Why is there a warning in this Java generic method definition?

I noticed that if I use generic method signature to complete something similar to the return type of common type, it works just as I thought, except that it will generate a warning:

interface Car {
    <T extends Car> T getCar();
}

class MazdaRX8 implements Car {
    public MazdaRX8 getCar() { // "Unchecked overriding" warning
        return this;
    }
}

Using the above code, my ide warns: "unchecked override: the return type requires an unchecked conversion. Find: 'mazdarx8', need't '

What does this warning mean?

It doesn't make any sense to me. Google doesn't come up with anything useful Why isn't this a warning free replacement for the following interface (which is also free because Java allows similar return types)?

interface Car {
    Car getCar();
}

Solution

You have made the method generic, so the caller gets what type to return (because the caller can specify the type parameter) This is a good interface and can be implemented correctly in Java. Here you won't find type parameters during execution

For example, consider:

Car mazda = new MazdaRX8();        
FordGalaxy galaxy = mazda.<FordGalaxy>getCar();

As far as the interface is concerned, this is perfectly legal, but it obviously won't work

Any reason why interfaces are not generic rather than methods? Then mazdarx8 will implement car < < mazdarx8 >:

interface Car<T extends Car> {
    T getCar();
}

class MazdaRX8 implements Car<MazdaRX8 > {
    public MazdaRX8 getCar() {
        return this;
    }
}