void with Generics in Java

Question

I have a function that returns void

public interface IProductService {
   void delete(String id);
}

Generic method

public interface IRequestHandler<C , R> {
    R handler(C c);
    Class<C> commandType();
}

Implementation of generic interface

 @Singleton
    public record DeleteProductCommandHandler(IProductService iProductService)
            implements IRequestHandler<DeleteProductCommand, Void> {


        @Override
        public Void handler(DeleteProductCommand deleteProductCommand) {
            return iProductService.delete(deleteProductCommand.id);
        }

        @Override
        public Class<DeleteProductCommand> commandType() {
            return DeleteProductCommand.class;
        }
    }

How can I use void in IRequestHandler<DeleteProductCommand, Void> so that I can map void from iProductService.delete(deleteProductCommand.id);

Make the `handler` method to contain two statements: 1) simple `iProductService.delete(...)`; 2) and `return null;`. — terrorrussia-keeps-killing, Dec 22 '21 at 15:17
Null is not the return type; `Void` is. But there is only one possible value of `Void` -- `null`. So that's what you return. (You can understand this by analogy to the other primitive wrappers. `Integer` has one value that corresponds to each value of `int`, plus the distinguished value `null`. Same is true for all the primitive wrappers. Since `void` has no values, `Void` has all (none) of those, plus `null`.) — Brian Goetz, Dec 22 '21 at 15:19
@BrianGoetz - Your explanation is a little confusing. It seems I can only return a `null` from a `Void` method. If `Void has all (none) of those, plus null` how come I cannot return any thing such as an `int`? — K.Nicholas, Dec 22 '21 at 15:53
@K.Nicholas `Integer` is the reference companion type for `int`. For every value of `int` (all 2^32) of them, there is a corresponding value of `Integer`, plus `Integer` also can take on the value `null`. `Void` is the reference companion for `void`. For every value of `void` (all zero of them), there is a corresponding value of `Void`, plus `Void` can also take on the value `null`. So `null` is the only valid value of `Void`. If you wanted to be able to return _anything_, make your return type `Object`. — Brian Goetz, Dec 22 '21 at 16:20
@BrianGoetz - Thanks. Sounds like you're using set theory (discrete math?) for your explanation. — K.Nicholas, Dec 22 '21 at 16:30
See [Cannot convert void to java.lang.Void](https://stackoverflow.com/q/28762715/2711488) — Holger, Dec 22 '21 at 17:11
Does this answer your question? [Java generics void/Void types](https://stackoverflow.com/questions/5568409/java-generics-void-void-types) — Nowhere Man, Dec 22 '21 at 22:57

score 3 · Accepted Answer · answered Dec 22 '21 at 21:00

Option 1:

Just return null:

@Override
public Void handler(DeleteProductCommand deleteProductCommand) {
    iProductService.delete(deleteProductCommand.id);
    return null;
}

Option 2:

Update the IProductService::delete method to return something meaningful, e.g. a boolean value like Collection::remove does:

public interface IProductService {
   boolean delete(String id);
}

@Singleton
public record DeleteProductCommandHandler(IProductService iProductService)
            implements IRequestHandler<DeleteProductCommand, Boolean> {

    @Override
    public Boolean handler(DeleteProductCommand deleteProductCommand) {
        return iProductService.delete(deleteProductCommand.id);
    }

    @Override
    public Class<DeleteProductCommand> commandType() {
        return DeleteProductCommand.class;
    }
}

void with Generics in Java

1 Answers1

Option 1:

Option 2: