绑定不匹配:Foo类型不是有效的替代

I need to do something like:

我需要做一些事情，比如:

class AbstractA<PARAM extends AbstractB<AbstractA<PARAM>>> {

    public AbstractA(PARAM arg) {
        arg.doStuff(this);
    }

}

class AbstractB<T extends AbstractA<?>> {

    public void doStuff(T onWhat) {
        System.out.println(onWhat);
    }

}

class A extends AbstractA<Foo> {

    public A(Foo arg) {
        super(arg);
    }

}

class Foo extends AbstractB<A> {

}

However, I'm getting this error at "class A extends AbstractA {":

但是，我在“类A扩展AbstractA{”中得到这个错误:

Bound mismatch: The type Foo is not a valid substitute for the bounded
parameter <PARAM extends AbstractB<AbstractA<PARAM>>> of the
type AbstractA<PARAM>

How's that possible? When i try to work out the generics, it looks correct.

那怎么可能?当我试着算出泛型时，它看起来是正确的。

1 个解决方案

#1

Let's draw it out. In

让我们画出来。在

class A extends AbstractA<Foo> {

the type argument Foo is supposed to match

类型参数Foo应该匹配

class AbstractA<PARAM extends AbstractB<AbstractA<PARAM>>> {
                  ^
                  |
                 Foo

where Foo is

Foo在哪里

class Foo extends AbstractB<A> {

Up to now, Foo extends AbstractB. Let's see for its type arguments.

到目前为止，Foo扩展了AbstractB。让我们看看它的类型参数。

class AbstractB<T extends AbstractA<?>> {
                ^
                |
                A

So what is A? Does it match those bounds

那么是什么呢?它符合这些界限吗?

class A extends AbstractA<Foo> {

It seems to since Foo can fit the wildcard.

似乎因为Foo可以匹配通配符。

However, when Foo is used as a type argument, the wildcard needs to be taken into account, so it kind of looks like the following inheritance (and generics) hierarchy

但是，当Foo用作类型参数时，需要考虑通配符，因此它看起来像下面的继承(和泛型)层次结构。

Foo extends AbstractB<A extends AbstractA<?>>

which does not fit the bigger picture of

哪个不符合大局

PARAM extends AbstractB<AbstractA<PARAM>>

So Foo cannot be used as a type argument there.

所以Foo不能用作类型参数。

#1