Builder returns wrong type for generics

[lludwa]

Lombok version: 1.16.18

Using Lombok builder on generics gives compilation error, related with wrong types:

public class Main {
    
    public static abstract class Animal {
    }

    public static class Dog extends Animal {
    }

    @Getter
    @Setter
    public static class SimpleBuilder<ANIMAL extends Animal> {
        private ANIMAL animal;
    }

    @Getter
    @lombok.Builder
    public static class LombokBuilder<ANIMAL extends Animal> {
        private ANIMAL animal;
    }

    public static void main(String[] args) {
        //COMPILES OK:
        final SimpleBuilder<Dog> simpleBuilder = new SimpleBuilder<>();
        simpleBuilder.setAnimal(new Dog());

        //COMPILATION ERROR:
        final LombokBuilder<Dog> lombokBuilder = LombokBuilder.builder().animal(new Dog()).build();
    }
}

LombokBuilder gives upcoming error:

Required: LombokBuilder <Dog>
Found: LombokBuilder <Animal>

[victorwss]

Possibly related with #1420.

[rspilker]

The problem has to do with the type inference. It doesn't carry as far as you seem to think. The call to LombokBuilder.builder() does not have any clue on what you're going to put in. So you need to help the compiler a bit to make it compile:

final LombokBuilder<Dog> lombokBuilder = LombokBuilder.<Dog>builder().animal(new Dog()).build();

I don't think that this is a lombok problem.

[laffuste]

You can get a slightly less dry syntax adding in your class with a @Builder:

public static <T extends Animal> Owner.OwnerBuilder<T> builder(Class<T> cls) { return new Owner.OwnerBuilder<>(); }

(I changed the example class name LombokBuilder to Owner for clarity)

Now you can use the builder like:

// Owner<Dog> owner = Owner.<Dog>builder().animal(new Dog()).build();
Owner<Dog> owner = Owner.builder(Dog.class).animal(new Dog()).build();

Wohoo, almost the same :/