[BUG] SuperBuilder's builder method with unbound generics forces casting down the route...

[lbreuss]

Describe the bug
The builder() method of @SuperBuilder returns a Builder with unbound generics, which causes problems down the lane.

@SuperBuilder
public class MembershipEntity {
    String topic;
}

a Test method:

    // compile error: 
    public List<MembershipEntity> generate() {
        return List.of(MembershipEntity.builder().topic("TEST").build());
    }

compile error:
Required type: List<MembershipEntity>
Provided: List<? extends MembershipEntity>

The problematic delomboked Vanilla code:

    public static MembershipEntityBuilder<?,?> builder() {
        return new MembershipEntityBuilderImpl();
    }

Workaround
I have to cast the result of the builder, which is quite ugly and cumbersome:

    public List<MembershipEntity> generateCasted() {
        return List.of(((MembershipEntity)(MembershipEntity.builder().topic("TEST").build())));
    }

Or I could provide my own implementation of builder without generics in every @SuperBuilder annotated class:

    public static MembershipEntityBuilder builder() {
        return new MembershipEntityBuilderImpl();
    }

This does not work for toBuilder() tough, which has the same problem...

Expected behavior
I would like to have no generics at all, or properly bound generics for the above case to work.

    public static MembershipEntityBuilder builder() {
        return new MembershipEntityBuilderImpl();
    }

Version info :

• Lombok version 1.18.26
• Platform: IntelliJ, JDK javac 11.0.19

[lbreuss]

#2708, eventually related to this issue

[Rawi01]

The build method is the important one, the builder doesn't matter. As far as I know it always returns a non-generic type. Can you please check your imports and verify that you use the jdk List?

[lbreuss]

Thanks for the prompt answer. Yes, it's java.util.List, I verified.

Indeed, the delomboked code shows that build() returns the desired class, but still I get the above error. Can you reproduce the compiler error stated above with the simple test method?

Let me show you the relevant code:

@SuperBuilder
public class MembershipEntity extends PersistenceEntity {
    private String topic;

    public static MembershipEntityBuilder<?, ?> builder() {
        return new MembershipEntityBuilderImpl();
    }

    public static abstract class MembershipEntityBuilder<C extends MembershipEntity, B extends MembershipEntityBuilder<C, B>> extends PersistenceEntityBuilder<C, B> {
        private String topic;

        public B topic(String topic) {
            this.topic = topic;
            return self();
        }

        protected B $fillValuesFrom(C instance) {
            super.$fillValuesFrom(instance);
            MembershipEntityBuilder.$fillValuesFromInstanceIntoBuilder(instance, this);
            return self();
        }
        protected abstract B self();
        public abstract C build();
    }

    private static final class MembershipEntityBuilderImpl extends MembershipEntityBuilder<MembershipEntity, MembershipEntityBuilderImpl> {
        protected MembershipEntityBuilderImpl self() {
            return this;
        }
        public MembershipEntity build() {
            return new MembershipEntity(this);
        }
    }

If the builder() method really needs generics, it's probably a bit more of generics acrobatics than just <?,?>. Didn't have time to investigate myself yet. generics sometimes require super powers. ;-)

[janrieke]

I cannot reproduce this. This code compiles perfectly with both Eclipse and javac:

@SuperBuilder
public class MembershipEntity {
    String topic;
    
    public List<MembershipEntity> generate() {
        return List.of(MembershipEntity.builder().topic("TEST").build());
    }
}

I'm not using IntelliJ, maybe it's a problem just for the IntelliJ compiler?

[lbreuss]

I'll double check in 3 weeks... thanks so far for retesting.