Question

我有一个通用接口，需要将其类型作为通用参数：

interface Base<X extends Base<X>> {
    X foo();
}
class Derived implements Base<Derived> {
    public Derived foo() { ... }
    public Derived bar() { ... }
}
class Derived2 implements Base<Derived2> {
    public Derived2 foo() { ... }
    public void quz() { ... }
}

我有另一个使用此接口作为通用参数的类。

interface Policy<B extends Base<B>> {
  B apply(B b);
}

我有一些Policy实现仅适用于特定的派生类：

class DerivedPolicy implements Policy<Derived> {
   public Derived apply(Derived d) {
     return d.foo().bar();
   }
}

但可以使用任何实现的其他人

class GeneralPolicy implements Policy {
     public Base apply(Base b) {
         return b.foo();
     }
}

以上代码编译，但在GeneralPolicy中提供有关未经检查的类型的警告，这是准确的，因为Base没有指定其泛型类型。第一个明显的修复是GeneralPolicy implements Policy<Base>，w

Test.java:26: error: type argument Base is not within bounds of type-variable B
class GeneralPolicy implements Policy<Base> {
                                      ^
  where B is a type-variable:
    B extends Base<B> declared in interface Policy

使用GeneralPolicy implements Policy<Base<?>>也不起作用：

Test.java:26: error: type argument Base<?> is not within bounds of type-variable B
class GeneralPolicy implements Policy<Base<?>> {
                                          ^
  where B is a type-variable:
    B extends Base<B> declared in interface Policy

我最后一次尝试：GeneralPolicy implements Policy<Base<? extends Base<?>>>

Test.java:26: error: type argument Base<? extends Base<?>> is not within bounds of type-variable B
class GeneralPolicy implements Policy<Base<? extends Base<?->- {
                                           ^
  where B is a type-variable:
    B extends Base<B> declared in interface Policy

有没有办法声明这个有效并且没有未经检查的类型？

Answer 1

在Java 5+中，返回类型可以是https://cors-anywhere.herokuapp.com/，所以你不需要使用泛型，一开始就有：

interface Base {
    Base foo();
}
class Derived implements Base {
    public Derived foo() { ... }
    public Derived bar() { ... }
}

然后我再也看不到仿制药的问题了。

递归泛型类型列表

1 个答案: