Java类型不匹配，无法转换为以通用类型返回自身

Question

It error's on the "return this;" line with this: Type mismatch: cannot convert from A to T.

public class A<T extends A<T>>{
    public T method() {
        return this;
    }
}

I don't understand why the compiler can't convert from A to T, when A is a suitable candidate for T.

Answer 1

Let us assume your code was valid...

You might have a subclass defined like this

public class B extends A<B> {

}

Here type parameter T is same as type of this .
Which is why I guess you said

A is a suitable candidate for T

---

Now consider this case

public class C extends A<B> {

}

which is perfectly valid since B passes all criteria required for T .

In this case your method declaration in class A becomes invalid.
(return type is B but you are returning instance of class C .)

---

So your assumption that

A is a suitable candidate for T

is invalid. Hence the error.

Hope this helps.
Good luck.