Kotlin:不能使用GenericTypeIndicator来调用Firebase数据库的getValue

Question

当使用Kotlin与Firebase数据库一起工作时,List<String>如果我使用GenericTypeIndicator如下所示,我似乎无法检索类型的值:

snap.getValue(object : GenericTypeIndicator<List<String>>() {})

它会从Firebase SDK中生成异常,如下所示:

com.google.firebase.database.DatabaseException: Generic wildcard types are not supported
    at com.google.android.gms.internal.zzbtg.zza(Unknown Source)
    at com.google.android.gms.internal.zzbtg.zza(Unknown Source)
    at com.google.android.gms.internal.zzbtg.zza(Unknown Source)
    at com.google.android.gms.internal.zzbtg.zza(Unknown Source)
    at com.google.firebase.database.DataSnapshot.getValue(Unknown Source)

但是,如果我从Java调用它,如下所示,它可以工作:

snap.getValue(new GenericTypeIndicator<List<String>>() {})

我猜它与类型reifying有关,所以我这样做:

inline fun <reified T> genericType() = object: GenericTypeIndicator<T>() {}
val stringListIndicator = genericType<List<String>>()

snap.getValue(stringListIndicator)

但同样的例外发生了.

为什么会这样？

---

编辑:我尝试使用jadx-0.6.1反编译Java和Kotlin版本.

Java来源:

public class Randommmm {
    private static final GenericTypeIndicator<List<String>> ti = new GenericTypeIndicator<List<String>>() {
    };

    public static List<String> x(DataSnapshot snap) {
        return snap.getValue(ti);
    }
}

反编译:

public class Randommmm {
    private static final GenericTypeIndicator<List<String>> ti = new C12761();

    static class C12761 extends GenericTypeIndicator<List<String>> {
        C12761() {
        }
    }

    public static List<String> m48x(DataSnapshot snap) {
        return (List) snap.getValue(ti);
    }
}

Kotlin来源(1):

object Randommmm {
    private val ti = object : GenericTypeIndicator<List<String>>() {
    }

    fun x(snap: DataSnapshot): List<String> {
        return snap.getValue(ti)
    }
}

反编译:

public final class Randommmm {
    public static final Randommmm INSTANCE = null;
    private static final Randommmm$ti$1 ti = null;

    static class C12761 extends GenericTypeIndicator<List<String>> {
        C12761() {
        }
    }

    static {
        Randommmm randommmm = new Randommmm();
    }

    private Randommmm() {
        INSTANCE = this;
        ti = new Randommmm$ti$1();
    }

    @NotNull
    public final List<String> m48x(@NotNull DataSnapshot snap) {
        Intrinsics.checkParameterIsNotNull(snap, "snap");
        Object value = snap.getValue(ti);
        Intrinsics.checkExpressionValueIsNotNull(value, "snap.getValue(ti)");
        return (List) value;
    }
}

public final class Randommmm$ti$1 extends GenericTypeIndicator<List<? extends String>> {
    Randommmm$ti$1() {
    }
}

Kotlin来源(2)使用ArrayListDoug的建议:

object Randommmm {
    private val ti = object : GenericTypeIndicator<ArrayList<String>>() {
    }

    fun x(snap: DataSnapshot): List<String> {
        return snap.getValue(ti)
    }
}

反编译:

public final class Randommmm {
    public static final Randommmm INSTANCE = null;
    private static final Randommmm$ti$1 ti = null;

    static class C12761 extends GenericTypeIndicator<List<String>> {
        C12761() {
        }
    }

    static {
        Randommmm randommmm = new Randommmm();
    }

    private Randommmm() {
        INSTANCE = this;
        ti = new Randommmm$ti$1();
    }

    @NotNull
    public final List<String> m48x(@NotNull DataSnapshot snap) {
        Intrinsics.checkParameterIsNotNull(snap, "snap");
        Object value = snap.getValue(ti);
        Intrinsics.checkExpressionValueIsNotNull(value, "snap.getValue(ti)");
        return (List) value;
    }
}

public final class Randommmm$ti$1 extends GenericTypeIndicator<ArrayList<String>> {
    Randommmm$ti$1() {
    }
}

Answer 1

在Kotlin,

val ti = object : GenericTypeIndicator<List<String>>() {}

生成为(在Java中):

SyntethicClass ti = new SyntethicClass();

public final class SyntethicClass extends GenericTypeIndicator<List<? extends String>> {}

注意通配符? extends String而不是普通的String.

有关说明,请参阅https://kotlinlang.org/docs/reference/java-to-kotlin-interop.html#variant-generics.

为了防止这种情况,请使用以下命令注释类型参数@JvmSuppressWildcards:

val ti = object : GenericTypeIndicator<List<@JvmSuppressWildcards String>>() {}

这变得非常难看,但它确实有效.或者,ArrayList根据Doug的回答使用.

Answer 2

尝试更改此行：

snap.getValue(object : GenericTypeIndicator<List<String>>() {})

对此：

snap.getValue(object : GenericTypeIndicator<ArrayList<String>>() {})

它对我有用，但我不知道为什么。