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如何使Java上界通配符类型参数符合Kotlin?

generics kotlin java

Abhijit Sarkar · 技术社区 · 5 年前

鉴于

public interface StubTransformer {
    AbstractStub<?> transform(String name, AbstractStub<?> stub);
}

和

public static <T extends AbstractStub<T>> T attachHeaders(T stub, Metadata extraHeaders) {...}

这个Kotlin类无法编译。

class MetadataInjectingStubTransformer: StubTransformer {
    override fun transform(name: String, stub: AbstractStub<*>): AbstractStub<*> {
        val header = Metadata()
        return MetadataUtils.attachHeaders(stub, header)
    }
}

错误

Type parameter bound for T in 

fun <T : AbstractStub<T!>!> attachHeaders(stub: T!, extraHeaders: Metadata!): T!
is not satisfied: inferred type AbstractStub<*>! is not a subtype of AbstractStub<AbstractStub<*>!>!

但是Java版本可能会被迫编译。

public class MetadataInjectingStubTransformer implements StubTransformer {
    @Override
    @SuppressWarnings("unchecked")
    public AbstractStub<?> transform(String name, AbstractStub stub) {
        Metadata header = new Metadata();    
        return MetadataUtils.attachHeaders(stub, header);
    }
}

我们如何编译Kotlin代码?