Question

我正在使用一个我无法控制的外部库。它需要我实现一个方法，但我不知道如何返回所需的类型：

@Override
public Optional<Class<? extends AnInterface<?>>> getCustomClass() {
    return Optional.of(MyClass.class);
    // compiler is giving error at the above line:
    // error: incompatible types: inference variable T has incompatible bounds
    // equality constraints: Class<? extends AnInterface<?>>
    // lower bounds: Class<MyClass>
    // where T is a type-variable:
    //   T extends Object declared in method <T>of(T)
}

MyClass的：

public MyClass implements AnInterface<String> {
    ...
}

我也尝试过这个，但它不起作用：

@Override
public Optional<Class<? extends AnInterface<?>>> getCustomClass() {
    Optional<Class<? extends AnInterface<?>> clazz = MyClass.class;
    // this time the compiler gives error at the above line:
    // error: incompatible types: Class<MyClass> cannot be converted to Class<? extends AnInterface<?>>
    return clazz;
}

还尝试了另一个不起作用的人：

@Override
public Optional<Class<? extends AnInterface<?>>> getCustomClass() {
    Optional<Class<? extends AnInterface> clazz = MyClass.class;
    return clazz;
    // this time the compiler gives error at the above line:
    // error: incompatible types: inference variable T has incompatible bounds
    // equality constraints: Class<? extends PropertyEditor<?>>
    // lower bounds: Class<CAP#1>
    // where T is a type-variable:
    //   T extends Object declared in method <T>of(T)
    // where CAP#1 is a fresh type-variable:
    //   CAP#1 extends PropertyEditor from capture of ? extends PropertyEditor
}

请知道如何退回自定义课程？

JDK版本：8u77

编辑：这些类是这样嵌套的：

public abstract class ParentClass<T> {
    public abstract T method();

    public class ImplementationClass implements ExternalLibraryInterface {
        @Override
        public Optional<Class<? extends AnotherExternalLibraryInterface<?>>> getCustomClass() {
            return Optional.of(MyClass.class);
            // compiler is giving error at the above line:
            // error: incompatible types: inference variable T has incompatible bounds
            // equality constraints: Class<? extends AnotherExternalLibraryInterface<?>>
            // lower bounds: Class<MyClass>
            // where T is a type-variable:
            //   T extends Object declared in method <T>of(T)
        }

        MyClass implements AnotherExternalLibraryInterface<String>{
            ...
        }
    }
}

因此，出于某种原因，在实现外部接口的重写方法时，编译器会被通配符搞糊涂。将所有类移动到自己的文件后，一切正常。

然而，@ Zefick的第一个解决方案实际上在嵌套类的情况下工作，我会将其标记为已解决：

return (Optional)(Optional.of(MyClass.class));

Answer 1

转换为原始的可选类型有效但会发出警告。

public Optional<Class<? extends AnInterface<?>>> getCustomClass() {
    return (Optional)(Optional.of(MyClass.class));
}

其他解决方案：

public Optional<Class<? extends AnInterface<?>>> getCustomClass() {
    return Optional.<Class<? extends AnInterface<?>>>of(MyClass.class);
}

返回班级

1 个答案: