在接口

Question

我一直在阅读Java中的Wildcards，但我无法弄清楚如何在接口方法声明的实现中解析Collection的Wildcard类型。您可以通过检查Collection中的一个对象从技术上发现类型，但这不允许您解析Collection中的类型，如果Collection为空，它将失败。

public interface SomeInterface {
    void addAThing(Object thing);
    void addAListOfThings(Collection< ?> things);
}

public class SomeInterfaceImplementation implements SomeInterface {
    @Override
    public void addAThing(Object thing) {
        if (thing instanceof Foo) {
            /* thing has been discovered to be of type Foo
            so now it can be assigned to an explicit Foo object */
            Foo fooThing = (Foo) thing;
        }
    }

    @Override
    public void addAListOfThings(Collection< ?> things) {
        //this fails if things is empty
        if (things.toArray()[0] instanceof Foo) {
            /* things type has been discovered(maybe) to be of type Foo
            but now we are unable cast without an unchecked cast exception */
            Collection<Foo> fooThings = (Collection<Foo>) things;
        }
    }
}

是否有一个我不知道这样做的正确方法？

Answer 1

如果您希望您的方法使用泛型，则应在签名或类/接口定义中定义。

public interface SomeInterface<T> {
    void addAThing(T thing);
    void addAListOfThings(Collection<T> things);
}

通用类型Foo的实现类：

public class SomeInterfaceImplementation implements SomeInterface<Foo> {

    @Override
    public void addAThing(Foo thing) {
      // thing is of type Foo
    }

    @Override
    public void addAListOfThings(Collection<Foo> things) {
      // things is a collection of Foo
    }
}