递归泛型类型列表

Question

I have a generic interface which needs to be have its type as a generic parameter:

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() { ... }
}

I have another class which uses this interface as a generic parameter.

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

I have some Policy implementations that only work with a specific derived class:

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

but others that can work with any implementation

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

The above code compiles, but gives warnings about unchecked types in GeneralPolicy , which is accurate since Base does not have its generic type specified. The first obvious fix is 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

Using GeneralPolicy implements Policy<Base<?>> also does not work:

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

I made one last try: GeneralPolicy implements Policy<Base<? extends Base<?>>> 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

Is there a way to declare this that works and does not have unchecked types?

Answer 1

In Java 5+ return type could be covariant , so you don't need to use generics, at the beginning you have:

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

then I don't see a problem with generics anymore.