravariance) — Dennis Kuypers, Dec 28 '17 at 15:48
"Can some one tell me how to fix this problem ?" -> Yes, deploy [PECS](https://stackoverflow.com/questions/2723397/what-is-pecs-producer-extends-consumer-super). TL;DR: generics are not covariant. — Turing85, Dec 28 '17 at 15:48
@azurefrog relevant, but it's the other way round: OP is asking why a list of animals can't be cast to a list of dogs. And of course, the answer is that a list of animals might contain cats. — Andy Turner, Dec 28 '17 at 15:48
@PrzmyslawMoskal same comment I made above to azurefrog. OP is asking about the reverse case. — Andy Turner, Dec 28 '17 at 15:50
@AndyTurner not really, is the same case. OP is trying to cast a `List` into a `List`. Even if he finds a way so that doesn't fail at compilation, it will fail in runtime when the `List` contains something that is not a dog. Like a cat. Or a whale. — Ray O'Kalahjan, Dec 28 '17 at 16:08
Why do you think you need to do this cast in the first place? Why can't the `role` set just be `Set`? In your edit, you even appear to be returning the set as a `Set`. — Radiodef, Dec 28 '17 at 16:09
@AndyTurner I deleted comment in which there was an information about that I suspect this question as duplicate. It seems that it was not the correct way of releasing the flag that I submitted, because now I have information that my flag was helpful although I reconsidered submitting it. Anyway, thank you for offering your point of view, I actually agree with your opinion now. — Przemysław Moskal, Dec 28 '17 at 20:23

score 0 · Answer 1 · answered Dec 28 '17 at 16:01

My advice is to declare the generic type in the interface:

interface AppUserRole {
    void m1();
}

class AppUserRoleEntity implements AppUserRole {

    @Override
    public void m1() {

    }

}

interface AppUser<R extends AppUserRole> {

    void m2(Set<R> abc);
}

class AppUserEntity implements AppUser<AppUserRoleEntity> {

    Set<AppUserRoleEntity> role;

    @Override
    public void m2(Set<AppUserRoleEntity> abc) {
        this.role = abc;// No cast needed

    }

}

Marcos Vasconcelos · Answer 2 · 2017-12-28T15:59:40.973

-1

You can erase type and do a unchecked cast for your type, as below:

@Override
public void m2(Set<AppUserRole> abc) {
    this.role = (Set<AppUserRoleEntity>) (Set<?>) abc;
}

Also if Set is typed as accepting a capture of extends AppUser you can do the cast without erasure.

@Override
public void m2(Set<? extends AppUserRole> abc) {
    this.role = (Set<AppUserRoleEntity>) abc;
}

That is just a unsafe cast. If you pass a item type that is not AppUserRoleEntity the program will throw ClassCastException.

Besides, the best solution is to use the interface and not the implementing type.

edited Dec 28 '17 at 15:59

answered Dec 28 '17 at 15:54

Marcos Vasconcelos

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which will fail horribly at runtime when `abc` contains other implementations than `AppUserRoleEntity` – Lino Dec 28 '17 at 15:55
An explicit cast will also fails – Marcos Vasconcelos Dec 28 '17 at 15:57
1

... which is why we have [PECS](https://stackoverflow.com/questions/2723397/what-is-pecs-producer-extends-consumer-super) – Turing85 Dec 28 '17 at 15:59

Unable to case Set to Set

2 Answers2